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Rules for the Storage of Calcium Carbide

RULES FOR THE STORAGE OF CALCIUM CARBIDE.

(a) Calcium carbide in quantities not to exceed six hundred (600) pounds may be stored, when contained in approved metal packages not to exceed one hundred (100) pounds each, inside insured property, provided that the place of storage be dry, waterproof and well ventilated, and also provided that all but one of the packages in any one building shall be sealed and the seals shall not be broken so long as there is carbide in excess of one (1) pound in any other unsealed package in the building.

(b) Calcium carbide in quantities in excess of six hundred (600) pounds must be stored above ground in detached buildings, used exclusively for the storage of calcium carbide, in approved metal packages, and such buildings shall be constructed to be dry, waterproof and well ventilated.

(c) Packages to be approved must be made of metal of sufficient strength to insure handling the package without rupture, and be provided with a screwed top or its equivalent.

They must be constructed so as to be water- and air-tight without the use of solder, and conspicuously marked "CALCIUM CARBIDE—DANGEROUS IF NOT KEPT DRY."

The following is a summary of the AUSTRIAN GOVERNMENT rules relating to the storage and handling of carbide:

(1) It must be sold and stored only in closed water-tight vessels, which, if the contents exceed 10 kilos., must be marked in plain letters "CALCIUM CARBIDE—TO BE KEPT CLOSED AND DRY." They must not be of copper and if soldered must be opened by mechanical means and not by unsoldering. They must be stored out of the reach of water.

(2) Quantities not exceeding 300 kilos. may be stored in occupied houses, provided the single drums do not exceed 100 kilos. nominal capacity. The storage-place must be dry and not underground.

(3) The limits specified in Rule 2 apply also to generator-rooms, with the proviso also that in general the amount stored shall not exceed five days' consumption.

(4) Quantities ranging from 300 to 1000 kilos. must be stored in special well-ventilated uninhabited non-basement rooms in which lights and smoking are not allowed.

(5) Quantities exceeding 1000 kilos. must be stored in isolated fireproof magazines with light water-tight roofs. The floors must be at least 8 inches above ground-level.

(6) Carbide in water-tight drums may be stored in the open in a fenced enclosure at least 30 feet from buildings, adjoining property, or inflammable materials. The drums must be protected from wet by a light roof.

(7) The breaking of carbide must be done by men provided with respirators and goggles, and care taken to avoid the formation of dust.

(8) Local or other authorities will issue from time to time special regulations in regard to carbide trade premises.

The ITALIAN GOVERNMENT rules relating to the storage and transport of carbide follow in the main those of the Austrian Government, but for quantities between 300 and 2000 kilos sanction is required from the local authorities, and for larger quantities from superior authorities. The storage of quantities ranging from 300 to 2000 kilos is forbidden in dwelling-houses and above the latter quantity the storage-place must be isolated and specially selected. No special permit is required for the storage of quantities not exceeding 300 kilos. Workmen exposed to carbide dust arising from the breaking of carbide or otherwise must have their eyes and respiratory organs suitably protected.

THE PURCHASE OF CARBIDE.—Since calcium carbide is only useful as a means of preparing acetylene, it should be bought under a guarantee (1) that it contains less impurities than suffice to render the crude gas dangerous in respect of spontaneous inflammability, or objectionable in a manner to be explained later on, when consumed; and (2) that it is capable of evolving a fixed minimum quantity of acetylene when decomposed by water. Such determination, however, cannot be carried out by the ordinary consumer for himself. A generator which is perfectly satisfactory in general behaviour, and which evolves a sufficient proportion of the possible total make of gas to be economical, does not of necessity decompose the carbide quantitatively; nor is it constructed in a fashion to render an exact measurement of the gas liberated at standard temperature and pressure easy to obtain. For obvious reasons the careful consumer of acetylene will keep a record of the carbide decomposed and of the acetylene generated—the latter perhaps only in terms of burner- hours, or the like; but in the event of serious dispute as to the gas- making capacity of his raw material, he must have a proper analysis made by a qualified chemist.

Calcium carbide is crushed by the makers into several different sizes, in each of which all the lumps exceed a certain size and are smaller than another size. It is necessary to find out by experiment, or from the maker, what particular size suits the generator best, for different types of apparatus require different sizes of carbide. Carbide cannot well be crushed by the consumer of acetylene. It is a difficult operation, and fraught with the production of dust which is harmful to the eyes and throat, and if done in open vessels the carbide deteriorates in gas- making power by its exposure to the moisture of the atmosphere. True dust in carbide is objectionable, and practically useless for the generation of acetylene in any form of apparatus, but carbide exceeding 1 inch in mesh is usually sold to satisfy the suggestions of the British Acetylene Association, which prescribes 5 per cent, of dust as the maximum. Some grades of carbide are softer than others, and therefore tend to yield more dust if exposed to a long journey with frequent unloadings.

There are certain varieties of ordinary carbide known as "treated carbide," the value of which is more particularly discussed in Chapter III. The treatment is of two kinds, or of a combination of both. In one process the lumps are coated with a strong solution of glucose, with the object of assisting in the removal of spent lime from their surface when the carbide is immersed in water. Lime is comparatively much more soluble in solutions of sugar (to which class of substances glucose belongs) than in plain water; so that carbide treated with glucose is not so likely to be covered with a closely adherent skin of spent lime when decomposed by the addition of water to it. In the other process, the carbide is coated with or immersed in some oil or grease to protect it from premature decomposition. The latter idea, at least, fulfils its promises, and does keep the carbide to a large extent unchanged if the lumps are exposed to damp air, while solving certain troubles otherwise met with in some generators (cf. Chapter III.); but both operations involve additional expense, and since ordinary carbide can be used satisfactorily in a good fixed generator, and can be preserved without serious deterioration by the exercise of reasonable care, treated carbide is only to be recommended for employment in holderless generators, of which table-lamps are the most conspicuous forms. A third variant of plain carbide is occasionally heard of, which is termed "scented" carbide. It is difficult to regard this material seriously. In all probability calcium carbide is odourless, but as it begins to evolve traces of gas immediately atmospheric moisture reaches it, a lump of carbide has always the unpleasant smell of crude acetylene. As the material is not to be stored in occupied rooms, and as all odour is lost to the senses directly the carbide is put into the generator, scented carbide may be said to be devoid of all utility.

THE REACTION BETWEEN CARBIDE AND WATER.—The reaction which occurs when calcium carbide and water are brought into contact belongs to the class that chemists usually term double decompositions. Calcium carbide is a chemical compound of the metal calcium with carbon, containing one chemical "part," or atomic weight, of the former united to two chemical parts, or atomic weights, of the latter; its composition expressed in symbols being CaC_2. Similarly, water is a compound of two chemical parts of hydrogen with one of oxygen, its formula being H_2O. When those two substances are mixed together the hydrogen of the water leaves its original partner, oxygen, and the carbon of the calcium carbide leaves the calcium, uniting together to form that particular compound of hydrogen and carbon, or hydrocarbon, which is known as acetylene, whose formula is C_2H_2; while the residual calcium and oxygen join together to produce calcium oxide or lime, CaO. Put into the usual form of an equation, the reaction proceeds thus—

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This equation not only means that calcium carbide and water combine to yield acetylene and lime, it also means that one chemical part of carbide reacts with one chemical part of water to produce one chemical part of acetylene and one of lime. But these four chemical parts, or molecules, which are all equal chemically, are not equal in weight; although, according to a common law of chemistry, they each bear a fixed proportion to one another. Reference to the table of "Atomic Weights" contained in any text-book of chemistry will show that while the symbol Ca is used, for convenience, as a contraction or sign for the element calcium simply, it bears a more important quantitative significance, for to it will be found assigned the number 40. Against carbon will be seen the number 12; against oxygen, 16; and against hydrogen, 1. These numbers indicate that if the smallest weight of hydrogen ever found in a chemical compound is called 1 as a unit of comparison, the smallest weights of calcium, carbon, and oxygen, similarly taking part in chemical reactions are 40, 12, and 16 respectively. Thus the symbol CaC_2, comes to convoy three separate ideas: (a) that the substance referred to is a compound of calcium and carbon only, and that it is therefore a carbide of calcium; (b) that it is composed of one chemical part or atom of calcium and two atoms of carbon; and (c) that it contains 40 parts by weight of calcium combined with twice twelve, or 24, parts of carbon. It follows from (c) that the weight of one chemical part, now termed a molecule as the substance is a compound, of calcium carbide is (40 + 2 x 12) = 64. By identical methods of calculation it will be found that the weight of one molecule of water is 18; that of acetylene, 26; and that of lime, 56. The general equation (1) given above, therefore, states in chemical shorthand that 64 parts by weight of calcium carbide react with 18 parts of water to give 26 parts by weight of acetylene and 56 parts of lime; and it is very important to observe that just as there are the same number of chemical parts, viz., 2, on each side, so there are the same number of parts by weight, for 64 + 18 = 56 + 26 = 82. Put into other words equation (1) shows that if 64 grammes, lb., or cwts. of calcium carbide are treated with 18 grammes, lb., or cwts. of water, the whole mass will be converted into acetylene and lime, and the residue will not contain any unaltered calcium carbide or any water; whence it may be inferred, as is the fact, that if the weights of carbide and water originally taken do not stand to one another in the ratio 64 : 18, both substances cannot be entirely decomposed, but a certain quantity of the one which was in excess will be left unattacked, and that quantity will be in exact accordance with the amount of the said excess—indifferently whether the superabundant substance be carbide or water.

Hitherto, for the sake of simplicity, the by-product in the preparation of acetylene has been described as calcium oxide or quicklime. It is, however, one of the leading characteristics of this body to be hygroscopic, or greedy of moisture; so that if it is brought into the presence of water, either in the form of liquid or as vapour, it immediately combines therewith to yield calcium hydroxide, or slaked lime, whose chemical formula is Ca(OH)_2. Accordingly, in actual practice, when calcium carbide is mixed with an excess of water, a secondary reaction takes place over and above that indicated by equation (1), the quicklime produced combining with one chemical part or molecule of water, thus—

CaO + H_2O = Ca(OH)_2.

As these two actions occur simultaneously, it is more usual, and more in agreement with the phenomena of an acetylene generator, to represent the decomposition of calcium carbide by the combined equation—

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2.

By the aid of calculations analogous to those employed in the preceding paragraph, it will be noticed that equation (2) states that 1 molecule of calcium carbide, or 64 parts by weight, combines with 2 molecules of water, or 36 parts by weight, to yield 1 molecule, or 26 parts by weight of acetylene, and 1 molecule, or 74 parts by weight of calcium hydroxide (slaked lime). Here again, if more than 36 parts of water are taken for every 64 parts of calcium carbide, the excess of water over those 36 parts is left undecomposed; and in the same fashion, if less than 36 parts of water are taken for every 64 parts of calcium carbide, some of the latter must remain unattacked, whilst, obviously, the amount of acetylene liberated cannot exceed that which corresponds with the quantity of substance suffering complete decomposition. If, for example, the quantity of water present in a generator is more than chemically sufficient to attack all the carbide added, however largo or small that excess may be, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 64 parts by weight of calcium carbide consumed. It is, however, not correct to invert the proposition, and to say that if the carbide is in excess of the water added, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 36 parts of water consumed, as might be gathered from equation (2); because equation (1) shows that 26 parts of acetylene may, on occasion, be produced by the decomposition of 18 parts by weight of water. From the purely chemical point of view this apparent anomaly is explained by the circumstance that of the 36 parts of water present on the left-hand aide of equation (2), only one-half, i.e., 18 parts by weight, are actually decomposed into hydrogen and oxygen, the other 18 parts remaining unattacked, and merely attaching themselves as "water of hydration" to the 56 parts of calcium oxide in equation (1) so as to produce the 74 parts of calcium hydroxide appearing on the right-hand side of equation (2). The matter is perhaps rendered more intelligible by employing the old name for calcium hydroxide or slaked lime, viz., hydrated oxide of calcium, and by writing its formula in the corresponding form, when equation (2) becomes

CaC_2 + 2H_2O = C_2H_2 + CaO.H_2O.

It is, therefore, absolutely correct to state that if the amount of calcium carbide present in an acetylene generator is more than chemically sufficient to decompose all the water introduced, no more, and theoretically speaking no less, acetylene can ever be liberated than 26 parts by weight of gas for every 18 parts by weight of water attacked. This, it must be distinctly understood, is the condition of affairs obtaining in the ideal acetylene generator only; since, for reasons which will be immediately explained, when the output of gas is measured in terms of the water decomposed, in no commercial apparatus, and indeed in no generator which can be imagined fit for actual employment, does that output of gas ever approach the quantitative amount; but the volume of water used, if not actually disappearing, is always vastly in excess of the requirements of equation (2). On the contrary, when the make of gas is measured in terms of the calcium carbide consumed, the said make may, and frequently does, reach 80, 90, or even 99 per cent. of what is theoretically possible. Inasmuch as calcium carbide is the one costly ingredient in the manufacture of acetylene, so long as it is not wasted— so long, that is to say, as nearly the theoretical yield of gas is obtained from it—an acetylene generator is satisfactory or efficient in this particular; and except for the matter of solubility discussed in the following chapter, the quantity of water consumed is of no importance whatever.

HEAT EVOLVED IN THE REACTION.—The chemical reaction between calcium carbide and water is accompanied by a large evolution of heat, which, unless due precautions are taken to prevent it, raises the temperature of the substances employed, and of the apparatus containing them, to a serious and often inconvenient extent. This phenomenon is the most important of all in connexion with acetylene manufacture; for upon a proper recognition of it, and upon the character of the precautions taken to avoid its numerous evil effects, depend the actual value and capacity for smooth working of any acetylene generator. Just as, by an immutable law of chemistry, a given weight of calcium carbide yields a given weight of acetylene, and by no amount of ingenuity can be made to produce either more or less; so, by an equally immutable law of physics, the decomposition of a given weight of calcium carbide by water, or the decomposition of a given weight of water by calcium carbide, yields a perfectly definite quantity of heat—a quantity of heat which cannot be reduced or increased by any artifice whatever. The result of a production of heat is usually to raise the temperature of the material in which it is produced; but this is not always the case, and indeed there is no necessary connexion or ratio between the quantity of heat liberated in any form of chemical reaction—of which ordinary combustion is the commonest type—and the temperature attained by the substances concerned. This matter has so weighty a bearing upon acetylene generation, and appears to be so frequently misunderstood, that a couple of illustrations may with advantage be studied. If a vessel full of cold water, and containing also a thermometer, is placed over a lighted gas-burner, at first the temperature of the liquid rises steadily, and there is clearly a ratio between the size of the flame and the speed at which the mercury mounts up the scale. Finally, however, the thermometer indicates a certain point, viz., 100° C, and the water begins to boil; yet although the burner is untouched, and consequently, although heat must be passing into the vessel at the same rate as before, the mercury refuses to move as long as any liquid water is left. By the use of a gas meter it might be shown that the same volume of gas is always consumed (a) in raising the temperature of a given quantity of cold water to the boiling- point, and another equally constant volume of gas is always consumed (b) in causing the boiling water to disappear as steam. Hence, as coal-gas is assumed for the present purpose to possess invariably the same heating power, it appears that the same quantity of heat is always needed to convert a given amount of cold water at a certain temperature into steam; but inasmuch as reference to the meter would show that about 5 times the volume of gas is consumed in changing the boiling water into steam as is used in heating the cold water to the boiling-point, it will be evident that the temperature of the mass is raised as high by the heat evolved during the combustion of one part of gas as it is by that liberated on the combustion of 6 times that amount.

A further example of the difference between quantity of heat and sensible temperature may be seen in the combustion of coal, for (say) one hundredweight of that fuel might be consumed in a very few minutes in a furnace fitted with a powerful blast of air, the operation might be spread over a considerable number of hours in a domestic grate, or the coal might be allowed to oxidise by exposure to warm air for a year or more. In the last case the temperature might not attain that of boiling water, in the second it would be about that of dull redness, and in the first it would be that of dazzling whiteness; but in all three cases the total quantity of heat produced by the time the coal was entirely consumed would be absolutely identical. The former experiment with water and a gas-burner, too, might easily be modified to throw light upon another problem in acetylene generation, for it would be found that if almost any other liquid than water were taken, less gas (i.e., a smaller quantity of heat) would be required to raise a given weight of it from a certain low to a certain high temperature than in the case of water itself; while if it were possible similarly to treat the same weight of iron (of which acetylene generators are constructed), or of calcium carbide, the quantity of heat used to raise it through a given number of thermometric degrees would hardly exceed one-tenth or one- quarter of that needed by water itself. In technical language this difference is due to the different specific heats of the substances mentioned; the specific heat of a body being the relative quantity of heat consumed in raising a certain weight of it a certain number of degrees when the quantity of heat needed to produce the same effect on the same weight of water is called unity. Thus, the specific heat of water being termed 1.0, that of iron or steel is 0.1138, and that of calcium carbide 0.247, [Footnote: This is Carlson's figure. Morel has taken the value 0.103 in certain calculations.] both measured at temperatures where water is a liquid. Putting the foregoing facts in another shape, for a given rise in temperature that substance will absorb the most heat which has the highest specific heat, and therefore, in this respect, 1 part by weight of water will do the work of roughly 9 parts by weight of iron, and of about 4 parts by weight of calcium carbide.

