Chemical and physical properties of acetylene

THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE

It will only be necessary for the purpose of this book to indicate the more important chemical and physical properties of acetylene, and, in particular, those which have any bearing on the application of acetylene for lighting purposes. Moreover, it has been found convenient to discuss fully in other chapters certain properties of acetylene, and in regard to such properties the reader is referred to the chapters mentioned.

PHYSICAL PROPERTIES.—Acetylene is a gas at ordinary temperatures, colourless, and, when pure, having a not unpleasant, so-called "ethereal" odour. Its density, or specific gravity, referred to air as unity, has been found experimentally by Leduc to be 0.9056. It is customary to adopt the value 0.91 for calculations into which the density of the gas enters (vide Chapter VII.). The density of a gas is important not only for the determination of the size of mains needed to convey it at a given rate of flow under a given pressure, as explained in Chapter VII., but also because the volume of gas which will pass through small orifices in a given time depends on its density. According to Graham's well-known law of the effusion of gases, the velocity with which a gas effuses varies directly as the square root of the difference of pressure on the two sides of the opening, and inversely as the square root of the density of the gas. Hence it follows that the volume of gas which escapes through a porous pipe, an imperfect joint, or a burner orifice is, provided the pressure in the gas-pipe is the same, a function of the square root of the density of the gas. Hence this density has to be taken into consideration in the construction of burners, i.e., a burner required to pass a gas of high density must have a larger orifice than one for a gas of low density, if the rate of flow of gas is to be the same under the same pressure. This, however, is a question for the burner manufacturers, who already make special burners for gases of different densities, and it need not trouble the consumer of acetylene, who should always use burners devised for the consumption of that gas. But the Law of effusion indicates that the volume of acetylene which can escape from a leaky supply-pipe will be less than the volume of a gas of lower density, e.g., coal-gas, if the pressure in the pipe is the same for both. This implies that on an extensive distributing system, in which for practical reasons leakage is not wholly avoidable, the loss of gas through leakage will be less for acetylene than for coal-gas, given the same distributing pressure. If v = the loss of acetylene from a distributing system and v' = the loss of coal-gas from a similar system worked at the same pressure, both losses being expressed in volumes (cubic feet) per hour, and the coal-gas being assumed to have a density of 0.04, then

(1) (v/v') = (0.40 / 0.91)^(1/2) = 0.663

or, v = 0.663_v'_,

which signifies that the loss of acetylene by leakage under the same conditions of pressure, &c., will be only 0.663 times that of the loss of coal-gas. In practice, however, the pressures at which the gases are usually sent through mains are not identical, being greater in the case of acetylene than in that of coal-gas. Formula (1) therefore requires correction whenever the pressures are different, and calling the pressure at which the acetylene exists in the main p, and the corresponding pressure of the coal-gas p', the relative losses by leakage are—

(2) (v/v') = (0.40 / 0.91)^(1/2) x (p/p')^(1/2)

v = 0.663_v'_ x (p/p')^(1/2)

It will be evident that whenever the value of the fraction (p/p')^(1/2), is less than 1.5, i.e., whenever the pressure of the acetylene does not exceed double that of the coal-gas present in pipes of given porosity or unsoundness, the loss of acetylene will be less than that of coal-gas. This is important, especially in the case of large village acetylene installations, where after a time it would be impossible to avoid some imperfect joints, fractured pipes, &c., throughout the extensive distributing mains. The same loss of gas by leakage would represent a far higher pecuniary value with acetylene than with coal-gas, because the former must always be more costly per unit of volume than the latter. Hence it is important to recognise that the rate of leakage, cteris paribus, is less with acetylene, and it is also important to observe the economical advantage, at least in terms of gas or calcium carbide, of sending the acetylene into the mains at as low a pressure as is compatible with the length of those mains and the character of the consumers' burners. As follows from what will be said in Chapter VII., a high initial pressure makes for economy in the prime cost of, and in the expense of laying, the mains, by enabling the diameter of those mains to be diminished; but the purchase and erection of the distributing system are capital expenses, while a constant expenditure upon carbide to meet loss by leakage falls upon revenue.

The critical temperature of acetylene, i.e., the temperature below which an abrupt change from the gaseous to the liquid state takes place if the pressure is sufficiently high, is 37° C., and the critical pressure, i.e., the pressure under which that change takes place at that temperature, is nearly 68 atmospheres. Below the critical temperature, a lower pressure than this effects liquefaction of the gas, i.e., at 13.5° C. a pressure of 32.77 atmospheres, at 0° C., 21.53 atmospheres (Ansdell, cf. Chapter XI.). These data are of comparatively little practical importance, owing to the fact that, as explained in Chapter XI., liquefied acetylene cannot be safely utilised.