From the practical aspect what has been said amounts to this: During the operation of an acetylene generator a large amount of heat is produced, the quantity of which is beyond human control. It is desirable, for various reasons, that the temperature shall be kept as low as possible. There are three substances present to which the heat may be compelled to transfer itself until it has opportunity to pass into the surrounding atmosphere: the material of which the apparatus is constructed, the gas which is in process of evolution, and whichever of the two bodies— calcium carbide or water—is in excess in the generator. Of these, the specific heat at constant pressure of acetylene has unfortunately not yet been determined, but its relative capacity for absorbing heat is undoubtedly small; moreover the gas could not be permitted to become sufficiently hot to carry off the heat without grave disadvantages. The specific heat of calcium carbide is also comparatively small, and there are similar disadvantages in allowing it to become hot; moreover it is deficient in heat-conducting power, so that heat communicated to one portion of the mass does not extend rapidly throughout, but remains concentrated in one spot, causing the temperature to rise objectionably. Steel has a sufficient amount of heat-conducting power to prevent undue concentration in one place; but, as has been stated, its specific heat is only one-ninth that of water. Water is clearly, therefore, the proper substance to employ for the dissipation of the heat generated, although it is strictly speaking almost devoid of heat-conducting power; for not only is the specific heat of water much greater than that of any other material present, but it possesses in a high degree the faculty of absorbing heat throughout its mass, by virtue of the action known as convection, provided that heat is communicated to it at or near the bottom, and not too near its upper surface. Moreover, water is a much more valuable substance for dissipating heat than appears from the foregoing explanation; for reference to the experiment with the gas- burner will show that six and a quarter times as much heat can be absorbed by a given weight of water if it is permitted to change into steam, as if it is merely raised to the boiling-point; and since by no urging of the gas-burner can the temperature be raised above 100° C. as long as any liquid water remains unevaporated, if an excess of water is employed in an acetylene generator, the temperature inside can never— except quite locally—exceed 100° C., however fast the carbide be decomposed. An indefinitely large consumption of water by evaporation in a generator matters nothing, for the liquid may be considered of no pecuniary value, and it can all be recovered by condensation in a subsequent portion of the plant.

It has been said that the quantity of heat liberated when a certain amount of carbide suffers decomposition is fixed; it remains now to consider what that quantity is. Quantities of heat are always measured in terms of the amount needed to raise a certain weight of water a certain number of degrees on the thermometric scale. There are several units in use, but the one which will be employed throughout this book is the "Large Calorie"; a large calorie being the amount of heat absorbed in raising 1 kilogramme of water 1° C. Referring for a moment to what has been said about specific heats, it will be apparent that if 1 large calorie is sufficient to heat 1 kilo, of water through 1° C. the same quantity will heat 1 kilo. of steel, whose specific heat is roughly 0.11, through (10/011) = 9° C., or, which comes to the same thing, will heat 9 kilos, of steel through 1° C.; and similarly, 1 large calorie will raise 4 kilos. of calcium carbide 1° C. in temperature, or 1 kilo. 4° C. The fact that a definite quantity of heat is manifested when a known weight of calcium carbide is decomposed by water is only typical; for in every chemical process some disturbance of heat, though not necessarily of sensible (or thermometric) character, occurs, heat being either absorbed or set free. Moreover, if when given weights of two or more substances unite to form a given weight of another substance, a certain quantity of heat is set free, precisely the same amount of heat is absorbed, or disappears, when the latter substance is decomposed to form the same quantities of the original substances; and, per contra, if the combination is attended by a disappearance of heat, exactly the same amount is liberated when the compound is broken up into its first constituents. Compounds are therefore of two kinds: those which absorb heat during their preparation, and consequently liberate heat when they are decomposed—such being termed endothermic; and those which evolve heat during their preparation, and consequently absorb heat when they are decomposed—such being called exothermic. If a substance absorbs heat during its formation, it cannot be produced unless that heat is supplied to it; and since heat, being a form of motion, is equally a form of energy, energy must be supplied, or work must be done, before that substance can be obtained. Conversely, if a substance evolves heat during its formation, its component parts evolve energy when the said substance is being produced; and therefore the mere act of combination is accompanied by a facility for doing work, which work may be applied in assisting some other reaction that requires heat, or may be usefully employed in any other fashion, or wasted if necessary. Seeing that there is a tendency in nature for the steady dissipation of energy, it follows that an exothermic substance is stable, for it tends to remain as it is unless heat is supplied to it, or work is done upon it; whereas, according to its degree of endothermicity, an endothermic substance is more or less unstable, for it is always ready to emit heat, or to do work, as soon as an opportunity is given to it to decompose. The theoretical and practical results of this circumstance will be elaborated in Chapter VI., when the endothermic nature of acetylene is more fully discussed.

A very simple experiment will show that a notable quantity of heat is set free when calcium carbide is brought into contact with water, and by arranging the details of the apparatus in a suitable manner, the quantity of heat manifested may be measured with considerable accuracy. A lengthy description of the method of performing this operation, however, scarcely comes within the province of the present book, and it must be sufficient to say that the heat is estimated by decomposing a known weight of carbide by means of water in a small vessel surrounded on all sides by a carefully jacketed receptacle full of water and provided with a sensitive thermometer. The quantity of water contained in the outer vessel being known, and its temperature having been noted before the reaction commences, an observation of the thermometer after the decomposition is finished, and when the mercury has reached its highest point, gives data which show that the reaction between water and a known weight of calcium carbide produces heat sufficient in amount to raise a known weight of water through a known thermometric distance; and from these figures the corresponding number of large calories may easily be calculated. A determination of this quantity of heat has been made experimentally by several investigators, including Lewes, who has found that the heat evolved on decomposing 1 gramme of ordinary commercial carbide with water is 0.406 large calorie. [Footnote: Lewes returns his result as 406 calories, because he employs the "small calorie." The small calorie is the quantity of heat needed to raise 1 gramme of water 1° C.; but as there are 1000 grammes in 1 kilogramme, the large calorie is equal to 1000 small calories. In many respects the former unit is to be preferred.] As the material operated upon contained only 91.3 per cent. of true calcium carbide, he estimates the heat corresponding with the decomposition of 1 gramme of pure carbide to be 0.4446 large calorie. As, however, it is better, and more in accordance with modern practice, to quote such data in terms of the atomic or molecular weight of the substance concerned, and as the molecular weight of calcium carbide is 64, it is preferable to multiply these figures by 64, stating that, according to Lewes' researches, the heat of decomposition of "1 gramme- molecule" (i.e., 64 grammes) of a calcium carbide having a purity of 91.3 per cent. is just under 26 calories, or that of 1 gramme-molecule of pure carbide 28.454 calories. It is customary now to omit the phrase "one gramme-molecule" in giving similar figures, physicists saying simply that the heat of decomposition of calcium carbide by water when calcium hydroxide is the by-product, is 28.454 large calories.

Assuming all the necessary data known, as happens to be the case in the present instance, it is also possible to calculate theoretically the heat which should be evolved on decomposing calcium carbide by means of water. Equation (2), given on page 24, shows that of the substances taking part in the reaction 1 molecular weight of calcium carbide is decomposed, and 1 molecular weight of acetylene is formed. Of the two molecules of water, only one is decomposed, the other passing to the calcium hydroxide unchanged; and the 1 molecule of calcium hydroxide is formed by the combination of 1 atom of free calcium, 1 atom of free oxygen, and 1 molecule of water already existing as such. Calcium hydroxide and water are both exothermic substances, absorbing heat when they are decomposed, liberating it when they are formed. Acetylene is endothermic, liberating heat when it is decomposed, absorbing it when it is produced. Unfortunately there is still some doubt about the heat of formation of calcium carbide, De Forcrand returning it as -0.65 calorie, and Gin as +3.9 calories. De Forcrand's figure means, as before explained, that 64 grammes of carbide should absorb 0.65 large calorie when they are produced by the combination of 40 grammes of calcium with 24 grammes of carbon; the minus sign calling attention to the belief that calcium carbide is endothermic, heat being liberated when it suffers decomposition. On the contrary, Gin's figure expresses the idea that calcium carbide is exothermic, liberating 3.9 calories when it is produced, and absorbing them when it is decomposed. In the absence of corroborative evidence one way or the other, Gin's determination will be accepted for the ensuing calculation. In equation (2), therefore, calcium carbide is decomposed and absorbs heat; water is decomposed and absorbs heat; acetylene is produced and absorbs heat; and calcium hydroxide is produced liberating heat. On consulting the tables of thermo-chemical data given in the various text-books on physical chemistry, all the other constants needed for the present purpose will be found; and it will appear that the heat of formation of water is +69 calories, that of acetylene -58.1 calories, and that of calcium hydroxide, when 1 atom of calcium, 1 atom of oxygen, and 1 molecule of water unite together, is +160.1 calories. [Footnote: When 1 atom of calcium, 2 atoms of oxygen, and 2 atoms of hydrogen unite to form solid calcium hydroxide, the heat of formation of the latter is 229.1 (cf. infra). This value is simply 160.1 + 69.0 = 229.1; 69.0 being the heat of formation of water.] Collecting the results into the form of a balance-sheet, the effect of decomposing calcium carbide with water is this:

Therefore when 64 grammes of calcium carbide are decomposed by water, or when 18 grammes of water are decomposed by calcium carbide (the by- product in each case being calcium hydroxide or slaked lime, for the formation of which a further 18 grammes of water must be present in the second instance), 29.1 large calories are set free. It is not possible yet to determine thermo-chemical data with extreme accuracy, especially on such a material as calcium carbide, which is hardly to be procured in a state of chemical purity; and so the value 28.454 calories experimentally found by Lewes agrees very satisfactorily, considering all things, with the calculated value 29.1 calories. It is to be noticed, however, that the above calculated value has been deduced on the assumption that the calcium hydroxide is obtained as a dry powder; but as slaked lime is somewhat soluble in water, and as it evolves 3 calories in so dissolving, if sufficient water is present to take up the calcium hydroxide entirely into the liquid form (i.e., that of a solution), the amount of heat set free will be greater by those 3 calories, i.e., 32.1 large calories altogether.

THE PROCESS OF GENERATION.—Taking 28 as the number of large calories developed when 64 grammes of ordinary commercial calcium carbide are decomposed with sufficient water to leave dry solid calcium hydroxide as the by-product in acetylene generation, this quantity of heat is capable of exerting any of the following effects. It is sufficient (1) to raise 1000 grammes of water through 28° C., say from 10° C. (50° F., which is roughly the temperature of ordinary cold water) to 38° C. It is sufficient (2) to raise 64 grammes of water (a weight equal to that of the carbide decomposed) through 438° C., if that were possible. It would raise (3) 311 grammes of water through 90° C., i.e., from 10° C. to the boiling-point. If, however, instead of remaining in the liquid state, the water were converted into vapour, the same quantity of heat would suffice (4) to change 44.7 grammes of water at 10° C. into steam at 100° C.; or (5) to change 46.7 grammes of water at 10° C. into vapour at the same temperature. It is an action of the last character which takes place in acetylene generators of the most modern and usual pattern, some of the surplus water being evaporated and carried away as vapour at a comparatively low temperature with the escaping gas; for it must be remembered that although steam, as such, condenses into liquid water immediately the surrounding temperature falls below 100° C., the vapour of water remains uncondensed, even at temperatures below the freezing- point, when that vapour is distributed among some permanent gas—the precise quantity of vapour so remaining being a function of the temperature and barometric height. Thus it appears that if the heat evolved during the decomposition of calcium carbide is not otherwise consumed, it is sufficient in amount to vaporise almost exactly 3 parts by weight of water for every 4 parts of carbide attacked; but if it were expended upon some substance such as acetylene, calcium carbide, or steel, which, unlike water, could not absorb an extra amount by changing its physical state (from solid to liquid, or from liquid to gas), the heat generated during the decomposition of a given weight of carbide would suffice to raise an equal weight of the particular substance under consideration to a temperature vastly exceeding 438° C. The temperature attained, indeed, measured in Centigrade degrees, would be 438 multiplied by the quotient obtained on dividing the specific heat of water by the specific heat of the substance considered: which quotient, obviously, is the "reciprocal" of the specific heat of the said substance.

The analogy to the combustion of coal mentioned on a previous page shows that although the quantity of heat evolved during a certain chemical reaction is strictly fixed, the temperature attained is dependent on the time over which the reaction is spread, being higher as the process is more rapid. This is due to the fact that throughout the whole period of reaction heat is escaping from the mass, and passing into the atmosphere at a fairly constant speed; so that, clearly, the more slowly heat is produced, the better opportunity has it to pass away, and the less of it is left to collect in the material under consideration. During the action of an acetylene generator, there is a current of gas constantly travelling away from the carbide, there is vapour of water constantly escaping with the gas, there are the walls of the generator itself constantly exposed to the cooling action of the atmosphere, and there is either a mass of calcium carbide or of water within the generator. It is essential for good working that the temperature of both the acetylene and the carbide shall be prevented from rising to any noteworthy extent; while the amount of heat capable of being dissipated into the air through the walls of the apparatus in a given time is narrowly limited, depending upon the size and shape of the generator, and the temperature of the surrounding air. If, then, a small, suitably designed generator is working quite slowly, the loss of heat through the external walls of the apparatus may easily be rapid enough to prevent the internal temperature from rising objectionably high; but the larger the generator, and the more rapidly it is evolving gas, the less does this become possible. Since of the substances in or about a generator water is the one which has by far the largest capacity for absorbing heat, and since it is the only substance to which any necessary quantity of heat can be safely or conveniently transmitted, it follows that the larger in size an acetylene generator is, or the more rapidly that generator is made to deliver gas, the more desirable is it to use water as the means for dissipating the surplus heat, and the more necessary is it to employ an apparatus in which water is in large chemical excess at the actual place of decomposition.

The argument is sometimes advanced that an acetylene generator containing carbide in excess will work satisfactorily without exhibiting an undesirable rise in internal temperature, if the vessel holding the carbide is merely surrounded by a large quantity of cold water. The idea is that the heat evolved in that particular portion of the charge which is suffering decomposition will be communicated with sufficient speed throughout the whole mass of calcium carbide present, whence it will pass through the walls of the containing vessel into the water all round. Provided the generator is quite small, provided the carbide container is so constructed as to possess the maximum of superficial area with the minimum of cubical capacity (a geometrical form to which the sphere, and in one direction the cylinder, are diametrically opposed), and provided the walls of the container do not become coated internally or externally with a coating of lime or water scale so as to diminish in heat- transmitting power, an apparatus designed in the manner indicated is undoubtedly free from grave objection; but immediately any of those provisions is neglected, trouble is likely to ensue, for the heat will not disappear from the place of actual reaction at the necessary speed. Apparent proof that heat is not accumulating unduly in a water-jacketed carbide container even when the generator is evolving gas at a fair speed is easy to obtain; for if, as usually happens, the end of the container through which the carbide is inserted is exposed to the air, the hand may be placed upon it, and it will be found to be only slightly warm to the touch. Such a test, however, is inconclusive, and frequently misleading, because if more than a pound or two of carbide is present as an undivided mass, and if water is allowed to attack one portion of it, that particular portion may attain a high temperature while the rest is comparatively cool: and if the bulk of the carbide is comparatively cool, naturally the walls of the containing vessel themselves remain practically unheated. Three causes work together to prevent this heat being dissipated through the walls of the carbide vessel with sufficient rapidity. In the first place, calcium carbide itself is a very bad conductor of heat. So deficient in heat-conducting power is it that a lump a few inches in diameter may be raised to redness in a gas flame at one spot, and kept hot for some minutes, while the rest of the mass remains sufficiently cool to be held comfortably in the fingers. In the second place, commercial carbide exists in masses of highly irregular shape, so that when they are packed into any vessel they only touch at their angles and edges; and accordingly, even if the material were a fairly good heat conductor of itself, the air or gas present between each lump would act as an insulator, protecting the second piece from the heat generated in the first. In the third place, the calcium hydroxide produced as the by-product when calcium carbide is decomposed by water occupies considerably more space than the original carbide—usually two or three times as much space, the exact figures depending upon the conditions in which it is formed—and therefore a carbide container cannot advisedly be charged with more than one-third the quantity of solid which it is apparently capable of holding. The remaining two-thirds of the space is naturally full of air when the container is first put into the generator, but the air is displaced by acetylene as soon as gas production begins. Whether that space, however, is occupied by air, by acetylene, or by a gradually growing loose mass of slaked lime, each separate lump of hot carbide is isolated from its neighbours by a material which is also a very bad heat conductor; and the heat has but little opportunity of distributing itself evenly. Moreover, although iron or steel is a notably better conductor of heat than any of the other substances present in the carbide vessel, it is, as a metal, only a poor conductor, being considerably inferior in this respect to copper. If heat dissipation were the only point to be studied in the construction of an acetylene apparatus, far better results might be obtained by the employment of copper for the walls of the carbide container; and possibly in that case a generator of considerable size, fitted with a water- jacketed decomposing vessel, might be free from the trouble of overheating. Nevertheless it will be seen in Chapter VI. that the use of copper is not permissible for such purposes, its advantages as a good conductor of heat being neutralised by its more important defects.

When suitable precautions are not taken to remove the heat liberated in an acetylene apparatus, the temperature of the calcium carbide occasionally rises to a remarkable degree. Investigating this point, Caro has studied the phenomena of heat production in a "dipping" generator— i.e., an apparatus in which a cage of carbide is alternately immersed in and lifted out of a vessel containing water. Using a generator designed to supply five burners, he has found a maximum recording thermometer placed in the gas space of the apparatus to give readings generally between 60° and 100° C.; but in two tests out of ten he obtained temperatures of about 160° C. To determine the actual temperature of the calcium carbide itself, he scattered amongst the carbide charge fragments of different fusible metallic alloys which were known to melt or soften at certain different temperatures. In all his ten tests the alloys melting at 120° C. were fused completely; in two tests other alloys melting at 216° and 240° C. showed signs of fusion; and in one test an alloy melting at 280° C. began to soften. Working with an experimental apparatus constructed on the "dripping" principle— i.e., a generator in which water is allowed to fall in single drops or as a fine stream upon a mass of carbide—with the deliberate object of ascertaining the highest temperatures capable of production when calcium carbide is decomposed in this particular fashion, and employing for the measurement of the heat a Le Chatelier thermo-couple, with its sensitive wires lying among the carbide lumps, Lewes has observed a maximum temperature of 674° C. to be reached in 19 minutes when water was dripped upon 227 grammes of carbide at a speed of about 8 grammes per minute. In other experiments he used a laboratory apparatus designed upon the "dipping" principle, and found maximum temperatures, in four different trials, of 703°, 734°, 754°, and 807° C., which were reached in periods of time ranging from 12 to 17 minutes. Even allowing for the greater delicacy of the instrument adopted by Lewes for measuring the temperature in comparison with the device employed by Caro, there still remains an astonishing difference between Caro's maximum of 280° and Lewes' maximum of 807° C. The explanation of this discrepancy is to be inferred from what has just been said. The generator used by Caro was properly made of metal, was quite small in size, was properly designed with some skill to prevent overheating as much as possible, and was worked at the speed for which it was intended—in a word, it was as good an apparatus as could be made of this particular type. Lewes' generator was simply a piece of glass and metal, in which provisions to avoid overheating were absent; and therefore the wide difference between the temperatures noted does not suggest any inaccuracy of observation or experiment, but shows what can be done to assist in the dissipation of heat by careful arrangement of parts. The difference in temperature between the acetylene and the carbide in Caro's test accentuates the difficulty of gauging the heat in a carbide vessel by mere external touch, and supplies experimental proof of the previous assertions as to the low heat-conducting power of calcium carbide and of the gases of the decomposing vessel. It must not be supposed that temperatures such as Lewes has found ever occur in any commercial generator of reasonably good design and careful construction; they must be regarded rather as indications of what may happen in an acetylene apparatus when the phenomena accompanying the evolution of gas are not understood by the maker, and when all the precautions which can easily be taken to avoid excessive heating have been omitted, either by building a generator with carbide in excess too large in size, or by working it too rapidly, or more generally by adopting a system of construction unsuited to the ends in view. The fact, however, that Lewes has noted the production of a temperature of 807° C. is important; because this figure is appreciably above the point 780° C., at which acetylene decomposes into its elements in the absence of air.

Nevertheless the production of a temperature somewhat exceeding 100° C. among the lumps of carbide actually undergoing decomposition can hardly be avoided in any practical generator. Based on a suggestion in the "Report of the Committee on Acetylene Generators" which was issued by the British Home Office in 1902, Fouché has proposed that 130° C., as measured with the aid of fusible metallic rods, [Footnote: An alloy made by melting together 55 parts by weight of commercial bismuth and 45 parts of lead fuses at 127° C., and should be useful in performing the tests.] should be considered the maximum permissible temperature in any part of a generator working at full speed for a prolonged period of time. Fouché adopts this figure on the ground that 130° C. sensibly corresponds with the temperature at which a yellow substance is formed in a generator by a process of polymerisation; and, referring to French conditions, states that few actual apparatus permit the development of so high a temperature. As a matter of fact, however, a fairly high temperature among the carbide is less important than in the gas, and perhaps it would be better to say that the temperature in any part of a generator occupied by acetylene should not exceed 100° C. Fraenkel has carried out some experiments upon the temperature of the acetylene immediately after evolution in a water-to-carbide apparatus containing the carbide in a subdivided receptacle, using an apparatus now frequently described as belonging to the "drawer" system of construction. When a quantity of about 7 lb. of carbide was distributed between 7 different cells of the receptacle, each cell of which had a capacity of 25 fluid oz., and the apparatus was caused to develop acetylene at the rate of 7 cubic feet per hour, maximum thermometers placed immediately over the carbide in the different cells gave readings of from 70° to 90° C., the average maximum temperature being about 80° C. Hence the Austrian code of rules issued in 1905 governing the construction of acetylene apparatus contains a clause to the effect that the temperature in the gas space of a generator must never exceed 80° C.; whereas the corresponding Italian code contains a similar stipulation, but quotes the maximum temperature as 100° C. (vide Chapter IV.).

It is now necessary to see why the production of an excessively high temperature in an acetylene generator has to be avoided. It must be avoided, because whenever the temperature in the immediate neighbourhood of a mass of calcium carbide which is evolving acetylene under the attack of water rises materially above the boiling-point of water, one or more of three several objectionable effects is produced—(a) upon the gas generated, (b) upon the carbide decomposed, and (c) upon the general chemical reaction taking place.

It has been stated above that in moat generators when the action between the carbide and the water is proceeding smoothly, it occurs according to equation (2)—

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2

rather than in accordance with equation (1)—

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This is because calcium oxide, or quicklime, the by-product in (1), has considerable affinity for water, evolving a noteworthy quantity of heat when it combines with one molecule of water to form one molecule of calcium hydroxide, or slaked lime, the by-product in (2). If, then, a small amount of water is added to a large amount of calcium carbide, the corresponding quantity of acetylene may be liberated on the lines of equation (1), and there will remain behind a mixture of unaltered calcium carbide, together with a certain amount of calcium oxide. Inasmuch as both these substances possess an affinity for water (setting heat free when they combine with it), when a further limited amount of water is introduced into the mixture some of it will probably be attracted to the oxide instead of to the carbide present. It is well known that at ordinary temperatures quicklime absorbs moisture, or combines with water, to produce slaked lime; but it is equally well known that in a furnace, at about a red heat, slaked lime gives up water and changes into quicklime. The reaction, in fact, between calcium oxide and water is reversible, and whether those substances combine or dissociate is simply a question of temperature. In other words, as the temperature rises, the heat of hydration of calcium oxide diminishes, and calcium hydroxide becomes constantly a less stable material. If now it should happen that the affinity between calcium carbide and water should not diminish, or should diminish in a lower ratio than the affinity between calcium oxide and water as the temperature of the mass rises from one cause or other, it is conceivable that at a certain temperature calcium carbide might be capable of withdrawing the water of hydration from the molecule of slaked lime, converting the latter into quicklime, and liberating one molecule of acetylene, thus—

(3) CaC_2 + Ca_2(OH) = C_2H_2 + 2CaO.

It has been proved that a reaction of this character does occur, the temperature necessary to determine it being given by Lewes as from 420° to 430° C., which is not much more than half that which he found in a generator having carbide in excess, albeit one of extremely bad design. Treating this reaction in the manner previously adopted, the thermo- chemical phenomena of equation (3) are:

[1 Footnote: Into its elements, Ca, O_2, and H_2; cf. footnote, p: 31.]

Or, since the calcium hydroxide is only dehydrated without being entirely decomposed, and only one molecule of water is broken up, it may be written:

which comes to the same thing. Putting the matter in another shape, it may be said that the reaction between calcium carbide and water is exothermic, evolving either 14.0 or 29.1 calories according as the byproduct is calcium oxide or solid calcium hydroxide; and therefore either reaction proceeds without external assistance in the cold. The reaction between carbide and slaked lime, however, is endothermic, absorbing 1.1 calories; and therefore it requires external assistance (presence of an elevated temperature) to start it, or continuous introduction of heat (as from the reaction between the rest of the carbide present and the water) to cause it to proceed. Of itself, and were it not for the disadvantages attending the production of a temperature remotely approaching 400° C. in an acetylene generator, which disadvantages will be explained in the following paragraphs, there is no particular reason why reaction (3) should not be permitted to occur, for it involves (theoretically) no loss of acetylene, and no waste of calcium carbide. Only one specific feature of the reaction has to be remembered, and due practical allowance made for it. The reaction represented by equation (2) proceeds almost instantaneously when the calcium carbide is of ordinarily good quality, and the acetylene resulting therefrom is wholly generated within a very few minutes. Equation (3), on the contrary, consumes much time for its completion, and the gas corresponding with it is evolved at a gradually diminishing speed which may cause the reaction to continue for hours—a circumstance that may be highly inconvenient or quite immaterial according to the design of the apparatus. When, however, it is desired to construct an automatic acetylene generator, i.e., an apparatus in which the quantity of gas liberated has to be controlled to suit the requirements of any indefinite number of burners in use on different occasions, equation (3) becomes a very important factor in the case. To determine the normal reaction (No. 2) of an acetylene generator, 64 parts by weight of calcium carbide must react with 36 parts of water to yield 26 parts by weight of acetylene, and apparently both carbide and water are entirely consumed; but if opportunity is given for the occurrence of reaction (3), another 64 parts by weight of carbide may be attacked, without the addition of any more water, producing, inevitably, another 26 parts of acetylene. If, then, water is in chemical excess in the generator, all the calcium carbide present will be decomposed according to equation (2), and the action will take place without delay; after a few minutes' interval the whole of the acetylene capable of liberation will have been evolved, and nothing further can possibly happen until another charge of carbide is inserted in the apparatus. If, on the other hand, calcium carbide is in chemical excess in the generator, all the water run in will be consumed according to equation (2), and this action will again take place without delay; but unless the temperature of the residual carbide has been kept well below 400° C., a further evolution of gas will occur which will not cease for an indeterminate period of time, and which, by strict theory, given the necessary conditions, might continue until a second volume of acetylene equal to that liberated at first had been set free. In practice this phenomenon of a secondary production of gas, which is known as "after-generation," is regularly met with in all generators where the carbide is in excess of the water added; but the amount of acetylene so evolved rarely exceeds one-quarter or one-third of the main make. The actual amount evolved and the rate of evolution depend, not merely upon the quantity of undecomposed carbide still remaining in contact with the damp lime, but also upon the rapidity with which carbide naturally decomposes in presence of liquid water, and the size of the lumps. Where "after-generation" is caused by the ascent of water vapour round lumps of carbide, the volume of gas produced in a given interval of time is largely governed by the temperature prevailing and the shape of the apparatus. It is evident that even copious "after-generation" is a matter of no consequence in any generator provided with a holder to store the gas, assuming that by some trustworthy device the addition of water is stopped by the time that the holder is two-thirds or three-quarters full. In the absence of a holder, or if the holder fitted is too small to serve its proper purpose, "aftergeneration" is extremely troublesome and sometimes dangerous, but a full discussion of this subject must be postponed to the next chapter.

EFFECT OF HEAT ON ACETYLENE.—The effect of excessive retention of heat in an acetylene generator upon the gas itself is very marked, as acetylene begins spontaneously to suffer change, and to be converted into other compounds at elevated temperatures. Being a purely chemical phenomenon, the behaviour of acetylene when exposed to heat will be fully discussed in Chapter VI. when the properties of the gas are being systematically dealt with. Here it will be sufficient to assume that the character of the changes taking place is understood, and only the practical results of those changes as affecting the various components of an acetylene installation have to be studied. According to Lewes, acetylene commences to "polymerise" at a temperature of about 600° C., when it is converted into other hydrocarbons having the same percentage composition, but containing more atoms of carbon and hydrogen in their molecules. The formula of acetylene is C_2H_2 which means that 2 atoms of carbon and 2 atoms of hydrogen unite to form 1 molecule of acetylene, a body evidently containing roughly 92.3 per cent. by weight of carbon and 7.7 per cent. by weight of hydrogen. One of the most noteworthy substances produced by the polymerisation of acetylene is benzene, the formula of which is C_6H_6, and this is formed in the manner indicated by the equation—

(4) 3C_2H_2 = C_6H_6.

Now benzene also contains 92.3 per cent. of carbon and 7.7 per cent. by weight of hydrogen in its composition, but its molecule contains 6 atoms of each element. When the chemical formula representing a compound body indicates a substance which is, or can be obtained as, a gas or vapour, it convoys another idea over and above those mentioned on a previous page. The formula "C_2H_2," for example, means 1 molecule, or 26 parts by weight of acetylene, just as "H_2" means 1 molecule, or 2 parts by weight of hydrogen; but both formulæ also mean equal parts by volume of the respective substances, and since H_2 must mean 2 volumes, being twice H, which is manifestly 1, C_2H_2 must mean 2 volumes of acetylene as well. Thus equation (4) states that 6 volumes of acetylene, or 3 x 26 parts by weight, unite to form 2 volumes of benzene, or 78 parts by weight. If these hydrocarbons are burnt in air, both are indifferently converted into carbon dioxide (carbonic acid gas) and water vapour; and, neglecting for the sake of simplicity the nitrogen of the atmosphere, the processes may be shown thus:

(5) 2C_2H_2 + 5O_2 = 4CO_2 + 2H_2O.

(6) 2C_6H_6 + 15O_2 = 12CO_2 + 6H_2O.

Equation (5) shows that 4 volumes of acetylene combine with 10 volumes of oxygen to produce 8 volumes of carbon dioxide and 4 of water vapour; while equation (6) indicates that 4 volumes of benzene combine with 30 volumes of oxygen to yield 24 volumes of carbon dioxide and 12 of water vapour. Two parts by volume of acetylene therefore require 5 parts by volume of oxygen for perfect combustion, whereas two parts by volume of benzene need 15—i.e., exactly three times as much. In order to work satisfactorily, and to develop the maximum of illuminating power from any kind of gas consumed, a gas-burner has to be designed with considerable skill so as to attract to the base of the flame precisely that volume of air which contains the quantity of oxygen necessary to insure complete combustion, for an excess of air in a flame is only less objectionable than a deficiency thereof. If, then, an acetylene burner is properly constructed, as most modern ones are, it draws into the flame air corresponding with two and a half volumes of oxygen for every one volume of acetylene passing from the jets; whereas if it were intended for the combustion of benzene vapour it would have to attract three times that quantity. Since any flame supplied with too little air tends to emit free carbon or soot, it follows that any well-made acetylene burner delivering a gas containing benzene vapour will yield a more or lens smoky flame according to the proportion of benzene in the acetylene. Moreover, at ordinary temperatures benzene is a liquid, for it boils at 81° C., and although, as was explained above in the case of water, it is capable of remaining in the state of vapour far below its boiling-point so long as it is suspended in a sufficiency of some permanent gas like acetylene, if the proportion of vapour in the gas at any given temperature exceeds a certain amount the excess will be precipitated in the liquid form; while as the temperature falls the proportion of vapour which can be retained in a given volume of gas also diminishes to a noteworthy extent. Should any liquid, be it water or benzene, or any other substance, separate from the acetylene under the influence of cold while the gas is passing through pipes, the liquid will run downwards to the lowest points in those pipes; and unless due precautions are taken, by the insertion of draw-off cocks, collecting wells, or the like, to withdraw the deposited water or other liquid, it will accumulate in all bends, angles, and dips till the pipes are partly or completely sealed against the passage of gas, and the lights will either "jump" or be extinguished altogether. In the specific case of an acetylene generator this trouble is very likely to arise, even when the gas is not heated sufficiently during evolution for polymerisation to occur and benzene or other liquid hydrocarbons to be formed, because any excess of water present in the decomposing vessel is liable to be vaporised by the heat of the reaction—as already stated it is desirable that water shall be so vaporised—and will remain safely vaporised as long as the pipes are kept warm inside or near the generator; but directly the pipes pass away from the hot generator the cooling action of the air begins, and some liquid water will be immediately produced. Like the phenomenon of after- generation, this equally inevitable phenomenon of water condensation will be either an inconvenience or source of positive danger, or will be a matter of no consequence whatever, simply as the whole acetylene installation, including the service-pipes, is ignorantly or intelligently built.