The mean coefficient of expansion of gaseous acetylene between 0° C. and 100° C., is, under constant pressure, 0.003738; under constant volume, 0.003724. This means that, if the pressure is constant, 0.003738 represents the increase in volume of a given mass of gaseous acetylene when its temperature is raised one degree (C.), divided by the volume of the same mass at 0° C. The coefficients of expansion of air are: under constant pressure, 0.003671; under constant volume, 0.003665; and those of the simple gases (nitrogen, hydrogen, oxygen) are very nearly the same. Strictly speaking the table given in Chapter XIV., for facilitating the correction of the volume of gas measured over water, is not quite correct for acetylene, owing to the difference in the coefficients of expansion of acetylene and the simple gases for which the table was drawn up, but practically no appreciable error can ensue from its use. It is, however, for the correction of volumes of gases measured at different temperatures to one (normal) temperature, and, broadly, for determining the change of volume which a given mass of the gas will undergo with change of temperature, that the coefficient of expansion of a gas becomes an important factor industrially.

Ansdell has found the density of liquid acetylene to range from 0.460 at -7° C. to 0.364 at +35.8° C., being 0.451 at 0° C. Taking the volume of the liquid at -7° as unity, it becomes 1.264 at 35.8°, and thence Ansdell infers that the mean coefficient of expansion per degree is 0.00489° for the total range of pressure." Assuming that the liquid was under the same pressure at the two temperatures, the coefficient of expansion per degree Centigrade would be 0.00605, which agrees more nearly with the figure 0.007 which is quoted, by Fouché As mentioned before, data referring to liquid (i.e., liquefied) acetylene are of no practical importance, because the substance is too dangerous to use. They are, however, interesting in so far as they indicate the differences in properties between acetylene converted into the liquid state by great pressure, and acetylene dissolved in acetone under less pressure; which differences make the solution fit for employment. It may be observed that as the solution of acetylene in acetone is a liquid, the acetylene must exist therein as a liquid; it is, in fact, liquid acetylene in a state of dilution, the diluent being an exothermic and comparatively stable body.

The specific heat of acetylene is given by M. A. Morel at 0.310, though he has not stated by whom the value was determined. For the purpose of a calculation in Chapter III. the specific heat at constant pressure was assumed to be 0.25, which, in the absence of precise information, appears somewhat more probable as an approximation to the truth. The ratio (k or C_p/C_v ) of the specific heat at constant pressure to that at constant volume has been found by Maneuvrier and Fournier to be 1.26; but they did not measure the specific heat itself. [Footnote: The ratio 1.26 k or (C_p/C_v) has been given in many text-books as the value of the specific heat of acetylene, whereas this value should obviously be only about one-fourth or one-fifth of 1.26.

By employing the ordinary gas laws it is possible approximately to calculate the specific heat of acetylene from Maneuvrier and Fournier's ratio. Taking the molecular weight of acetylene as 26, we have

26 C_p - 26 C_v = 2 cal.,

and

C_p = 1.26 C_v.

From this it follows that C_p, i.e., the specific heat at constant pressure of acetylene, should be 0.373.] It will be seen that this value for k differs considerably from the corresponding ratio in the case of air and many common gases, where it is usually 1.41; the figure approaches more closely that given for nitrous oxide. For the specific heat of calcium carbide Carlson quotes the following figures:

0° 1000° 1500° 2000° 2500° 3000° 3500° 0.247 0.271 0.296 0.325 0.344 0.363 0.381

The molecular volume of acetylene is 0.8132 (oxygen = 1).

According to the international atomic weights adopted in 1908, the molecular weight of acetylene is 26.016 if O = 16; in round numbers, as ordinarily used, it is 26. Employing the latest data for the weight of 1 litre of dry hydrogen and of dry normal air containing 0.04 per cent. of carbon dioxide at a temperature of 0° C. and a barometric pressure of 760 mm. in the latitude of London, viz., 0.089916 and 1.29395 grammes respectively (Castell-Evans), it now becomes possible to give the weight of a known volume of dry or moist acetylene as measured under stated conditions with some degree of accuracy. Using 26.016 as the molecular weight of the gas (O = 16), 1 litre of dry acetylene at 0° C. and 760 mm. weighs 1.16963 grammes, or 1 gramme measures 0.854973 litre. From this it follows that the theoretical specific gravity of the gas at 0°/0° C. is 0.9039 (air = 1), a figure which may be compared with Leduc's experimental value of 0.9056. Taking as the coefficient of expansion at constant pressure the figure already given, viz., 0.003738, the weights and measures of dry and moist acetylene observed under British conditions (60° F. and 30 inches of mercury) become approximately:

Dry. Saturated.
1 litre . . . 1.108 grm. . . 1.102 grm.
1 gramme . . . 0.902 litre. . . 0.907 litre.
1000 cubic feet . 69.18 lb. . . . 68.83 lb.

It should be remembered that unless the gas has been passed through a chemical drier, it is always saturated with aqueous vapour, the amount of water present being governed by the temperature and pressure. The 1 litre of moist acetylene which weighs 1.102 gramme at 60° F. and 30 inches of mercury, contains 0.013 gramme of water vapour; and therefore the weight of dry acetylene in the 1 litre of moist gas is 1.089 gramme. Similarly, the 68.83 pounds which constitute the weight of 1000 cubic feet of moist acetylene, as measured under British standard conditions, are composed of almost exactly 68 pounds of dry acetylene and 0.83 pound of water vapour. The data required in calculating the mass of vapour in a known volume of a saturated gas at any observed temperature and pressure, i.e., in reducing the figures to those which represent the dry gas at any other (standard) temperature and pressure, will be found in the text-books of physical chemistry. It is necessary to recollect that since coal-gas is measured wet, the factors given in the table quoted in Chapter XIV. from the "Notification of the Gas Referees" simply serve to convert the volume of a wet gas observed under stated conditions to the equivalent volume of the same wet gas at the standard conditions mentioned.

HEAT OF COMBUSTION, &C—Based on Berthelot and Matignon's value for the heat of combustion which is given on a subsequent page, viz., 315.7 large calories per molecular weight of 26.016 grammes, the calorific power of acetylene under different conditions is shown in the following table:

Dry. Dry. Saturated. 0° C. & 760 mm. 60° F & 30 ins. 60° F. & 30 ins.

1 gramme 12.14 cals. 12.14 cals. 12.0 cals. 1 litre 14.l9 " 13.45 " 13.22 " 1 cubic foot 40.19 " 380.8 " 374.4 "

The figures in the last column refer to the dry acetylene in the gas, no correction having been made for the heat absorbed by the water vapour present. As will appear in Chapter X., the average of actual determinations of the calorific value of ordinary acetylene is 363 large calories or 1440 B.Th.U. per cubic foot. The temperature of ignition of acetylene has been generally stated to be about 480° C. V. Meyer and Münch in 1893 found that a mixture of acetylene and oxygen ignited between 509° and 515° C. Recent (1909) investigations by H. B. Dixon and H. F. Coward show, however, that the ignition temperature in neat oxygen is between 416° and 440° (mean 428° C.) and in air between 406° and 440°, with a mean of 429° C. The corresponding mean temperature of ignition found by the same investigators for other gases are: hydrogen, 585°; carbon monoxide, moist 664°, dry 692°; ethylene, in oxygen 510°, in air 543°; and methane, in oxygen between 550° and 700°, and in air, between 650° and 750° C.

Numerous experiments have been performed to determine the temperature of the acetylene flame. According to an exhaustive research by L. Nichols, when the gas burns in air it attains a maximum temperature of 1900° C. ± 20°, which is 120° higher than the temperature he found by a similar method of observation for the coal-gas flame (fish-tail burner). Le Chatelier had previously assigned to the acetylene flame a temperature between 2100° and 2400°, while Lewes had found for the dark zone 459°, for the luminous zone 1410°, and for the tip 1517° C, Féry and Mahler have also made measurements of the temperatures afforded by acetylene and other fuels, some of their results being quoted below. Féry employed his optical method of estimating the temperature, Mahler a process devised by Mallard and Le Chatelier. Mahler's figures all relate to flames supplied with air at a temperature of 0° C. and a constant pressure of 760 mm.

Hydrogen . . . . . . . . . . . 1900 1960
Carbon monoxide . . . . . . . . . — 2100
Methane . . . . . . . . . . . — 1850
Coal-gas (luminous) . . . . . . . . 1712 |
" (atmospheric, with deficient supply of air) . 1812 | 1950
" (atmospheric, with full supply of air) . . 1871 |
Water-gas . . . . . . . . . . — 2000
Oxy-coal-gas blowpipe . . . . . . . 2200 —
Oxy-hydrogen blowpipe . . . . . . . 2420 —
Acetylene . . . . . . . . . . 2548 2350
Alcohol . . . . . . . . . . . 1705 1700
Alcohol (in Denayrouze Bunsen) . . . . . 1862 —
Alcohol and petrol in equal parts . . . . 2053 —
Crude petroleum (American) . . . . . . — 2000
Petroleum spirit " . . . . . . . — 1920
Petroleum oil " . . . . . . . — 1660

Catani has published the following determinations of the temperature yielded by acetylene when burnt with cold and hot air and also with oxygen:

Acetylene and cold air . . . . . . 2568° C. " air at 500° C . . . . 2780° C. " air at 1000° C . . . . 3000° C. " oxygen . . . . . . 4160° C.