As long as nothing but pure polymerisation happens to the acetylene, as long, that is to say, as it is merely converted into other hydrocarbons also having the general formula C_(2n)H_(2n), no harm will be done to the gas as regards illuminating power, for benzene burns with a still more luminous flame than acetylene itself; nor will any injury result to the gas if it is required for combustion in heating or cooking stoves beyond the fact that the burners, luminous or atmospheric, will be delivering a material for the consumption of which they are not properly designed. But if the temperature should rise much above the point at which benzene is the most conspicuous product of polymerisation, other far more complicated changes occur, and harmful effects may be produced in two separate ways. Some of the new hydrocarbons formed may interact to yield a mixture of one or more other hydrocarbons containing a higher proportion of carbon than that which is present in acetylene and benzene, together with a corresponding proportion of free hydrogen; the former will probably be either liquids or solids, while the latter burns with a perfectly non-luminous flame. Thus the quantity of gas evolved from the carbide and passed into the holder is less than it should be, owing to the condensation of its non-gaseous constituents. To quote an instance of this, Haber has found 15 litres of acetylene to be reduced in volume to 10 litres when the gas was heated to 638° C. By other changes, some "saturated hydrocarbons," i.e., bodies having the general formula C_nH_(2n+2), of which methane or marsh-gas, CH_4 is the best known, may be produced; and those all possess lower illuminating powers than acetylene. In two of those experiments already described, where Lewes observed maximum temperatures ranging from 703° to 807° C., samples of the gas which issued when the heat was greatest were submitted to chemical analysis, and their illuminating powers were determined. The figures he gives are as follows:

I. II.
Per Cent. Per Cent.
Acetylene 70.0 69.7
Saturated hydrocarbons 11.3 11.4
Hydrogen 18.7 18.9
_____ _____

100.0 100.0

The average illuminating power of these mixed gases is about 126 candles per 5 cubic feet, whereas that of pure acetylene burnt under good laboratory conditions is 240 candles per 5 cubic feet. The product, it will be seen, had lost almost exactly 50 per cent. of its value as an illuminant, owing to the excessive heating to which it had been, exposed. Some of the liquid hydrocarbons formed at the same time are not limpid fluids like benzene, which is less viscous than water, but are thick oily substances, or even tars. They therefore tend to block the tubes of the apparatus with great persistence, while the tar adheres to the calcium carbide and causes its further attack by water to be very irregular, or even altogether impossible. In some of the very badly designed generators of a few years back this tarry matter was distinctly visible when the apparatus was disconnected for recharging, for the spent carbide was exceptionally yellow, brown, or blackish in colour, [Footnote: As will be pointed out later, the colour of the spent lime cannot always be employed as a means for judging whether overheating has occurred in a generator.] and the odour of tar was as noticeable as that of crude acetylene.

There is another effect of heat upon acetylene, more calculated to be dangerous than any of those just mentioned, which must not be lost sight of. Being an endothermic substance, acetylene is prone to decompose into its elements—

(7) C_2H_2 -> C_2 + H_2

whenever it has the opportunity; and the opportunity arrives if the temperature of the gas risen to 780° C., or if the pressure under which the gas is stored exceeds two atmospheres absolute (roughly 30 lb. per square inch). It decomposes, be it carefully understood, in the complete absence of air, directly the smallest spark of red-hot material or of electricity, or directly a gentle shock, such as that of a fall or blow on the vessel holding it, is applied to any volume of acetylene existing at a temperature exceeding 780° or at a gross pressure of 30 lb. per square inch; and however large that volume may be, unless it is contained in tubes of very small diameter, as will appear hereafter, the decomposition or dissociation into its elements will extend throughout the whole of the gas. Equation (7) states that 2 volumes of acetylene yield 2 volumes of hydrogen and a quantity of carbon which would measure 2 volumes were it obtained in the state of gas, but which, being a solid, occupies a space that may be neglected. Apparently, therefore, the dissociation of acetylene involves no alteration in volume, and should not exhibit explosive effects. This is erroneous, because 2 volumes of acetylene only yield exactly 2 volumes of hydrogen when both gases are measured at the same temperature, and all gases increase in volume as their temperature rises. As acetylene is endothermic and evolves much heat on decomposition, and as that heat must primarily be communicated to the hydrogen, it follows that the latter must be much hotter than the original acetylene; the hydrogen accordingly strives to fill a much larger space than that occupied by the undecomposed gas, and if that gas is contained in a closed vessel, considerable internal pressure will be set up, which may or may not cause the vessel to burst.

What has been said in the preceding paragraph about the temperature at which acetylene decomposes is only true when the gas is free from any notable quantity of air. In presence of air, acetylene inflames at a much lower temperature, viz., 480° C. In a manner precisely similar to that of all other combustible gases, if a stream of acetylene issues into the atmosphere, as from the orifices of a burner, the gas catches fire and burns quietly directly any substance having a temperature of 480° C. or upwards is brought near it; but if acetylene in bulk is mixed with the necessary quantity of air to support combustion, and any object exceeding 480° C. in temperature comes in contact with it, the oxidation of the hydrocarbon proceeds at such a high rate of speed as to be termed an explosion. The proportion of air needed to support combustion varies with every combustible material within known limits (cf. Chapter VI.), and according to Eitner the smallest quantity of air required to make acetylene burn or explode, as the case may be, is 25 per cent. If, by ignorant design or by careless manipulation, the first batches of acetylene evolved from a freshly charged generator should contain more than 25 per cent. of air; or if in the inauguration of a new installation the air should not be swept out of the pipes, and the first makes of gas should become diluted with 25 to 50 per cent. of air, any glowing body whose temperature exceeds 480° C. will fire the gas; and, as in the former instance, the flame will extend all through the mass of acetylene with disastrous violence and at enormous speed unless the gas is stored in narrow pipes of extremely small diameter. Three practical lessons are to be learnt from this circumstance: first, tobacco-smoking must never be permitted in any building where an escape of raw acetylene is possible, because the temperature of a lighted cigar, &c., exceeds 480° C.; secondly, a light must never be applied to a pipe delivering acetylene until a proper acetylene burner has been screwed into the aperture; thirdly, if any appreciable amount of acetylene is present in the air, no operation should be performed upon any portion of an acetylene plant which involves such processes as scraping or chipping with the aid of a steel tool or shovel. If, for example, the iron or stoneware sludge-pipe is choked, or the interior of the dismantled generator is blocked, and attempts are made to remove the obstruction with a hard steel tool, a spark is very likely to be formed which, granting the existence of sufficient acetylene in the air, is perfectly able to fire the gas. For all such purposes wooden implements only are best employed; but the remark does not apply to the hand-charging of a carbide-to-water generator through its proper shoot. Before passing to another subject, it may be remarked that a quantity of air far less than that which causes acetylene to become dangerous is objectionable, as its presence is apt to reduce the illuminating power of the gas unduly.

EFFECT OF HEAT ON CARBIDE.—Chemically speaking, no amount of heat possible of attainment in the worst acetylene generator can affect calcium carbide in the slightest degree, because that substance may be raised to almost any temperature short of those distinguishing the electric furnace, without suffering any change or deterioration. In the absence of water, calcium carbide is as inert a substance as can well be imagined: it cannot be made to catch fire, for it is absolutely incombustible, and it can be heated in any ordinary flame for reasonable periods of time, or thrown into any non-electrical furnace without suffering in the least. But in presence of water, or of any liquid containing water, matters are different. If the temperature of an acetylene generator rises to such an extent that part of the gas is polymerised into tar, that tar naturally tends to coat the residual carbide lumps, and, being greasy in character, more or less completely protects the interior from further attack. Action of this nature not only means that the acetylene is diminished in quantity and quality by partial decomposition, but it also means that the make is smaller owing to imperfect decomposition of the carbide: while over and above this is the liability to nuisance or danger when a mass of solid residue containing some unaltered calcium carbide is removed from the apparatus and thrown away. In fact, whenever the residue of a generator is not so saturated with excess of water as to be of a creamy consistency, it should be put into an uncovered vessel in the open air, and treated with some ten times its volume of water before being run into any drain or closed pipe where an accumulation of acetylene may occur. Even at temperatures far below those needed to determine a production of tar or an oily coating on the carbide, if water attacks an excess of calcium carbide somewhat rapidly, there is a marked tendency for the carbide to be "baked" by the heat produced; the slaked lime adhering to the lumps as a hard skin which greatly retards the penetration of more water to the interior.

COLOUR OF SPENT CARBIDE.—In the early days of the industry, it was frequently taken for granted that any degradation in the colour of the spent lime left in an acetylene generator was proof that overheating had taken place during the decomposition of the carbide. Since both calcium oxide and hydroxide are white substances, it was thought that a brownish, greyish, or blackish residue must necessarily point to incipient polymerisation of the gas. This view would be correct if calcium carbide were prepared in a state of chemical purity, for it also is a white body. Commercial carbide, however, is not pure; it usually contains some foreign matter which tints the residue remaining after gasification. When a manufacturer strives to give his carbide the highest gas-making power possible he frequently increases the proportion of carbon in the charge submitted to electric smelting, until a small excess is reached, which remains in the free state amongst the finished carbide. After decomposition the fine particles of carbon stain the moist lime a bluish grey tint, the depth of shade manifestly depending upon the amount present. If such a sludge is copiously diluted with water, particles of carbon having the appearance and gritty or flaky nature of coke often rise to the surface or fall to the bottom of the liquid; whence they can easily be picked out and identified as pure or impure carbon by simple tests. Similarly the lime or carbon put into the electric furnace may contain small quantities of compounds which are naturally coloured; and which, reappearing in the sludge either in their original or in a different state of combination, confer upon the sludge their characteristic tinge. Spent lime of a yellowish brown colour is frequently to be met with in circumstances that are clearly no reproach to the generator. Doubtless the tint is due to the presence of some coloured metallic oxide or other compound which has escaped reduction in the electric furnace. The colour which the residual lime afterwards assumes may not be noticeable in the dry carbide before decomposition, either because some change in the colour-giving impurity takes place during the chemical reactions in the generator or because the tint is simply masked by the greyish white of the carbide and its free carbon. Hence it follows that a bad colour in the waste lime removed from a generator only points to overheating and polymerisation of the acetylene when corroborative evidence is obtained—such as a distinct tarry smell, the actual discovery of oily or tarry matters elsewhere, or a grave reduction in the illuminating power of the gas.

MAXIMUM ATTAINABLE TEMPERATURES.—In order to discover the maximum temperature which can be reached in or about an acetylene generator when an apparatus belonging to one of the best types is fed at a proper rate with calcium carbide in lumps of the most suitable size, the following calculation may be made. In the first place, it will be assumed that no loss of heat by radiation occurs from the walls of the generator; secondly, the small quantity of heat taken up by the calcium hydroxide produced will be ignored; and, thirdly, the specific heat of acetylene will be assumed to be 0.25, which is about its most probable value. Now, a hand-fed carbide-to-water generator will work with half a gallon of water for every 1 lb. of carbide decomposed, quantities which correspond with 320 grammes of water per 64 grammes (1 molecular weight) of carbide. Of those 320 grammes of water, 18 are chemically destroyed, leaving 302. The decomposition of 64 grammes of commercial carbide evolves 28 large calories of heat. Assuming all the heat to be absorbed by the water, 28 calories would raise 302 grammes through (28 X 1000 / 302) = 93° C., i.e., from 44.6° F. to the boiling-point. Assuming all the heat to be communicated to the acetylene, those 28 calories would raise the 26 grammes of gas liberated through (28 X 1000 / 26 / 0.25) = 4308° C., if that were possible. But if, as would actually be the case, the heat were distributed uniformly amongst the 302 grammes of water and the 20 grammes of acetylene, both gas and water would be raised through the same number of degrees, viz., 90.8° C. [Footnote: Let x = the number of large calories absorbed by the water; then 28 - x = those taken up by the gas. Then—

1000x / 302 = 1000 (28 - x) / (26 X 0.25)

whence x = 27.41; and 28 - x = 0.59.

Therefore, for water, the rise in temperature is—

27.41 X 1000 / 302 = 90.8° C.;

and for acetylene the rise is—

0.59 X 1000 / 26 / 0.25 = 90.8° C.]

If the generator were designed on lines to satisfy the United States Fire Underwriters, it would contain 8.33 lb. of water to every 1 lb. of carbide attacked; identical calculations then showing that the original temperature of the water and gas would be raised through 53.7° C. Provided the carbide is not charged into such an apparatus in lumps of too large a size, nor at too high a rate, there will be no appreciable amount of local overheating developed; and nowhere, therefore, will the rise in temperature exceed 91° in the first instance, or 54° C. in the second. Indeed it will be considerably smaller than this, because a large proportion of the heat evolved will be lost by radiation through the generator walls, while another portion will be converted from sensible into latent heat by causing part of the water to pass off as vapour with the acetylene.

EFFECT OF HIGH TEMPERATURES ON GENERATORS.—As the temperature amongst the carbide in any generator in which water is not present in large excess may easily reach 200° C. or upwards, no material ought to be employed in the construction of such generators which is not competent to withstand a considerable amount of heat in perfect safety. The ordinary varieties of soft solder applied with the bitt in all kinds of light metal-work usually melt, according to their composition, at about 180° C.; and therefore this method of making joints is only suitable for objects that are never raised appreciably in temperature above the boiling-point of water. No joint in an acetylene generator, the partial or complete failure of which would radically affect the behaviour of the apparatus, by permitting the charges of carbide and of water to come into contact at an abnormal rate of speed, by allowing the acetylene to escape directly through the crack into the atmosphere, or by enabling the water to run out of the seal of any vessel containing gas so as to set up a free communication between that vessel and the air, ought ever to be made of soft solder—every joint of this character should be constructed either by riveting, by bolting, or by doubly folding the metal sheets. Apparently, a joint constantly immersed in water on one side cannot rise in temperature above the boiling-point of the liquid, even when its other side is heated strongly; but since, even if a generator is not charged with naturally hard water, its fluid contents soon become "hard" by dissolution of lime, there is always a liability to the deposition of water scale over the joint. Such water scale is a very bad heat conductor, as is seen in steam boilers, so that a seam coated with an exceedingly thin layer of scale, and heated sharply on one side, will rise above the boiling-point of water even if the liquid on its opposite side is ice-cold. For a while the film of scale may be quite water-tight, but after it has been heated by contact with the hot metal several times it becomes brittle and cracks without warning. But there is a more important reason for avoiding the use of plumbers' solder. It might seem that as the natural hard, protective skin of the metal is liable to be injured or removed by the bending or by the drilling or punching which precedes the insertion of the rivets or studs, an application of soft solder to such a joint should be advantageous. This is not true because of the influence of galvanic action. As all soft solders consist largely of lead, if a joint is soldered, a "galvanic couple" of lead and iron, or of lead and zinc (when the apparatus is built of galvanised steel), is exposed to the liquid bathing it; and since in both cases the lead is highly electro-negative to the iron or zinc, it is the iron or zinc which suffers attack, assuming the liquid to possess any corrosive properties whatever. Galvanised iron which has been injured during the joint-making presents a zinc-iron couple to the water, but the zinc protects the iron; if a lead solder is present, the iron will begin to corrode immediately the zinc has disappeared. In the absence of lead it is the less important metal, but in the presence of lead it is the more important (the foundation) metal which is the soluble element of the couple. Where practicable, joints in an acetylene generator may safely be made by welding or by autogenous soldering ("burning"), because no other metal is introduced into the system; any other process, except that of riveting or folding, only hastens destruction of the plant. The ideal method of making joints about an acetylene generator is manifestly that of autogenous soldering, because, as will appear in Chapter IX. of this book, the most convenient and efficient apparatus for performing the operation is the oxy-acetylene blow-pipe, which can be employed so as to convert two separate pieces of similar metal into one homogeneous whole.