EXPLOSIVE LIMITS.—The range of explosibility of mixtures of acetylene and air has been determined by various observers. Eitner's figures for the lower and upper explosive limits, when the mixture, at 62.6° F., is in a tube 19 mm. in diameter, and contains 1.9 per cent. of aqueous vapour, are 3.35 and 52.3 per cent. of acetylene (cf. Chapter X.). In this case the mixture was fired by electric spark. In wider vessels, the upper explosive limit, when the mixture was fired by a Bunsen flame, was found to be as high as 75 per cent. of acetylene. Eitner also found that when 13 of the 21 volumes of oxygen in air are displaced by carbon dioxide, a mixture of such "carbon dioxide air" with acetylene is inexplosive in all proportions. Also that when carbon dioxide is added to a mixture of acetylene and air, an explosion no longer occurs when the carbon dioxide amounts to 46 volumes or more to every 54 volumes of air, whatever may be the proportion of acetylene in the mixture. [Footnote: According to Caro, if acetylene is added to a mixture composed of 55 per cent. by volume of air and 45 per cent. of carbon dioxide, the whole is only explosive when the proportion of acetylene lies between 5.0 and 5.8 per cent. Caro has also quoted the effect of various inflammable vapours upon the explosive limits of acetylene, his results being referred to in Chapter X.] These figures are valuable in connexion with the prevention of the formation of explosive mixtures of air and acetylene when new mains or plant are being brought into operation (cf. Chapter VII.). Eitner has also shown, by direct investigation on mixtures of other combustible gases and air, that the range of explosibility is greatly reduced by increase in the proportion of aqueous vapour present. As the proportion of aqueous vapour in gas standing over water increases with the temperature the range of explosibility of mixtures of a combustible gas and air is naturally and automatically reduced when the temperature rises, provided the mixture is in contact with water. Thus at 17.0° C., mixtures of hydrogen, air, and aqueous vapour containing from 9.3 to 65.0 per cent, of hydrogen are explosive, whereas at 78.1° C., provided the mixture is saturated with aqueous vapour, explosion occurs only when the percentage of hydrogen in the mixture is between 11.2 and 21.9. The range of explosibility of mixtures of acetylene and air is similarly reduced by the addition of aqueous vapour (though the exact figures have not been experimentally ascertained); and hence it follows that when the temperature in an acetylene generator in which water is in excess, or in a gasholder, rises, the risk of explosion, if air is mixed with the gas, is automatically reduced with the rise in temperature by reason of the higher proportion of aqueous vapour which the gas will retain at the higher temperature. This fact is alluded to in Chapter II. Acetone vapour also acts similarly in lowering the upper explosive limit of acetylene (cf. Chapter XI.).

It may perhaps be well to indicate briefly the practical significance of the range of explosibility of a mixture of air and a combustible gas, such as acetylene. The lower explosive limit is the lowest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in less than that percentage explosion is impossible. The upper explosive limit is the highest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in more than that percentage explosion is impossible. Mixtures, however, in which the percentage of combustible gas lies between these two limits will explode when a light or spark is applied to them; and the comprehensive term "range of explosibility" is used to cover all lying between the two explosive limits. If, then, a naked light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is below the lower limit of explosibility, the gas will not take fire, but the light will continue to burn, deriving its necessary oxygen from the excess of air present. On the other hand, if a light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is above the upper limit of explosibility, the light will be extinguished, and within the vessel the gaseous mixture will not burn; but it may burn at the open mouth of the vessel as it comes in contact with the surrounding air, until by diffusion, &c., sufficient air has entered the vessel to form, with the remaining gas, a mixture lying within the explosive limits, when an explosion will occur. Again, if a gaseous mixture containing less of its combustible constituent than is necessary to attain the lower explosive limit escapes from an open-ended pipe and a light is applied to it, the mixture will not burn as a useful compact flame (if, indeed, it fires at all); if the mixture contains more of its combustible constituent than is required to attain the upper explosive limit, that mixture will burn quietly at the mouth of the pipe and will be free from any tendency to fire back into the pipe—assuming, of course, that the gaseous mixture within the pipe is constantly travelling towards the open end. If, however, a gaseous mixture containing a proportion of its combustible constituent which lies between the lower and the upper explosive limit of that constituent escapes from an open- ended pipe and a light is applied, the mixture will fire and the flame will pass back into the pipe, there to produce an explosion, unless the orifice of the said pipe is so small as to prevent the explosive wave passing (as is the case with a proper acetylene burner), or unless the pipe itself is so narrow as appreciably to alter the range of explosibility by lowering the upper explosive limit from its normal value.

By far the most potent factor in altering the range of explosibility of any gas when mixed with air is the diameter of the vessel containing or delivering such mixture. Le Chatelier has investigated this point in the case of acetylene, and his values are reproduced overleaf; they are comparable among themselves, although it will be observed that his absolute results differ somewhat from those obtained by Eitner which are quoted later:

Explosive Limits of Acetylene mixed with Air.—(Le Chatelier.)

___________________________________________________________ | | | | | | Explosive Limits. | | | Diameter of Tube |_______________________| Range of | | in Millimetres. | | | Explosibility. | | | Lower. | Upper. | | |__________________|___________|___________|________________| | | | | | | | Per Cent. | Per Cent. | Per Cent. | | 40 | 2.9 | 64 | 61.1 | | 30 | 3.1 | 62 | 58.9 | | 20 | 3.5 | 55 | 51.5 | | 6 | 4.0 | 40 | 36.0 | | 4 | 4.5 | 25 | 20.5 | | 2 | 5.0 | 15 | 10.0 | | 0.8 | 7.7 | 10 | 2.3 | | 0.5 | … | … | … | |__________________|___________|___________|________________|

Thus it appears that past an orifice or constriction 0.5 mm. in diameter no explosion of acetylene can proceed, whatever may be the proportions between the gas and the air in the mixture present.

With every gas the explosive limits and the range of explosibility are also influenced by various circumstances, such as the manner of ignition, the pressure, and other minor conditions; but the following figures for mixtures of air and different combustible gases were obtained by Eitner under similar conditions, and are therefore strictly comparable one with another. The conditions were that the mixture was contained in a tube 19 mm. (3/4-inch) wide, was at about 60° to 65° F., was saturated with aqueous vapour, and was fired by electric spark.

Table giving the Percentage by volume of Combustible Gas in a Mixture of that Gas and Air corresponding with the Explosive Limits of such a Mixture.—(Eitner.)

____________________________________________________________________ | | | | | | Description of | Lower | Upper | Difference between the | | Combustible Gas. | Explosive | Explosive | Lower and Upper Limits, | | | Limit. | Limit. | showing the range | | | | | covered by the | | | | | Explosive Mixtures. | |__________________|___________|___________|_________________________| | | | | | | | Per Cent. | Per Cent. | Per Cent. | | Carbon monoxide | 16.50 | 74.95 | 58.45 | | Hydrogen | 9.45 | 66.40 | 57.95 | | Water-gas | | | | | (uncarburetted) | 12.40 | 66.75 | 54.35 | | ACETYLENE | 3.35 | 52.30 | 48.95 | | Coal-gas | 7.90 | 19.10 | 11.20 | | Ethylene | 4.10 | 14.60 | 10.50 | | Methane | 6.10 | 12.80 | 6.70 | | Benzene (vapour) | 2.65 | 6.50 | 3.85 | | Pentane " | 2.40 | 4.90 | 2.50 | | Benzoline " | 2.40 | 4.90 | 2.50 | |__________________|___________|___________|_________________________|

These figures are of great practical significance. They indicate that a mixture of acetylene and air becomes explosive (i.e., will explode if a light is applied to it) when only 3.35 per cent. of the mixture is acetylene, while a similar mixture of coal-gas and air is not explosive until the coal-gas reaches 7.9 per cent. of the mixture. And again, air may be added to coal-gas, and it does not become explosive until the coal-gas is reduced to 19.1 per cent. of the mixture, while, on the contrary, if air is added to acetylene, the mixture becomes explosive as soon as the acetylene has fallen to 52.3 per cent. Hence the immense importance of taking precautions to avoid, on the one hand, the escape of acetylene into the air of a room, and, on the other hand, the admixture of air with the acetylene in any vessel containing it or any pipe through which it passes. These precautions are far more essential with acetylene than with coal-gas. The table shows further how great is the danger of explosion if benzene, benzoline, or other similar highly volatile hydrocarbons [Footnote: The nomenclature of the different volatile spirits is apt to be very confusing. "Benzene" is the proper name for the most volatile hydrocarbon derived from coal-tar, whose formula is C_6H_6. Commercially, benzene is often known as "benzol" or "benzole"; but it would be generally advantageous if those latter words were only used to mean imperfectly rectified benzene, i.e., mixtures of benzene with toluene, &c., such as are more explicitly understood by the terms "90.s benzol" and "50.s benzol." "Gasoline," "carburine," "petroleum ether," "benzine," "benzoline," "petrol," and "petroleum spirit" all refer to more or less volatile (the most volatile being mentioned first) and more or less thoroughly rectified products obtained from petroleum. They are mixtures of different hydrocarbons, the greater part of them having the general chemical formula C_nH_2n+2 where n = 5 or more. None of them is a definite chemical compound as is benzene; when n = 5 only the product is pentane. These hydrocarbons are known to chemists as "paraffins," "naphthenes" being occasionally met with; while a certain proportion of unsaturated hydrocarbons is also present in most petroleum spirits. The hydrocarbons of coal-tar are "aromatic hydrocarbons," their generic formula being C_nH_2^n-6, where n is never less than 6.] are allowed to vaporise in a room in which a light may be introduced. Less of the vapour of these hydrocarbons than of acetylene in the air of a room brings the mixture to the lower explosive limit, and therewith subjects it to the risk of explosion. This tact militates strongly against the use of such hydrocarbons within a house, or against the use of air-gas, which, as explained in Chapter I., is air more or less saturated with the vapour of volatile hydrocarbons. Conversely, a combustible gas, such as acetylene, may be safely "carburetted" by these hydrocarbons in a properly constructed apparatus set up outside the dwelling-house, as explained in Chapter X., because there would be no air (as in air-gas) in the pipes, &c., and a relatively large escape of carburetted acetylene would be required to produce an explosive atmosphere in a room. Moreover, the odour of the acetylene itself would render the detection of a leak far easier with carburetted acetylene than with air-gas.