In less critical situations in an acetylene plant, such as the partitions of a carbide container, &c., where the collapse of the seam or joint would not be followed by any of the effects previously suggested, there is less cause for prohibiting the use of unfortified solder; but even here, two or three rivets, just sufficient to hold the metal in position if the solder should give way, are advisedly put into all apparatus. In other portions of an acetylene installation where a merely soldered joint is exposed to warm damp gas which is in process of cooling, instead of being bathed in hard water, an equal, though totally dissimilar, danger is courted. The main constituent of such solders that are capable of being applied with the bitt is lead; lead is distinctly soluble in soft or pure water; and the water which separates by condensation out of a warm damp gas is absolutely soft, for it has been distilled. If condensation takes place at or near a soldered joint in such a way that water trickles over the solder, by slow degrees the metallic lead will be dissolved and removed, and eventually a time will come when the joint is no longer tight to gas. In fact, if an acetylene installation is of more than very small dimensions, e.g., when it is intended to supply any building as large as, or larger than, the average country residence, if it is to give satisfaction to both constructor and purchaser by being quite trustworthy and, possessed of a due lease of life, say ten or fifteen years, it must be built of stouter materials than the light sheets which alone are suitable for manipulation with the soldering-iron or for bending in the ordinary type of metal press. Sound cast-iron, heavy sheet-metal, or light boiler-plate is the proper substance of which to construct all the important parts of a generator, and the joints in wrought metal must be riveted and caulked or soldered autogeneously as mentioned above. So built, the installation becomes much more costly to lay down than an apparatus composed of tinplate, zinc, or thin galvanised iron, but it will prove more economical in the long run. It is not too much to say that if ignorant and short-sighted makers in the earliest days of the acetylene industry had not recommended and supplied to their customers lightly built apparatus which has in many instances already begun to give trouble, to need repairs, and to fail by thorough corrosion—apparatus which frequently had nothing but cheapness in its favour—the use of the gas would have spread more rapidly than it has done, and the public would not now be hearing of partial or complete failures of acetylene installations. Each of these failures, whether accompanied by explosions and injury to persons or not, acts more powerfully to restrain a possible new customer from adopting the acetylene light, than several wholly successful plants urge him to take it up; for the average member of the public is not in a position to distinguish properly between the collapse of a certain generator owing to defective design or construction (which reflects no discredit upon the gas itself), and the failure of acetylene to show in practice those advantages that have been ascribed to it. One peculiar and noteworthy feature of acetylene, often overlooked, is that the apparatus is constructed by men who may have been accustomed to gas-making plant all their lives, and who may understand by mere habit how to superintend a chemical operation; but the same apparatus is used by persons who generally have no special acquaintance with such subjects, and who, very possibly, have not even burnt coal-gas at any period of their lives. Hence it happens that when some thoughtless action on the part of the country attendant of an acetylene apparatus is followed by an escape of gas from the generator, and by an accumulation of that gas in the house where the plant is situated, or when, in disregard of rules, he takes a naked light into the house and an explosion follows, the builder dismisses the episode as a piece of stupidity or wilful misbehaviour for which he can in nowise be held morally responsible; whereas the builder himself is to blame for designing an apparatus from which an escape of gas can be accompanied by sensible risks to property or life. However unpalatable this assertion may be, its truth cannot be controverted; because, short of criminal intention or insanity on the part of the attendant, it is in the first place a mere matter of knowledge and skill so to construct an acetylene plant that an escape of gas is extremely unlikely, even when the apparatus is opened for recharging, or when it is manipulated wrongly; and in the second place, it is easy so to arrange the plant that any disturbance of its functions which may occur shall be followed by an immediate removal of the surplus gas into a place of complete safety outside and above the generator-house.

GENERATION AT LOW TEMPERATURES.—In all that has been said hitherto about the reaction between calcium carbide and water being instantaneous, it has been assumed that the two substances are brought together at or about the usual temperature of an occupied room, i.e., 15 degrees C. If, however, the temperature is materially lower than this, the speed of the reaction falls off, until at -5 degrees C., supposing the water still to remain liquid, evolution of acetylene practically ceases. Even at the freezing-point of pure water gas is produced but slowly; and if a lump of carbide is thrown on to a block of ice, decomposition proceeds so gently that the liberated acetylene may be ignited to form a kind of torch, while heat is generated with insufficient rapidity to cause the carbide to sink into the block. This fact has very important bearings upon the manipulation of an acetylene generator in winter time. It is evident that unless precautions are taken those portions of an apparatus which contain water are liable to freeze on a cold night; because, even if the generator has been at work producing gas (and consequently evolving heat) till late in the evening, the surplus heat stored in the plant may escape into the atmosphere long before more acetylene has to be made, and obviously while frost is still reigning in the neighbourhood. If the water freezes in the water store, in the pipes leading therefrom, in the holder seal, or in the actual decomposing chamber, a fresh batch of gas is either totally incapable of production, because the water cannot be brought into contact with the calcium carbide in the apparatus, or it can only be generated with excessive slowness because the carbide introduced falls on to solid ice. Theoretically, too, there is a possibility that some portion of the apparatus—a pipe in particular—may be burst by the freezing, owing to the irresistible force with which water expands when it changes into the solid condition. Probably this last contingency, clearly accompanied as it would be by grave risk, is somewhat remote, all the plant being constructed of elastic material; but in practice even a simple interference with the functions of a generator by freezing, ideally of no special moment, is highly dangerous, because of the great likelihood that hurried and wholly improper attempts to thaw it will be made by the attendant. As it has been well known for many years that the solidifying point of water can be lowered to almost any degree below normal freezing by dissolving in it certain salts in definite proportions, one of the first methods suggested for preventing the formation of ice in an acetylene generator was to employ such a salt, using, in fact, for the decomposition of the carbide some saline solution which remains liquid below the minimum night temperature of the winter season. Such a process, however, has proved unsuitable for the purpose in view; and the explanation of that fact is found in what has just been stated: the "water" of the generator may admittedly be safely maintained in the fluid state, but from so cold a liquid acetylene will not be generated smoothly, if at all. Moreover, were it not so, a process of this character is unnecessarily expensive, although suitable salts are very cheap, for the water of the generator is constantly being consumed, [Footnote: It has already been said that most generators "consume" a much larger volume of water than the amount corresponding with the chemical reaction involved: the excess of water passing into the sludge or by- product. Thus a considerable quantity of any anti-freezing agent must be thrown aside each time the apparatus is cleaned out or its fluid contents are run off.] and as constantly needs renewal; which means that a fresh batch of salt would be required every time the apparatus was recharged, so long as frost existed or might be expected. A somewhat different condition obtains in the holder of an acetylene installation. Here, whenever the holder is a separate item in the plant, not constituting a portion of the generating apparatus, the water which forms the seal of a rising holder, or which fills half the space of a displacement holder, lasts indefinitely; and it behaves equally well, whatever its temperature may be, so long as it retains a fluid state. This matter will be discussed with greater detail at the end of Chapter III. At present the point to be insisted on is that the temperature in any constituent of an acetylene installation which contains water must not be permitted to fall to the freezing-point; while the water actually used for decomposition must be kept well above that temperature.

GENERATION AT HIGH TEMPERATURES.—At temperatures largely exceeding those of the atmosphere, the reaction between calcium carbide and water tends to become irregular; while at a red heat steam acts very slowly upon carbide, evolving a mixture of acetylene and hydrogen in place of pure acetylene. But since at pressures which do not materially exceed that of the atmosphere, water changes into vapour at 100° C., above that temperature there can be no question of a reaction between carbide and liquid water. Moreover, as has been pointed out, steam or water vapour will continue to exist as such at temperatures even as low as the freezing-point so long as the vapour is suspended among the particles of a permanent gas. Between calcium carbide and water vapour a double decomposition occurs chemically identical with that between carbide and liquid water; but the physical effect of the reaction and its practical bearings are considerably modified. The quantity of heat liberated when 30 parts by weight of steam react with 64 parts of calcium carbide should be essentially unaltered from that evolved when the reagent is in the liquid state; but the temperature likely to be attained when the speed of reaction remains the same as before will be considerably higher for two conspicuous reasons. In the first place, the specific heat of steam in is only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat which is sufficient to raise the temperature of a given weight of liquid water through n thermometric degrees, will raise the temperature of the same weight of water vapour through rather more than 2 n degrees. In the second place, that relatively large quantity of heat which in the case of liquid water merely changes the liquid into a vapour, becoming "latent" or otherwise unrecognisable, and which, as already shown, forms roughly five-sixths of the total heat needed to convert cold water into steam, has no analogue if the water has previously been vaporised by other means; and therefore the whole of the heat supplied to water vapour raises its sensible temperature, as indicated by the thermometer. Thus it appears that, except for the sufficient amount of cooling that can be applied to a large vessel containing carbide by surrounding it with a water jacket, there is no way of governing its temperature satisfactorily if water vapour is allowed to act upon a mass of carbide—assuming, of course, that the reaction proceeds at any moderate speed, e.g., at a rate much above that required to supply one or two burners with gas.

The decomposition which with perfect chemical accuracy has been stated to occur quantitatively between 36 parts by weight, of water and 64 parts of calcium carbide scarcely ever takes place in so simple a fashion in an actual generator. Owing to the heat developed when carbide is in excess, about half the water is converted into vapour; and so the reaction proceeds in two stages: half the water added reacting with the carbide as a liquid, the other half, in a state of vapour, afterwards reacting similarly, [Footnote: This secondary reaction is manifestly only another variety of the phenomenon known as "after-generation" (cf. ante). After-generation is possible between calcium carbide and mechanically damp slaked lime, between carbide and damp gas, or between carbide and calcium hydroxide, as opportunity shall serve. In all cases the carbide must be in excess.] or hardly reacting at all, as the case may be. Suppose a vessel, A B, somewhat cylindrical in shape, is charged with carbide, and that water is admitted at the end called A. Suppose now (1) that the exit for gas is at the opposite end, B. As the lumps near A are attacked by half the liquid introduced, while the other half is changed into steam, a current, of acetylene and water vapour travels over the charge lying between the decomposing spot and the end B. During its passage the second half of the water, as vapour, reacts with the excess of carbide, the first make of acetylene being dried, and more gas being produced. Thus a second quantity of heat is developed, equal by theory to that previously evolved; but a second elevation in temperature, far more serious, and far less under control, than the former also occurs; and this is easily sufficient to determine some of those undesirable effects already described. Digressing for a moment, it may be admitted that the desiccation of the acetylene produced in this manner is beneficial, even necessary; but the advantages of drying the gas at this period of its treatment are outweighed by the concomitant disadvantages and by the later inevitable remoistening thereof. Suppose now (2) that both the water inlet and the gas exit of the carbide cylinder are at the same end, A. Again half the added water, as liquid, reacts with the carbide it first encounters, but the hot stream of damp gas is not permitted to travel over the rest of the lumps extending towards B: it is forced to return upon its steps, leaving B practically untouched. The gas accordingly escapes from the cylinder at A still loaded with water vapour, and for a given weight of water introduced much less acetylene is evolved than in the former case. The gas, too, needs drying somewhere else in the plant; but these defects are preferable to the apparent superiority of the first process because overheating is, or can be, more thoroughly guarded against.

PRESSURE IN GENERATORS.—Inasmuch as acetylene is prone to dissociate or decompose into its elements spontaneously whenever its pressure reaches 2 atmospheres or 30 lb. per square inch, as well as when its temperature at atmospheric pressure attains 780° C., no pressure approaching that of 2 atmospheres is permissible in the generator. A due observance of this rule, however, unlike a proper maintenance of a low temperature in an acetylene apparatus, is perfectly easy to arrange for. The only reason for having an appreciable positive pressure in any form of generating plant is that the gas may be compelled to travel through the pipes and to escape from the burner orifices; and since the plant is only installed to serve the burners, that pressure which best suits the burners must be thrown by the generator or its holder. Therefore the highest pressure it is ever requisite to employ in a generator is a pressure sufficient (a) to lift the gasholder bell, or to raise the water in a displacement holder, (b) to drive the gas through the various subsidiary items in the plant, such as washers and purifiers, (c) to overcome the friction in the service-pipes, [Footnote: This friction manifestly causes a loss of pressure, i.e., a fall in pressure, as a gas travels along a pipe; and, as will be shown in Chapter VII., it is the fall in pressure in a pipe rather than the initial pressure at which a gas enters a pipe that governs the volume of gas passing through that pipe. The proper behaviour and economic working of a burner (acetylene or other, luminous or incandescent) naturally depend upon the pressure in the pipe to which the burner is immediately attached being exactly suited to the design of that burner, and have nothing to do with the fall in pressure occurring in the delivery pipes. It is therefore necessary to keep entirely separate the ideas of proper burner pressure and of maximum desirable fall in pressure within the service due to friction.] and (d) to give at the points of combustion a pressure which is required by the particular burners adopted. In all except village or district installations, (c) may be virtually neglected. When the holder has a rising bell, (a) represents only an inch or so of water; but if a displacement holder is employed the pressure needed to work it is entirely indeterminate, being governed by the size and shape of the said holder. It will be argued in Chapter III. that a rising holder is always preferable to one constructed on the displacement principle. The pressure (d) at the burners may be taken at 4 inches of water as a maximum, the precise figure being dependent upon the kind of burners—luminous, incandescent, boiling, &c.—attached to the main. The pressure (b) also varies according to circumstances, but averages 2 or 3 inches. Thus a pressure in the generator exceeding that of the atmosphere by some 12 inches of water—i.e., by about 7 oz., or less than half a pound per square inch—is amply sufficient for every kind of installation, the less meritorious generators with displacement holders only excepted. This pressure, it should be noted, is the net or effective pressure, the pressure with which the gas raises the liquid in a water-gauge glass out of the level while the opposite end of the water column is exposed to the atmosphere. The absolute pressure in a vessel containing gas at an effective pressure of 12 inches of water is 7 oz. plus the normal, insensible pressure of the atmosphere itself—say 15-1/4 lb. per square inch. The liquid in a barometer which measures the pressure of the atmosphere stands at a height of 30 inches only, because that liquid is mercury, 13.6 times as heavy as water. Were it filled with water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet, approximately. Gas pressures are always measured in inches of water column, because expressed either as pounds per square inch or as inches of mercury, the figures would be so small as to give decimals of unwieldy length.

It would of course be perfectly safe so to arrange an acetylene plant that the pressure in the generating chamber should reach the 100 inches of water first laid down by the Home Office authorities as the maximum allowable. There is, however, no appreciable advantage to be gained by so doing, or by exceeding that pressure which feeds the burners best. Any higher original pressure involves the use of a governor at the exit of the plant, and a governor is a costly and somewhat troublesome piece of apparatus that can be dispensed with in most single installations by a proper employment of a well-balanced rising holder.

Physics and cheistry of the reaction between carbide and water - Acetylene

THE PHYSICS AND CHEMISTRY OF THE REACTION BETWEEN CARBIDE AND WATER

THE NATURE OF CALCIUM CARBIDE.—The raw material from which, by interaction with water, acetylene is obtained, is a solid body called calcium carbide or carbide of calcium. Inasmuch as this substance can at present only be made on a commercial scale in the electric furnace—and so far as may be foreseen will never be made on a large scale except by means of electricity—inasmuch as an electric furnace can only be worked remuneratively in large factories supplied with cheap coal or water power; and inasmuch as there is no possibility of the ordinary consumer of acetylene ever being able to prepare his own carbide, all descriptions of this latter substance, all methods of winning it, and all its properties except those which concern the acetylene-generator builder or the gas consumer have been omitted from the present book. Hitherto calcium carbide has found but few applications beyond that of evolving acetylene on treatment with water or some aqueous liquid, hygroscopic solid, or salt containing water of crystallisation; but it has possibilities of further employment, should its price become suitable, and a few words will be devoted to this branch of the subject in Chapter XII. Setting these minor uses aside, calcium carbide has no intrinsic value except as a producer of acetylene, and therefore all its characteristics which interest the consumer of acetylene are developed incidentally throughout this volume as the necessity for dealing with them arises.

It is desirable, however, now to discuss one point connected with solid carbide about which some misconception prevails. Calcium carbide is a body which evolves an inflammable, or on occasion an explosive, gas when treated with water; and therefore its presence in a building has been said to cause a sensible increase in the fire risk because attempts to extinguish a fire in the ordinary manner with water may cause evolution of acetylene which should determine a further production of flame and heat. In the absence of water, calcium carbide is absolutely inert as regards fire; and on several occasions drums of it have been recovered uninjured from the basement of a house which has been totally destroyed by fire. With the exception of small 1-lb. tins of carbide, used only by cyclists, &c., the material is always put into drums of stout sheet-iron with riveted or folded seams. Provided the original lid has not been removed, the drums are air- and water-tight, so that the fireman's hose may be directed upon them with impunity. When a drum has once been opened, and not all of its contents have been put into the generator, ordinary caution—not merely as regards fire, but as regards the deterioration of carbide when exposed to the atmosphere—suggests either that the lid must be made air-tight again (not by soldering it), [Footnote: Carbide drums are not uncommonly fitted with self-sealing or lever-top lids, which are readily replaced hermetically tight after opening and partial removal of the contents of the drum.] or preferably that the rest of the carbide shall be transferred to some convenient receptacle which can be perfectly closed. [Footnote: It would be a refinement of caution, though hardly necessary in practice, to fit such a receptacle with a safety-valve. If then the vessel were subjected to sudden or severe heating, the expansion of the air and acetylene in it could not possibly exert a disruptive effect upon the walls of the receptacle, which, in the absence of the safety-valve, is imaginable.] Now, assuming this done, the drums are not dependent upon soft solder to keep them sound, and so they cannot open with heat. Fire and water, accordingly, cannot affect them, and only two risks remain: if stored in the basement of a tall building, falling girders, beams or brickwork may burst them; or if stored on an upper floor, they may fall into the basement and be burst with the shock—in either event water then having free access to the contents. But drums of carbide would never be stored in such positions: a single one would be kept in the generator-house; several would be stored in a separate room therein, or in some similar isolated shed. The generator-house or shed would be of one story only; the drums could neither fall nor have heavy weights fall on them during a fire; and therefore there is no reason why, if a fire should occur, the firemen should not be permitted to use their hose in the ordinary fashion. Very similar remarks apply to an active acetylene generator. Well built, such plant will stand much heat and fire without failure; if it is non-automatic, and of combustible materials contains nothing but gas in the holder, the worst that could happen in times of fire would be the unsealing of the bell or its fracture, and this would be followed, not at all by any explosion, but by a fairly quiet burning of the escaping gas, which would be over in a very short time, and would not add to the severity of the conflagration unless the generator-house were so close to the residence that the large flame of burning gas could ignite part of the main building. Even if the heat were so great near the holder that the gas dissociated, it is scarcely conceivable that a dangerous explosion should arise. But it is well to remember, that if the generator-house is properly isolated from the residence, if it is constructed of non-inflammable materials, if the attendant obeys instructions and refrains from taking a naked light into the neighbourhood of the plant, and if the plant itself is properly designed and constructed, a fire at or near an acetylene generator is extremely unlikely to occur. At the same time, before the erection of plant to supply any insured premises is undertaken, the policy or the company should be consulted to ascertain whether the adoption of acetylene lighting is possibly still regarded by the insurers as adding an extra risk or even as vitiating the whole insurance.

REGULATIONS FOR THE STORAGE OF CARBIDE: BRITISH.—There are also certain regulations imposed by many local authorities respecting the storage of carbide, and usually a licence for storage has to be obtained if more than 5 lb. is kept at a time. The idea of the rule is perfectly justifiable, and it is generally enforced in a sensible spirit. As the rules may vary in different localities, the intending consumer of acetylene must make the necessary inquiries, for failure to comply with the regulations may obviously be followed by unpleasantness.

Having regard to the fact that, in virtue of an Order in Council dated July 7, 1897, carbide may be stored without a licence only in separate substantial hermetically closed metal vessels containing not more than 1 lb. apiece and in quantities not exceeding 5 lb. in the aggregate, and having regard also to the fact that regulations are issued by local authorities, the Fire Offices' Committee of the United Kingdom has not up to the present deemed it necessary to issue special rules with reference to the storage of carbide of calcium.

The following is a copy of the rules issued by the National Board of Fire Underwriters of the UNITED STATES OF AMERICA for the storage of calcium carbide on insured premises:

Cost Per Hour And Hygienic Effect Of Lighting By Different Means - Acetylene

COST PER HOUR AND HYGIENIC EFFECT OF LIGHTING BY DIFFERENT MEANS

The data (except in the column headed "cost per 100 candle-hours") refer to the illumination afforded by medium-sized (0.5 to 0.7 cubic foot per hour) acetylene burners yielding together a light of about 100 candle- power, and to the approximately equivalent illumination as afforded by other means of illumination, when the lighting-units or sources of light are rationally distributed.

Interest and depreciation charges on the outlay on piping or wiring a house, on brackets, fittings, lamps, candelabra, and storage accommodation (for carbide and oil) have been taken as equivalent for all modes of lighting, and omitted in computing the total cost. The cost of labour for attendance on acetylene plant, oil lamps, and candles is an uncertain and variable item—approximately equal for all these modes of lighting, but saved in coal-gas and electric lighting from public supply mains.

______________________________________________________________________ | | | | | | | | | |Candle- | Number |Aggregate| Cost | | | |Power of| of | Candle- | per | | | Description of | each |Lighting | Power | 100 | |Illuminant. | Burner or Lamp. |Lighting| Units |Afforded.|Candle-| | | | Unit. |Required.|(About.) |Hours. | | | |(About.)| | |Pence. | |____________|____________________|________|_________|_________|_______| | | | | | | | | |Self-luminous; 0.5 | | | | | | | cubic foot per hour| 18 | 5 | 90 | 1.11 | | |Self-luminous; 0.7 | | | | | | Acetylene | cubic foot per hour| 27 | 4 | 108 | 1.02 | | |Self-luminous; 1.0 | | | | | | | cubic foot per hour| 45.5 | 3 | 136 | 0.85 | | |Incandescent; 0.5 | | | | | | | cubic foot per hour| 50 | 3 | 150 | 0.49 | |____________|____________________|________|_________|_________|_______| | | | | | | | | Petroleum | Large lamp . . . . | 20 | 5 | 100 | 0.84 | | (paraffin | | | | | | | oil) | Small lamp . . . . | 5 | 14 | 70 | 1.31 | |____________|____________________|________|_________|_________|_______| | | | | | | | | |Flat flame (bad) 5 | | | | | | | cubic feet per hour| 8 | 10 | 80 | 3.75 | | |Flat flame (good) 6 | | | | | | Coal Gas | cubic feet per hour| 16 | 6 | 96 | 2.25 | | |Incandescent (No. 1 | | | | | | | Kern or Bijou In- | 25 | 4 | 100 | 0.38 | | | verted); 1-1/2 | | | | | | | cubic feet per hour| | | | | |____________|____________________|________|_________|_________|_______| | | | | | | | | Candles |"Wax" (so-called) . | 1.2 | 30 | 35 | 6.14 | |____________|____________________|________|_________|_________|_______| | | | | | | | | | Small glow . . . . | 7 | 11 | 77 | 2.81 | | | Large glow . . . . | 13 | 7 | 91 | 2.90 | | Electricity| | | | | | | | Tantalum . . . . . | 19 | 5 | 95 | 1.52 | | | Osram . . . . . . | 14 | 7 | 98 | 1.00 | |____________|____________________|________|_________|_________|_______|

___________________________________________________________________ | | | | | | | | | | | | | | Equivalent | | | Description of | Assumed Cost | Illumin- | |Illuminant. | Burner or Lamp. | of Illuminant. | ation. | | | | | Pence. | | | | | | |____________|____________________|____________________|____________| | | | | | | |Self-luminous; 0.5 | Calcium carbide | | | | cubic foot per hour| (yielding 5 | 1.00 | | |Self-luminous; 0.7 | cubic feet of | | | Acetylene | cubic foot per hour| acetylene per | 1.10 | | |Self-luminous; 1.0 | lb.) at 15s. | | | | cubic foot per hour| per cwt., inclu- | 1.16 | | |Incandescent; 0.5 | ding delivery | | | | cubic foot per hour| charges. | 0.74 | |____________|____________________|____________________|____________| | | | | | | Petroleum | Large lamp . . . . | Oil, 9d. per gal- | 0.84 | | (paraffin | | lon, including | | | oil) | Small lamp . . . . | delivery charges. | 0.92 | |____________|____________________|____________________|____________| | | | | | | |Flat flame (bad) 5 | | | | | cubic feet per hour| Public supply | 3.00 | | |Flat flame (good) 6 | from small | | | Coal Gas | cubic feet per hour| country works, | 2.16 | | |Incandescent (No. 1 | at 5s. per 1000 | | | | Kern or Bijou In- | cubic feet. | 0.38 | | | verted); 1-1/2 | | | | | cubic feet per hour| | | |____________|____________________|____________________|____________| | | | | | | Candles |"Wax" (so-called) . | 5d. per lb. | 2.60 | |____________|____________________|____________________|____________| | | | | | | | Small glow . . . . | Public supply | 2.16 | | | Large glow . . . . | from small | 2.64 | | Electricity| | town works | | | | Tantalum . . . . . | at 6d. per | 1.45 | | | Osram . . . . . . | B.O.T. unit. | 0.98 | |____________|____________________|____________________|____________|

_______________________________________________________________________ | | | | | | | | | |Inci- | Exhaus- |Vitiation | Heat | | | | den- | tion of | of Air. |Produced.| | | Description of | tal |Air.Cubic|Cubic Feet|Number of| |Illuminant. | Burner or Lamp. |Expen-|Feet Dep-| of Car- |Units of | | | | ces. |rived of |bonic Acid| Heat. | | | | | Oxygen. | Formed. |Calories.| |____________|____________________|______|_________|__________|_________| | | | | | | | | |Self-luminous; 0.5 | | | | | | | cubic foot per hour| [1] | 29.8 | 5.0 | 900 | | |Self-luminous; 0.7 | | | | | | Acetylene | cubic foot per hour| | 33.3 | 5.6 | 1010 | | |Self-luminous; 1.0 | | | | | | | cubic foot per hour| | 35.7 | 6.0 | 1000 | | |Incandescent; 0.5 | | | | | | | cubic foot per hour| [2] | 17.9 | 3.0 | 545 | |____________|____________________|______|_________|__________|_________| | | | | | | | | Petroleum | Large lamp . . . . | | 140.0 | 19.6 | 3630 | | (paraffin | | [3] | | | | | oil) | Small lamp . . . . | | 154.0 | 21.6 | 4000 | |____________|____________________|______|_________|__________|_________| | | | | | | | | |Flat flame (bad) 5 | | | | | | | cubic feet per hour| Nil | 270.0 | 27.0 | 7750 | | |Flat flame (good) 6 | | | | | | Coal Gas | cubic feet per hour| Nil | 195.0 | 19.5 | 5580 | | |Incandescent (No. 1 | | | | | | | Kern or Bijou In- | [4] | 27.0 | 2.7 | 775 | | | verted); 1-1/2 | | | | | | | cubic feet per hour| | | | | |____________|____________________|______|_________|__________|_________| | | | | | | | | Candles |"Wax" (so-called) . | Nil | 100.5 | 13.7 | 2700 | |____________|____________________|______|_________|__________|_________| | | | | | | | | | Small glow . . . . |2s.6d.| Nil | Nil | 285 | | | Large glow . . . . |2s.6d.| " | " | 360 | | Electricity| | [5] | | | | | | Tantalum . . . . . |7s.6d.| " | " | 172 | | | Osram . . . . . . | 6s. | " | " | 96 | |____________|____________________|______|_________|__________|_________|

[Footnote 1: Interest and depreciation charges on generating and purifying plant = 0.15 penny. Purifying material and burner renewals = 0.05 penny.]

[Footnote 2: Mantle renewals as for coal-gas.]

[Footnote 3: Renewals of wicks and chimneys = 0.02 penny.]

[Footnote 4: Renewals and mantles (and chimneys) at contract rate of 3s. per burner per annum.]

[Footnote 5: Renewals of lamps and fuses, at price indicated per lamp per annum.]

The conventional method of making pecuniary comparisons between different sources of artificial light consists in simply calculating the cost of developing a certain number of candle-hours of light—i.e., a certain amount of standard candle-power for a given number of hours—on the assumption that as many separate sources of light are employed as may be required to bring the combined illuminating power up to the total amount wanted. In view of the facts as to dissemination and diffusion, or the difference between sheer illuminating power and useful illuminating effect, which have just been elaborated, and in view of the different intensities of the different unit sources of light (which range from the single candle to a powerful large incandescent gas-burner or a metallic filament electric lamp), such a method of calculation is wholly illusory. The plan adopted in the following table may also appear unnecessarily complicated; but it is not so to the reader if he remembers that the apparently various amount of illumination is corrected by the different numbers of illuminating units until the amount of simple candle-power developed, whatever illuminant be employed, suffices to light a room having an area of about 300 square feet (i.e., a room, 17-1/2 feet square, or one 20 feet long by 15 feet wide), so that ordinary print may be read comfortably in any part of the room, and the titles of books, engravings, &c., in any position on the walls up to a height of 8 feet from the ground may be distinguished with ease. The difference in cost, &c., of a greater or less degree of illumination, or of lighting a larger or smaller room by acetylene or any other of the illuminants named, will be almost directly proportional to the cost given for the stated conditions. Nevertheless, it should be recollected that when the conventional system is retained—useful illuminating effect being sacrificed to absolute illuminating power—acetylene is made to appear cheaper in comparison with all weaker unit sources of light, and dearer in comparison with all stronger unit sources of light than the accompanying table indicates it to be. In using the comparative figures given in the table, it should be borne in mind that they refer to more general and more brilliant illumination of a room than is commonly in vogue where the lighting is by means of electric light, candles, or oil- lamps. The standard of illumination adopted for the table is one which is only gaining general recognition where incandescent gas or acetylene lighting is available, though in exceptional cases it has doubtless been attained by means of oil-lamps or flat-flame gas-burners, but very rarely if ever by means of carbon-filament electric glow-lamps, or candles. It assumes that the occupants of a room do not wish to be troubled to bring work or book "to the light," but wish to be able to work or read wheresoever in the room they will, without consideration of the whereabouts of the light or lights.

It should, perhaps, be added that so high a price as 5s. per 1000 cubic feet for coal-gas rarely prevails in Great Britain, except in small outlying towns, whereas the price of 6d. per Board of Trade unit for electricity is not uncommonly exceeded in the few similar country places in which there is a public electricity supply.

Acetylene lighting - The Cost and Advantages of Acetylene Lighting

INTRODUCTORY—THE COST AND ADVANTAGES OF ACETYLENE LIGHTING

Acetylene is a gas [Footnote: For this reason the expression, "acetylene gas," which is frequently met with, would be objectionable on the ground of tautology, even if it were not grammatically and technically incorrect. "Acetylene-gas" is perhaps somewhat more permissible, but it is equally redundant and unnecessary.] of which the most important application at the present time is for illuminating purposes, for which its properties render it specially well adapted. No other gas which can be produced on a commercial scale is capable of giving, volume for volume, so great a yield of light as acetylene. Hence, apart from the advantages accruing to it from its mode of production and the nature of the raw material from which it is produced, it possesses an inherent advantage over other illuminating gases in the smaller storage accommodation and smaller mains and service-pipes requisite for the maintenance of a given supply of artificial light. For instance, if a gasholder is required to contain sufficient gas for the lighting of an establishment or district for twenty-four hours, its capacity need not be nearly so great if acetylene is employed as if oil-gas, coal-gas, or other illuminating gas is used. Consequently, for an acetylene supply the gasholder can be erected on a smaller area and for considerably less outlay than for other gas supplies. In this respect acetylene has an unquestionable economical advantage as a competitor with other varieties of illuminating gas for supplies which have generally been regarded as lying peculiarly within their preserves. The extent of this advantage will be referred to later.

The advantages that accrue to acetylene from its mode of production, and the nature of the raw material from which it is obtained, are in reality of more importance. Acetylene is readily and quickly produced from a raw material—calcium carbide—which, relatively to the yield of light of the gaseous product, is less bulky than the raw materials of other gases. In comparison also with oils and candles, calcium carbide is capable of yielding, through the acetylene obtainable from it, more light per unit of space occupied by it. This higher light-yielding capacity of calcium carbide, ready to be developed through acetylene, gives the latter gas a great advantage over all other illuminants in respect of compactness for transport or storage. Hence, where facilities for transport or storage are bad or costly, acetylene may be the most convenient or cheapest illuminant, notwithstanding its relatively high cost in many other cases. For example, in a district to which coal and oil must be brought great distances, the freight on them may be so heavy that—regarding the question as simply one of obtaining light in the cheapest manner—it may be more economical to bring calcium carbide an equal or even greater distance and generate acetylene from it on the spot, than to use oil or make coal-gas for lighting purposes, notwithstanding that acetylene may not be able to compete on equal terms with oil—or coal-gas at the place from which the carbide is brought. Likewise where storage accommodation is limited, as in vehicles or in ships or lighthouses, calcium carbide may be preferable to oil or other illuminants as a source of light. Disregarding for the moment intrinsic advantages which the light obtainable from acetylene has over other lights, there are many cases where, owing to saving in cost of carriage, acetylene is the most economical illuminant; and many other cases where, owing to limited space for storage, acetylene far surpasses other illuminants in convenience, and is practically indispensable.

The light of the acetylene flame has, however, some intrinsic advantages over the light of other artificial illuminants. In the first place, the light more closely resembles sunlight in composition or "colour." It is more nearly a pure "white" light than is any other flame or incandescent body in general use for illuminating purposes. The nature or composition of the light of the acetylene flame will be dealt with more exhaustively later, and compared with that afforded by other illuminants; but, speaking generally, it may be said that the self-luminous acetylene light is superior in tint, to all other artificial lights, for which reason it is invaluable for colour-judging and shade-matching. In the second place, when the gas issues from a suitable self-luminous burner under proper pressure, the acetylene flame is perfectly steady; and in this respect it in preferable to most types of electric light, to all self- luminous coal-gas flames and candles, and to many varieties of oil-lamp. In steadiness and freedom from flicker it is fully equal to incandescent coal-gas light, but it in distinctly superior to the latter by virtue of its complete freedom from noise. The incandescent acetylene flame emits a slight roaring, but usually not more than that coming from an atmospheric coal-gas burner. With the exception of the electric arc, self-luminous acetylene yields a flame of unsurpassed intensity, and yet its light is agreeably soft. In the third place, where electricity is absent, a brilliancy of illumination which can readily be obtained from self-luminous acetylene can otherwise only be procured by the employment of the incandescent system applied either to coal-gas or to oil; and there are numerous situations, such as factories, workshops, and the like, where the vibration of the machinery or the prevalence of dust renders the use of mantles troublesome if not impossible. Anticipating what will be said later, in cases like these, the cost of lighting by self-luminous acetylene may fairly be compared with self-luminous coal- gas or oil only; although in other positions the economy of the Welsbach mantle must be borne in mind.