N. Teclu has investigated the explosive limits of mixtures of air with certain combustible gases somewhat in the same manner as Eitner, viz.: by firing the mixture in an eudiometer tube by means of an electric spark. He worked, however, with the mixture dry instead of saturated with aqueous vapour, which doubtless helps to account for the difference between his and Eitner's results.

Table giving the Percentages by volume of Combustible Gas in a Dehydrated Mixture of that Gas and Air between which the Explosive Limits of such a Mixture lie.—(Teclu).

____________________________________________________________________ | | | | | | Lower Explosive Limit. | Upper Explosive Limit. | | Description of |________________________|________________________| | Combustible Gas. | | | | | Per Cent. of Gas. | Per Cent. of Gas. | |__________________|________________________|________________________| | | | | | ACETYLENE | 1.53-1.77 | 57.95-58.65 | | Hydrogen | 9.73-9.96 | 62.75-63.58 | | Coal-gas | 4.36-4.82 | 23.35-23.63 | | Methane | 3.20-3.67 | 7.46- 7.88 | |__________________|________________________|________________________|

Experiments have been made at Lechbruch in Bavaria to ascertain directly the smallest proportion of acetylene which renders the air of a room explosive. Ignition was effected by the flame resulting when a pad of cotton-wool impregnated with benzoline or potassium chlorate was fired by an electrically heated wire. The room in which most of the tests were made was 8 ft. 10 in. long, 6 ft. 7 in. wide, and 6 ft. 8 in. high, and had two windows. When acetylene was generated in this room in normal conditions of natural ventilation through the walls, the volume generated could amount to 3 per cent. of the air-space of the room without explosion ensuing on ignition of the wool, provided time elapsed for equable diffusion, which, moreover, was rapidly attained. Further, it was found that when the whole of the acetylene which 2 kilogrammes or 4.4 lb. of carbide (the maximum permissible charge in many countries for a portable lamp for indoor use) will yield was liberated in a room, a destructive explosion could not ensue on ignition provided the air-space exceeded 40 cubic metres or 1410 cubic feet, or, if the evolved gas were uniformly diffused, 24 cubic metres or 850 cubic feet. When the walls of the room were rendered impervious to air and gas, and acetylene was liberated, and allowed time for diffusion, in the air of the room, an explosion was observed with a proportion of only 2-1/2 per cent. of acetylene in the air.

Solubility of Acetylene in Various Liquids.

_____________________________________________________________________ | | | | | | | | Volumes of | | | | Tem- | Acetylene | | | Solvent. |perature.|dissolved by| Authority. | | | | 100 Vols. | | | | | of Solvent.| | |___________________________|_________|____________|__________________| | | | | | | | Degs. C | | | | Acetone . . . . | 15 | 2500 | Claude and Hess | | " . . . . | 50 | 1250 | " | | Acetic acid; alcohol . | 18 | 600 | Berthelot | | Benzoline; chloroform . | 18 | 400 | " | | Paraffin oil . . . | 0 | 103.3 | E. Muller | | " . . . | 18 | 150 | Berthelot | | Olive oil . . . . | — | 48 | Fuchs and Schiff | | Carbon bisulphide . . | 18 | 100 | Berthelot | | " tetrachloride . | 0 | 25 | Nieuwland | | Water (at 4 65 atmospheres| | | | | pressure) . . | 0 | 160 | Villard | | " (at 755 mm. pressure)| 12 | 118 | Berthelot | | " (760 mm. pressure) . | 12 | 106.6 | E. Müller | | " " . | 15 | 110 | Lewes | | " " . | 18 | 100 | Berthelot | | " " . | — | 100 | E. Davy (in 1836)| | " " . | 19.5 | 97.5 | E. Müller | | Milk of lime: about 10 | | | | | grammes of calcium hy- | 5 | 112 | Hammerschmidt | | droxide per 100 c.c. . | | | and Sandmann | | " " " | 10 | 95 | " | | " " " | 20 | 75 | " | | " " " | 50 | 38 | " | | " " " | 70 | 20 | " | | " " " | 90 | 6 | " | | Solution of common salt,5%| 19 | 67.9 | " | | (sodium chloride) " | 25 | 47.7 | " | | " 20%| 19 | 29.6 | " | | " " | 25 | 12.6 | " | | "(nearly saturated, | | | | | 26%) . . | 15 | 20.6 | " | | "(saturated, sp. gr.| | | | | 1-21) . . | 0 | 22.0 | E. Müller | | " " " | 12 | 21.0 | " | | " " " | 18 | 20.4 | " | | Solution of calcium | | | Hammerschmidt | | chloride (saturated) . | 15 | 6.0 | and Sandmann | | Bergé and Reychler's re- | | | | | agent . . . . | — | 95 | Nieuwland | |___________________________|_________|____________|__________________|

SOLUBILITY.—Acetylene is readily soluble in many liquids. It is desirable, on the one hand, as indicated in Chapter III., that the liquid in the seals of gasholders, &c., should be one in which acetylene is soluble to the smallest degree practically attainable; while, on the other hand, liquids in which acetylene is soluble in a very high degree are valuable agents for its storage in the liquid state. Hence it is important to know the extent of the solubility of acetylene in a number of liquids. The tabular statement (p. 179) gives the most trustworthy information in regard to the solubilities under the normal atmospheric pressure of 760 mm. or thereabouts.

The strength of milk of lime quoted in the above table was obtained by carefully allowing 50 grammes of carbide to interact with 550 c.c. of water at 5° C. A higher degree of concentration of the milk of lime was found by Hammerschmidt and Sandmann to cause a slight decrease in the amount of acetylene held in solution by it. Hammerschmidt and Sandmann's figures, however, do not agree well with others obtained by Caro, who has also determined the solubility of acetylene in lime-water, using first, a clear saturated lime-water prepared at 20° C. and secondly, a milk of lime obtained by slaking 10 grammes of quicklime in 100 c.c. of water. As before, the figures relate to the volumes of acetylene dissolved at atmospheric pressure by 100 volumes of the stated liquid.

_________________________________________________ | | | | | Temperature. | Lime-water. | Milk of Lime. | |_______________|_______________|_________________| | | | | | Degs C. | | | | 0 | 146.2 | 152.6 | | 5 | 138.5 | — | | 15 | 122.8 | 134.8 | | 50 | 43.9 | 62.6 | | 90 | 6.2 | 9.2 | |_______________|_______________|_________________|

Figures showing the solubility of acetylene in plain water at different temperatures have been published in Landolt-Börnstein's Physico- Chemical Tables. These are reproduced below. The "Coefficient of Absorption" is the volume of the gas, measured at 0° C. and a barometric height of 760 mm. taken up by one volume of water, at the stated temperature, when the gas pressure on the surface, apart from the vapour pressure of the water itself, is 760 mm. The "Solubility" is the weight of acetylene in grammes taken up by 100 grammes of water at the stated temperature, when the total pressure on the surface, including that of the vapour pressure of the water, is 760 mm.

_____________________________________________ | | | | | Temperature. | Coefficient of | Solubility. | | | Absorption. | | |______________|________________|_____________| | | | | | Degs. C. | | | | 0 | 1.73 | 0.20 | | 1 | 1.68 | 0.19 | | 2 | 1.63 | 0.19 | | 3 | 1.58 | 0.18 | | 4 | 1.53 | 0.18 | | 5 | 1.49 | 0.17 | | 6 | 1.45 | 0.17 | | 7 | 1.41 | 0.16 | | 8 | 1.37 | 0.16 | | 9 | 1.34 | 0.15 | | 10 | 1.31 | 0.15 | | 11 | 1.27 | 0.15 | | 12 | 1.24 | 0.14 | | 13 | 1.21 | 0.14 | | 14 | 1.18 | 0.14 | | 15 | 1.15 | 0.13 | | 16 | 1.13 | 0.13 | | 17 | 1.10 | 0.13 | | 18 | 1.08 | 0.12 | | 19 | 1.05 | 0.12 | | 20 | 1.03 | 0.12 | | 21 | 1.01 | 0.12 | | 22 | 0.99 | 0.11 | | 23 | 0.97 | 0.11 | | 24 | 0.95 | 0.11 | | 25 | 0.93 | 0.11 | | 26 | 0.91 | 0.10 | | 27 | 0.89 | 0.10 | | 28 | 0.87 | 0.10 | | 29 | 0.85 | 0.10 | | 30 | 0.84 | 0.09 | |______________|________________|_____________|

Advantage is taken, as explained in Chapter XI., of the high degree of solubility of acetylene in acetone, to employ a solution of the gas in that liquid when acetylene is wanted in a portable condition. The solubility increases very rapidly with the pressure, so that under a pressure of twelve atmospheres acetone dissolves about 300 times its original volume of the gas, while the solubility also increases greatly with a reduction in the temperature, until at -80° C. acetone takes up 2000 times its volume of acetylene under the ordinary atmospheric pressure. Further details of the valuable qualities of acetone as a solvent of acetylene are given in Chapter XI., but it may here be remarked that the successful utilisation of the solvent power of acetone depends to a very large extent on the absolute freedom from moisture of both the acetylene and the acetone, so that acetone of 99 per cent. strength is now used as the solvent.