Acetylene lighting presents also certain important hygienic advantages over other forms of flame lighting, in that it exhausts, vitiates, and heats the air of a room to a less degree, for a given yield of light, than do either coal-gas, oils, or candles. This point in favour of acetylene is referred to here only in general terms; the evidence on which the foregoing statement is based will be recorded in a tabular comparison of the cost and qualities of different illuminants. Exhaustion of the air means, in this connexion, depletion of the oxygen normally present in it. One volume of acetylene requires 2-1/2 volumes of oxygen for its complete combustion, and since 21 volumes of oxygen are associated in atmospheric air with 79 volumes of inert gases—chiefly nitrogen—which do not actively participate in combustion, it follows that about 11.90 volumes of air are wholly exhausted, or deprived of oxygen, in the course of the combustion of one volume of acetylene. If the light which may be developed by the acetylene is brought into consideration, it will be found that, relatively to other illuminants, acetylene causes less exhaustion of the air than any other illuminating agent except electricity. For instance, coal-gas exhausts only about 6- 1/2 times its volume of air when it is burnt; but since, volume for volume, acetylene ordinarily yields from three to fifteen times as much light as coal-gas, it follows that the same illuminative value is obtainable from acetylene by considerably less exhaustion of the air than from coal-gas. The exact ratio depends on the degree of efficiency of the burners, or of the methods by which light is obtained from the gases, as will be realised by reference to the table which follows. Broadly speaking, however, no illuminant which evolves light by combustion (oxidation), and which therefore requires a supply of oxygen or air for its maintenance, affords light with so little exhaustion of the air as acetylene. Hence in confined, ill-ventilated, or crowded rooms, the air will suffer less exhaustion, and accordingly be better for breathing, if acetylene is chosen rather than any other illuminant, except electricity.

Next, in regard to vitiation of the air, by which is meant the alteration in its composition resulting from the admixture of products of combustion with it. Electric lighting is as superior to other modes of lighting in respect of direct vitiation as of exhaustion of the air, because it does not depend on combustion. Putting it aside, however, light is obtainable by means of acetylene with less attendant vitiation of the air than by means of any other gas or of oil or candles. The principal vitiating factor in all cases is the carbonic acid produced by the combustion. Now one volume of acetylene on combustion yields two volumes of carbonic acid, whereas one volume of coal-gas yields about 0.6 volume of carbonic acid. But even assuming that the incandescent system of lighting is applied in the case of coal-gas and not of acetylene, the ratio of the consumption of the two gases for the development of a given illuminative effect will be such that no more carbonic acid will be produced by the acetylene; and if the incandescent system is applied either in both cases or in neither, the ratio will be greatly in favour of acetylene. The other factors which determine the vitiation of the air of a room in which the gas is burning are likewise under ordinary conditions more in favour of acetylene. They are not, however, constant, since the so-called "impurities," which on combustion cause vitiation of the air, vary greatly in amount according to the extent to which the gases have been purified. London coal-gas, which was formerly purified to the highest degree practically attainable, used to contain on the average only 10 to 12 grains of sulphur per 100 cubic feet, and virtually no other impurity. But now coal-gas, in London and most provincial towns, contains 40 to 50 grains of sulphur per 100 cubic foot. At least 5 grains of ammonia per 100 cubic foot in also present in coal-gas in some towns. Crude acetylene also contains sulphur and ammonia, that coming from good quality calcium carbide at the present day including about 31 grains of the former and 25 grains of the latter per 100 cubic feet. But crude acetylene is also accompanied by a third impurity, viz., phosphoretted hydrogen or phosphine, which in unknown in coal-gas, and which is considerably more objectionable than either ammonia or sulphur. The formation, behaviour, and removal of those various impurities will be discussed in Chapter V.; but here it may be said that there is no reason why, if calcium carbide of a fair degree of purity has been used, and if the gas has been generated from it in a properly designed and smoothly working apparatus— this being quite as important as, or even more important than, the purity of the original carbide—the gas should not be freed from phosphorus, sulphur, and ammonia to the utmost necessary or desirable extent, by processes which are neither complicated nor expensive. And if this is done, as it always should be whenever the acetylene is required for domestic lighting, the vitiation of the air of a room due to the "impurities" in the gas will become much less in the case of acetylene than in that of even well-purified coal-gas; taking equal illuminating effect as the basis for comparison.

Acetylene is similarly superior, speaking generally, to petroleum in respect of impurities, though the sulphur present in petroleum oils, such as are sold in this country for household use, though very variable, is often quite small in amount, and seldom is responsible for serious vitiation of the atmosphere.

Regarding somewhat more closely the relative convenience and safety of acetylene and paraffin for the illumination of country residences, it may be remarked that an extraordinarily great amount of care must he bestowed upon each separate lamp if the whole house is to be kept free from an odour which is very offensive to the nostrils; and the time occupied in this process, which of itself is a disagreeable one, reaches several hours every day. Habit has taught the country dweller to accept as inevitable this waste of time, and largely to ignore the odour of petroleum in his abode; but the use of acetylene entirely does away with the daily cleaning of lamps, and, if the pipe-fitting work has been done properly, yields light absolutely unaccompanied by smell. Again, unless most carefully managed, the lamp-room of a large house, with its store of combustible oil, and its collection of greasy rags, must unavoidably prove a sensible addition to the risk of fire. The analogue of the lamp- room when acetylene is employed is the generator-house, and this is a separate building at some distance from the residence proper. There need be no appreciable odour in the generator-house, except during the times of charging the apparatus; but if there is, it passes into the open air instead of percolating into the occupied apartments.

The amount of heat developed by the combustion of acetylene also is less for a given yield of light than that developed by most other illuminants. The gas, indeed, is a powerful heating gas, but owing to the amount consumed being so small in proportion to the light developed, the heat arising from acetylene lighting in a room is less than that from most other illuminating agents, if the latter are employed to the extent required to afford equally good illumination. The ratio of the heat developed in acetylene lighting to that developed in, e.g., lighting by ordinary coal-gas, varies considerably according to the degree of efficiency of the burners, or, in other words, of the methods by which light is obtained from the gases. Volume for volume, acetylene yields on combustion about three and a half times as much heat as coal- gas, yet, owing to its superior efficiency as an illuminant, any required light may be obtained through it with no greater evolution of heat than the best practicable (incandescent) burners for coal-gas produce. The heat evolved by acetylene burners adequate to yield a certain light is very much less than that evolved by ordinary flat-flame coal-gas burners or by oil-lamps giving the same light, and is not more than about three times as much as that from ordinary electric lamps used in numbers sufficient to give the same light. More exact figures for the ratio between the heat developed in acetylene lighting and that in other modes of lighting are given in the table already referred to.

In connexion with the smaller amount of heat developed per unit of light when acetylene is the illuminant, the frequently exaggerated claim that acetylene does not blacken ceilings at all may be studied. Except it be a carelessly manipulated petroleum-lamp, no form of artificial illuminant employed nowadays ever emits black smoke, soot, or carbon, in spite of the fact that all luminous flames commercially capable of utilisation do contain free carbon in the elemental state. The black mark on a ceiling over a source of light is caused by a rising current of hot air and combustion products set up by the heat accompanying the light, which current of hot gas carries with it the dust and dirt always present in the atmosphere of an inhabited room. As this current of air and burnt gas travels in a fairly concentrated vertical stream, and as the ceiling is comparatively cool and exhibits a rough surface, that dust and dirt are deposited on the ceiling above the flame, but the stain is seldom or never composed of soot from the illuminant itself. Proof of this statement may be found in the circumstance that a black mark is eventually produced over an electric glow-lamp and above a pipe delivering hot water. Clearly, therefore, the depth and extent of the mark will depend on the volume and temperature of the hot gaseous current; and since per unit of light acetylene emits a far smaller quantity of combustion products and a far smaller amount of heat than any other flame illuminant except incandescent coal-gas, the inevitable black mark over its flame takes very much longer to appear. Quite roughly speaking, as may be deduced from what has already been said on this subject, the luminous flame of acetylene "blackens" a ceiling at about the same rate as a coal-gas burner of the best Welsbach type.

There is one respect in which acetylene and other flame illuminants are superior to electric lighting, viz., that they sterilise a larger volume of air. All the air which is needed to support combustion, as well as the excess of air which actually passes through the burner tube and flame in incandescent burners, is obviously sterilised; but so also is the much larger volume of air which, by virtue of the up-current due to the heat of the flame, is brought into anything like close proximity with the light. The electric glow-lamp, and the most popular and economical modern enclosed electric arc-lamp, sterilise only the much smaller volume of air which is brought into direct contact with their glass bulbs. Moreover, when large numbers of persons are congregated in insufficiently ventilated buildings—and many public rooms are insufficiently ventilated—the air becomes nauseous to inspire and positively detrimental to the health of delicate people, by reason of the human effluvia which arise from soiled raiment and uncleansed or unhealthy bodies, long before the proportion of carbonic acid by itself is high enough to be objectionable. Thus a certain proportion of carbonic acid coming from human lungs and skin is more harmful than the same proportion of carbonic acid derived from the combustion of gas or oil. Hence acetylene and flame illuminants generally have the valuable hygienic advantages over electric lighting, not only of killing a far larger number of the micro-organisms that may be present in the air, but, by virtue of their naked flames, of burning up and destroying a considerable quantity of the aforesaid odoriferous matter, thus relieving the nose and materially assisting in the prevention of that lassitude and anæmia occasionally follow the constant inspiration of air rendered foul by human exhalations.

The more important advantages of acetylene as an illuminant have now been indicated, and it remains to discuss the cost of acetylene lighting in comparison with other modes of procuring artificial light. At the outset it may be stated that a very much greater reduction in the price of calcium carbide—from which acetylene is produced—than is likely to ensue under the present methods and conditions of manufacture will be required to make acetylene lighting as cheap as ordinary gas lighting in towns in this country, provided incandescent burners are used for the gas. On the score of cheapness (and of convenience, unless the acetylene were delivered to the premises from some central generating station) acetylene cannot compete as an illuminant with coal-gas where the latter costs, say, not more than 5s. per 1000 cubic feet, if only reasonable attention is given to the gas-burners, and at least a quarter of them are on the incandescent system. If, on the other hand, coal-gas is misused and wasted through the employment only of interior or worn-out flat-flame burners, while the best types of burner are used for acetylene, the latter gas may prove as cheap for lighting as coal-gas at, say, 2s. 6d. per 1000 cubic feet (and be far better hygienically); whereas, contrariwise, if coal-gas is used only with good and properly maintained incandescent burners, it may cost over 10s. per 1000 cubic feet, and be cheaper than acetylene burned in good burners (and as good from the hygienic standpoint). More precise figures on the relative costs of coal-gas lighting and acetylene lighting are given in the tabular statement at the close of this chapter.

With regard to electric lighting it is somewhat difficult to lay down a fair basis of comparison, owing to the wide variations in the cost of current, and in the efficiency of lamps, and to the undoubted hygienic and aesthetic claims of electric lighting to precedence. But in towns in this country where there is a public electricity supply, electric lighting will be used rather than acetylene for the same reasons that it is preferred to coal-gas. Cost is only a secondary consideration in such cases, and where coal-gas is reasonably cheap, and nevertheless gives place to electric lighting, acetylene clearly cannot hope to supplant the latter. [Footnote: Where, however, as is frequently the case with small public electricity-supply works, the voltage of the supply varies greatly, the fluctuations in the light of the lamps, and the frequent destruction of fuses and lamps, are such manifest inconveniences that acetylene is in fact now being generally preferred to electric lighting in such circumstances.] But where current cannot be had from an electricity-supply undertaking, and it is a question, in the event of electric lighting being adopted, of generating current by driving a dynamo, either by means of a gas-engine supplied from public gas-mains, by means of a special boiler installation, or by means of an oil-engine or of a power gas-plant and gas-engine, the claims of acetylene to preference are very strong. An important factor in the estimation of the relative advantages of electricity and acetylene in such cases is the cost of labour in looking after the generating plant. Where a gas-engine supplied from public gas-mains is used for driving the dynamo, electric lighting can be had at a relatively small expenditure for attendance on the generating plant. But the cost of the gas consumed will be high, and actually light could be obtained directly from the gas by means of incandescent mantles at far loss cost than by consuming the gas in a motor for the indirect production of light by means of electric current. Therefore electric lighting, if adopted under these conditions, must be preferred to gas lighting from considerations which are deemed to outweigh those of a much higher cost, and acetylene does not present so great advantages over coal-gas as to affect the choice of electric lighting. But in the cases where there is no public gas-supply, and current must be generated from coal or coke or oil consumed on the spot, the cost of the skilled labour required to look after either a boiler, steam-engine and dynamo, or a power gas-plant and gas-engine or oil- engine and dynamo, will be so heavy that unless the capacity of the installation is very great, acetylene will almost certainly prove a cheaper and more convenient method of obtaining light. The attention required by an acetylene installation, such as a country house of upwards of thirty rooms would want, is limited to one or two hours' labour per diem at any convenient time during daylight. Moreover, the attendant need not be highly paid, as he will not have required an engineman's training, as will the attendant on an electric lighting plant. The latter, too, must be present throughout the hours when light is wanted unless a heavy expenditure has been incurred on accumulators. Furthermore, the capital outlay on generating plant will be very much less for acetylene than for electric lighting. General considerations such as these lead to the conclusion that in almost all country districts in this country a house or institution could be lighted more cheaply by means of acetylene than by electricity. In the tabular statement of comparative costs of different modes of lighting, electric lighting has been included only on the basis of a fixed cost per unit, as owing to the very varied cost of generating current by small installations in different parts of the country it would be futile to attempt to give the cost of electric lighting on any other basis, such as the prime cost of coal or coke in a particular district. Where current is supplied by a public electricity- supply undertaking, the cost per unit is known, and the comparative costs of electric light and acetylene can be arrived at with tolerable precision. It has not been thought necessary to include in the tabular statement electric arc-lamps, as they are only suitable for the lighting of large spaces, where the steadiness and uniformity of the illumination are of secondary importance. Under such conditions, it may be stated parenthetically, the electric arc-light is much less costly than acetylene lighting would be, but it is now in many places being superseded by high-pressure gas or oil incandescent lights, which are steady and generally more economical than the arc light.

The illuminant which acetylene is best fitted to supersede on the score of convenience, cleanliness, and hygienic advantages is oil. By oil is meant, in this connection, the ordinary burning petroleum, kerosene, or paraffin oil, obtained by distilling and refining various natural oils and shales, found in many countries, of which the United States (principally Pennsylvania), Russia (the Caucasus chiefly), and Scotland are practically the only ones which supply considerable quantities for use in Great Britain. Attempts are often made to claim superiority for particular grades of these oils, but it may be at once stated that so for as actual yield of light is concerned, the same weight of any of the commercial oils will give practically the same result. Hence in the comparative statement of the cost of different methods of lighting, oil will be taken at the cheapest rate at which it could ordinarily be obtained, including delivery charges, at a country house, when bought by the barrel. This rate at the present time is about ninepence per gallon. A higher price may be paid for grades of mineral oil reputed to be safer or to give a "brighter" or "clearer" light; but as the quantity of light depends mainly upon the care and attention bestowed on the burner and glass fittings of the lamp, and partly upon the employment of a suitable wick, while the safety of each lamp depends at least as much upon the design of that lamp, and the accuracy with which the wick fits the burner tube, as upon the temperature at which the oil "flashes," the extra expense involved in burning fancy-priced oils will not be considered here.

The efficiency (i.e., the light yielded per pint or other unit volume consumed) of oil-lamps varies greatly, and, speaking broadly, increases with the power of the lamp. But as large or high-power lamps are not needed throughout a house, it is fairer to assume that the light obtainable from oil in ordinary household use is the mean of that afforded by large and that afforded by small lamps. A large oil-lamp as commonly used in country houses will give a light of about 20 candle- power, while a convenient small lamp will give a light of not more than about 5 candle-power. The large lamp will burn about 55 hours for every gallon of oil consumed, or give an illuminating duty of about 1100 candle-hours (i.e., the product of candle-power by burning-hours) per gallon. The small lamp, on the other hand, will burn about 140 hours for every gallon of oil consumed, or give an illuminating duty of about 700 candle-hours per gallon. Actually large lamps would in most country houses be used only in the entrance hall, living-rooms, and kitchen, while passages and minor rooms on the lower floors would be lighted by small lamps. Hence, making due allowance for the lower rate of consumption of the small lamps, it will be seen that, given equal numbers of large and small lamps in use, the mean illuminating duty of a gallon of oil as burnt in country houses will be 987, or, in round figures, 990 candle-hours. Usually candles are used in the bedrooms of country houses where the lower floors are lighted by means of petroleum lamps; but when acetylene is installed in such a house it will frequently be adopted in the principal bed- and dressing-rooms as well as in the living-rooms, as, unless candles are employed very lavishly, they are really totally inadequate to meet the reasonable demands for light of, e.g., a lady dressing for dinner. Where acetylene displaces candles as well as lamps in a country house, it is necessary, in comparing the cost of the new illuminant with that of the candles and oil, to bear in mind the superior degree of illumination which is secured in all rooms, at least where candles were formerly used.

In regard to exhaustion and vitiation of the air, and to heat evolved, self-luminous petroleum lamps stand on much the same footing as coal-gas when the latter is burned in flat-flame burners, if the comparison is based on a given yield of light. A large lamp, owing to its higher illuminating efficiency, is better in this respect than a small one— light for light, it is more hygienic than ordinary flat-flame coal-gas burners, while a small lamp is less hygienic. It will therefore be understood at once, from what has already been said about the superiority on hygienic grounds of acetylene to flat-flame coal-gas lighting, that acetylene is in this respect far superior to petroleum lamps. The degree of its superiority is indicated more precisely by the figures quoted in the tabular statement which concludes this chapter.

Before giving the tabular statement, however, it is necessary to say a few words in regard to one method of lighting which, may possibly develop into a more serious competitor with acetylene for the lighting of the better class of country house than any of the illuminating agents and modes of lighting so far referred to. The method in question is lighting by so-called air-gas used for raising mantles to incandescence in upturned or inverted burners of the Welsbach-Kern type. "Air-gas" is ordinary atmospheric air, more or less completely saturated with the vapour of some highly volatile hydrocarbon. The hydrocarbons practically applied have so far been only "petroleum spirit" or "carburine," and "benzol." "Petroleum spirit" or "carburine" consists of the more highly volatile portion of petroleum, which is removed by distillation before the kerosene or burning oil is recovered from the crude oil. Several grades of this highly volatile petroleum distillate are distinguished in commerce; they differ in the temperature at which they begin to distil and the range of temperature covered by their distillation, and, speaking more generally, in their degree of volatility, uniformity, and density. If the petroleum distillate is sufficiently volatile and fairly uniform in character, good air-gas may be produced merely by allowing air to pass over an extended surface of the liquid. The vapour of the petroleum spirit is of greater density than air, and hence, if the course of the air-gas is downward from the apparatus at which it is produced, the flow of air into the apparatus and over the surface of the spirit will be automatically maintained by the "pull" of the descending air-gas when once the flow has been started until the outlet for the air-gas is stopped or the spirit in the apparatus is exhausted. Hence, if the apparatus for saturating air with the vapour of the light petroleum is placed well above all the points at which the air-gas is to be burnt— e.g., on the roof of the house—the production of the air-gas may by simple devices become automatic, and the only attention the apparatus will require will be the replenishing of its reservoir from time to time with light petroleum. But a number of precautions are required to make this simple process operate without interruption or difficulty. For instance, the evaporation of the spirit must not be so rapid relatively to its total bulk as to lower its temperature, and thereby that of the overflowing air, too much; the reservoir must be protected from extreme cold and extreme heat; and the risk of fire from the presence of a highly volatile and highly inflammable liquid on or near the roof of the house must be met. This risk is one to which fire insurance companies take exception.

More commonly, however, air-gas is made non-automatically, or more or less automatically by the employment of some mechanical means. The light petroleum, benzol, or other suitable volatile hydrocarbon is volatilised, where necessary, by the application of gentle heat, while air is driven over or through it by means of a small motor, which in some cases is a hot-air engine operated by heat supplied by a flame of the air-gas produced. These air-gas producers, or at least the reservoir of volatile hydrocarbon, may be placed in an outbuilding, so that the risk of fire in the house itself is minimised. They require, however, as much attention as an acetylene generator, usually more. It is difficult to give reliable data as to the cost of air-gas, inclusive of the expenses of production. It varies considerably with the description of hydrocarbon employed, and its market price. Air-gas is only slightly inferior hygienically to acetylene, and the colour of its light is that of the incandescent light as produced by coal-gas or acetylene. Air-gas of a certain grade may be used for lighting by flat-flame burners, but it has been available thus for very many years, and has failed to achieve even moderate success. But the advent of the incandescent burner has completely changed its position relatively to most other illuminants, and under certain conditions it seems likely to be the most formidable competitor with acetylene. Since air-gas, and the numerous chemically identical products offered under different proprietary names, is simply atmospheric air more or less loaded with the vapour of a volatile hydrocarbon which is normally liquid, it possesses no definite chemical constitution, but varies in composition according to the design of the generating plant, the atmospheric temperature at the time of preparation, the original degree of volatility of the hydrocarbon, the remaining degree of volatility after the more volatile portions have been vaporised, and the speed at which the air is passed through the carburettor. The illuminating power and the calorific value of air-gas, unless the manufacture is very precisely controlled, are apt to be variable, and the amount of light, emitted, either in self-luminous or in incandescent burners, is somewhat indeterminate. The generating plant must be so constructed that the air cannot at any time be mixed with as much hydrocarbon vapour as constitutes an explosive mixture with it, otherwise the pipes and apparatus will contain a gas which will forthwith explode if it is ignited, i.e., if an attempt is made to consume it otherwise than in burners with specially small orifices. The safely permissible mixtures are (1) air with less hydrocarbon vapour than constitutes an explosive mixture, and (2) air with more hydrocarbon vapour than constitutes an explosive mixture. The first of these two mixtures is available for illuminating purposes only with incandescent mantles, and to ensure a reasonable margin of safety the mixing apparatus must be so devised that the proportion of hydrocarbon vapour in the air-gas can never exceed 2 per cent. From Chapter VI. it will be evident that a little more than 2 per cent. of benzene, pentane or benzoline vapour in air forms an explosive mixture. What is the lowest proportion of such vapours in admixture with air which will serve on combustion to maintain a mantle in a state of incandescence, or even to afford a flame at all, does not appear to have been precisely determined, but it cannot be much below 1- 1/2 per cent. Hence the apparatus for producing air-gas of this first class must be provided with controlling or governing devices of such nicety that the proportion of hydrocarbon vapour in the air-gas is maintained between about 1-1/2 and 2 per cent. It is fair to say that in normal working conditions a number of devices appear to fulfil this requirement satisfactorily. The second of the two mixtures referred to above, viz., air with more hydrocarbon vapour than constitutes an explosive mixture, is primarily suitable for combustion in self-luminous burners, but may also be consumed in properly designed incandescent burners. But the generating apparatus for such air-gas must be equipped with some governing or controlling device which will ensure the proportion of hydrocarbon vapour in the mixture never falling below, say, 7 per cent. On the other hand, if saturation of the air with the vapour is practically attained, should the temperature of the gas fall before it arrives at the point of combustion, part of the spirit will condense out, and the product will thus lose part of its illuminating or calorific intensity, besides partially filling the pipes with liquid products of condensation. The loss of intensity in the gas during cold weather may or may not be inconvenient according to circumstances; but the removal of part of the combustible material brings the residual air-gas nearer to its limit of explosibility—for it is simply a mixture of combustible vapour with air, which, normally, is not explosive because the proportion of spirit is too high—and thus, when led into an atmospheric burner, the extra amount of air introduced at the injector jets may cause the mixture to be an explosive mixture of air and spirit, so that it will take fire within the burner tube instead of burning quietly at the proper orifice. This matter will be made clearer on studying what is said about explosive limits in Chapter VI., and what is stated about incandescent acetylene (carburetted or not) in Chapters IX. and X. Clearly, however, high-grade air-gas is only suitable for preparation at the immediate spot where it is to be consumed; it cannot be supplied to a complete district unless it is intentionally made of such lower intensity that the proportion of spirit is too small ever to allow of partial deposition in the mains during the winter.

It is perhaps necessary to refer to the more extended use of candles for lighting in some few houses in which lamps are disliked on aesthetic, or, in some cases, ostensibly on hygienic grounds. Candle lighting, speaking broadly, is either very inadequate so far as ordinary living-rooms are concerned, or, if adequate, is very costly. Tests specially carried out by one of the authors to determine some of the figures required in the ensuing table show that ordinary paraffin or "wax" candles usually emit about 20 per cent. more light than that given by the standard spermaceti candle, whose luminosity is the unit by which the intensity of other lights is reckoned in Great Britain; and also that the light so emitted by domestic candles is practically unaffected by the sizes—"sixes," "eights," or "twelves"—burnt. In the sizes examined the light evolved has varied between 1.145 and 1.298 "candles," perhaps tending to increase slightly with the diameter of the candle tested. Hence, to obtain illumination in a room equal on the average to that afforded by 100 standard candles, or some other light or lights aggregating 100 candle- power, would require the use of only 80 to 85 ordinary paraffin, ozokerite, or wax candles. But actually the essential objects in a room could be equally well illuminated by, say, 30 candles well distributed, as by two or three incandescent gas-burners, or four or five large oil- lamps. Lights of high intensity, such as powerful gas-burners or oil- lamps, must give a higher degree of illumination in their immediate vicinity than is really necessary, if they are to illuminate adequately the more distant objects. The dissemination and diffusion of their light can be greatly aided by suitable colouring of ceilings, walls and drapings; but unless the illumination by means of lights of relatively high intensity is made almost wholly indirect, candles or other lights of low intensity, such as small electric glow-lamps, can, by proper distribution, be made to give more uniform or more suitably apportioned illumination. In this respect candles have an economical and, in some measure, a material advantage over acetylene also. (But when the method of lighting is by flames—candle or other—the multiplication of the number of units which is involved when they are of low intensity, seriously increases the risk of fire through accidental contact of inflammable material with any one of the flames. This risk is much greater with naked flames, such as candles, than with, say, inverted incandescent gas flames, which are to all intents and purposes fully protected by a closed glass globe.) Hence, in the tabular statement which follows of the comparative cost, &c., of different illuminants, it will be assumed that 30 good candles would in practice be equally efficient in regard to the illumination of a room as large oil-lamps, acetylene flames, or incandescent gas-burners aggregating 100 candle-power.

For the same reason it will be assumed that electric glow-lamps of low intensity (nominally of 8 candle-power or less), aggregating 70-80 candle-power, will practically serve, if suitably distributed, equally as well as 100 candle-power obtained from more powerful sources of light. Electric glow-lamps of a nominal intensity of 16 candles or thereabouts, and good flat-flame gas-burners, aggregating 90-95 candle-power, will similarly be taken as equivalent, if suitably distributed, to 100 candle- power from more powerful sources of light. Of the latter it will be assumed that each source has an intensity between 20 and 30 candle-power, such as is afforded by a large oil-lamp, a No. 1 Welsbach-Kern upturned, or a "Bijou" inverted incandescent gas-burner, or a 0.70-cubic-foot-per- hour acetylene burner. Either of these sources of light, when used in sufficient numbers, so that with proper distribution they light a room adequately, will be taken in the tabular statement which follows as affording, per candle-power evolved, the standard illuminating effect required in that room. The same illuminating effect will be regarded as attainable by means of candles aggregating only 35 per cent., or small electric glow-lamps aggregating 77 per cent., or large electric glow- lamps and flat-flame gas-burners aggregating 90 to 95 per cent. of this candle-power; while if sources of light of higher intensity are used, such as Osram or Tantalum electric lamps, or the larger incandescent gas- burners (the Welsbach "C" or "York," or the Nos. 3 or 4 Welsbach-Kern upturned, or the No. 1 or larger size inverted burners) or incandescent acetylene burners, it will be assumed that their aggregate candle-power must be in excess by about 15 per cent., in order to compensate for the impossibility of obtaining equally well distributed illumination. These assumptions are based on general considerations and data as to the effect of sources of light of different intensities in giving practically the same degree of illumination in a room; it would occupy too much space here to discuss more fully the grounds on which they have been made. It must suffice to say that they have been adopted with the object of being perfectly fair to each means of illumination.

the war in its relation to slavery

THE WAR IN ITS RELATION TO SLAVERY.

To THE EDITOR OF THE NEW YORK TRIBUNE:

SIR,—Our country is opening up a new page in the history of governments. The world has never witnessed such a spontaneous uprising of any people in support of free institutions as that now exhibited by the citizens of our Northern States. I observe that the vexed question of slavery still has to be met, both in the Cabinet and in the field. It has been met by former Presidents, by former Cabinets, and by former military officers. They have established a train of precedents that may be well followed at this day. I write now for the purpose of inviting attention to those principles of international law which are regarded by publicists and jurists as proper guides in the exercise of that despotic and almost unlimited authority called the "war power." A synopsis of these doctrines was given by Major General Gaines, at New Orleans, in 1838.

General Jessup had captured many fugitive slaves and Indians in Florida, and had ordered them to be sent west of the Mississippi. At New Orleans, they were claimed by the owners, under legal process; but Gen. Gaines, commanding that military district, refused to deliver them to the sheriff, and appeared in court, stating his own defence.

He declared that these people (men, women and children) were captured in wars and held as prisoners of war: that as commander of that military department or district, he held them subject only to the order of the National Executive: that he could recognize no other power in time of war, or by the laws of war, as authorized to take prisoners from his possession.

He asserted that, in time of war, all slaves were belligerents as much as their masters. The slave men, said he, cultivate the earth and supply provisions. The women cook the food, nurse the wounded and sick, and contribute to the maintenance of the war, often more than the same number of males. The slave children equally contribute whatever they are able to the support of the war. Indeed, he well supported General Butler's declaration, that slaves are contraband of war.

The military officer, said he, can enter into no judicial examination of the claim of one man to the bone and muscle of another as property. Nor could he, as a military officer, know what the laws of Florida were while engaged in maintaining the Federal Government by force of arms. In such case, he could only be guided by the laws of war; and whatever may be the laws of any State, they must yield to the safety of the Federal Government. This defence of General Gaines may be found in House Document No. 225, of the Second Session of the 25th Congress. He sent the slaves West, where they became free.

Louis, the slave of a man named Pacheco, betrayed Major Dade's battalion, in 1836, and when he had witnessed their massacre, he joined the enemy. Two years subsequently, he was captured, Pacheco claimed him; General Jessup said if he had time, he would try him before a court-martial and hang him, but would not deliver him to any man. He however sent him West, and the fugitive slave became a free man, and is now fighting the Texans. General Jessup reported his action to the War Department, and Mr. Van Buren, then President, with his Cabinet, approved it. Pacheco then appealed to Congress, asking that body to pay him for the loss of his slave; and Mr. Greeley will recollect that he and myself, and a majority of the House of Representatives, voted against the bill, which was rejected. All concurred in the opinion that General Jessup did right in emancipating the slave, instead of returning him to his master.

In 1838, General Taylor captured a number of negroes said to be fugitive slaves. Citizens of Florida, learning what had been done, immediately gathered around his camp, intending to secure the slaves who had escaped from them. General Taylor told them that he had no prisoners but "prisoners of war." The claimants then desired to look at them, in order to determine whether he was holding their slaves as prisoners. The veteran warrior replied that no man should examine his prisoners for such a purpose; and he ordered them to depart. This action being reported to the War Department, was approved by the Executive. The slaves, however, were sent West, and set free.

In 1836, General Jessup wanted guides and men to act as spies. He therefore engaged several fugitive slaves to act as such, agreeing to secure the freedom of themselves and families if they served the Government faithfully. They agreed to do so, fulfilled their agreement, were sent West, and set free. Mr. Van Buren's Administration approved the contract, and Mr. Tyler's Administration approved the manner in which General Jessup fulfilled it by setting the slaves free.

In December, 1814, General Jackson impressed a large number of slaves at and near New Orleans, and kept them at work erecting defences, behind which his troops won such glory on the 8th of January, 1815. The masters remonstrated. Jackson disregarded their remonstrances, and kept the slaves at work until many of them were killed by the enemy's shots; yet his action was approved by Mr. Madison and Cabinet, and by Congress, which has ever refused to pay the masters for their losses.

But in all these cases, the masters were professedly friends of the Government; and yet our Presidents and Cabinets and Generals have not hesitated to emancipate their slaves whenever in time of war it was supposed to be for the interest of the country to do so. This was done in the exercise of the "war power" to which Mr. Adams referred in Congress, and for which he had the most abundant authority. But I think no records of this nation, nor of any other nation, will show an instance in which a fugitive slave has been sent back to a master who was in rebellion against the very Government who held his slave as captive.

From these precedents I deduce the following doctrines:—

1. That slaves belonging to an enemy are now and have ever been regarded as belligerents; may be lawfully captured and set free, sent out of the State, or otherwise disposed of at the will of the Executive.

2. That as slaves enable an enemy to continue and carry on the war now waged against our Government, it becomes the duty of all officers and loyal citizens to use every proper means to induce the slaves to leave their masters, and cease lending aid and comfort to the rebels.

3. That in all cases it becomes the duty of the Executive, and of all Executive officers and loyal citizens, to aid, assist and encourage those slaves who have escaped from rebel masters to continue their flight and maintain their liberty.

4. That to send back a fugitive slave to a rebel master would be lending aid and assistance to the rebellion. That those who arrest and send back such fugitives identify themselves with the enemies of our Government, and should be indicted as traitors.
J. R. GIDDINGS.

MONTREAL, June 6, 1861.

Pages

Rules for the Storage of Calcium Carbide

RULES FOR THE STORAGE OF CALCIUM CARBIDE.

(4) 3C_2H_2 = C_6H_6.

(5) 2C_2H_2 + 5O_2 = 4CO_2 + 2H_2O.

(6) 2C_6H_6 + 15O_2 = 12CO_2 + 6H_2O.

27.41 X 1000 / 302 = 90.8° C.;

0.59 X 1000 / 26 / 0.25 = 90.8° C.]

Physics and cheistry of the reaction between carbide and water - Acetylene

THE PHYSICS AND CHEMISTRY OF THE REACTION BETWEEN CARBIDE AND WATER

Cost Per Hour And Hygienic Effect Of Lighting By Different Means - Acetylene

Acetylene lighting - The Cost and Advantages of Acetylene Lighting

INTRODUCTORY—THE COST AND ADVANTAGES OF ACETYLENE LIGHTING

the war in its relation to slavery

the war in its relation to slavery