Turning to the other end of the scale of solubility, the most valuable liquids for serving as seals of gasholders, &c., are readily discernible. Far superior to all others is a saturated solution of calcium chloride, and this should be selected as the confining liquid whenever it is important to avoid dissolution of acetylene in the liquid as far as may be. Brine comes next in order of merit for this purpose, but it is objectionable on account of its corrosive action on metals. Olive oil should, according to Fuchs and Schiff, be of service where a saline liquid is undesirable; mineral oil seems useless. Were they concordant, the figures for milk of lime would be particularly useful, because this material is naturally the confining liquid in the generating chambers of carbide-to-water apparatus, and because the temperature of the liquid rises through the heat evolved during the generation of the gas (vide Chapters II. and III.). It will be seen that these figures would afford a means of calculating the maximum possible loss of gas by dissolution when a known volume of sludge is run off from a carbide-to- water generator at about any possible temperature.

According to Garelli and Falciola, the depression in the freezing-point of water caused by the saturation of that liquid with acetylene is 0.08° C., the corresponding figure for benzene in place of water being 1.40° C. These figures indicate that 100 parts by weight of water should dissolve 0.1118 part by weight of acetylene at 0° C., and that 100 parts of benzene should dissolve about 0.687 part of acetylene at 5° C. In other words, 100 volumes of water at the freezing-point should dissolve 95 volumes of acetylene, and 100 volumes of benzene dissolve some 653 volumes of the gas. The figure calculated for water in this way is lower than that which might be expected from the direct determinations at other temperatures already referred to; that for benzene may be compared with Berthelot's value of 400 volumes at 18° C. Other measurements of the solubility of acetylene in water at 0° C. have given the figure 0.1162 per cent. by weight.

TOXICITY.—Many experiments have been made to determine to what extent acetylene exercises a toxic action on animals breathing air containing a large proportion of it; but they have given somewhat inconclusive results, owing probably to varying proportions of impurities in the samples of acetylene used. The sulphuretted hydrogen and phosphine which are found in acetylene as ordinarily prepared are such powerful toxic agents that they would always, in cases of "acetylene" poisoning, be largely instrumental in bringing about the effects observed. Acetylene per se would appear to have but a small toxic action; for the principal toxic ingredient in coal-gas is carbon monoxide, which does not occur in sensible quantity in acetylene as obtained from calcium carbide. The colour of blood is changed by inhalation of acetylene to a bright cherry-red, just as in cases of poisoning by carbon monoxide; but this is due to a more dissolution of the gas in the haemoglobin of the blood, so that there is much more hope of recovery for a subject of acetylene poisoning than for one of coal-gas poisoning. Practically the risk of poisoning by acetylene, after it has been purified by one of the ordinary means, is nil. The toxic action of the impurities of crude acetylene is discussed in Chapter V.

Acetylene is an "endothermic" compound, as has been mentioned in Chapter II., where the meaning of the expression endothermic is explained. It has there been indicated that by reason of its endothermic nature it is unsafe to have acetylene at either a temperature of 780° C. and upwards, or at a pressure of two atmospheres absolute, or higher. If that temperature or that pressure is exceeded, dissociation (i.e., decomposition into its elements), if initiated at any spot, will extend through the whole mass of acetylene. In this sense, acetylene at or above 780° C., or at two or more atmospheres pressure, is explosive in the absence of air or oxygen, and it is thereby distinguished from the majority of other combustible gases, such as the components of coal-gas. But if, by dilution with another gas, the partial pressure of the acetylene is reduced, then the mixture may be subjected to a higher pressure than that of two atmospheres without acquiring explosiveness, as is fully shown in Chapter XI. Thus it becomes possible safely to compress mixtures of acetylene and oil-gas or coal-gas, whereas unadmixed acetylene cannot be safely kept under a pressure of two atmospheres absolute or more. In a series of experiments carried out by Dupré on behalf of the British Home Office, and described in the Report on Explosives for 1897, samples of moist acetylene, free from air, but apparently not purified by any chemical process, were exposed to the influence of a bright red-hot wire. When the gas was held in the containing vessel at the atmospheric pressure then obtaining, viz., 30.34 inches (771 mm.) of mercury, no explosion occurred. When the pressure was raised to 45.34 inches (1150 mm.), no explosion occurred; but when the pressure was further raised to 59.34 inches (1505 mm., or very nearly two atmospheres absolute) the acetylene exploded, or dissociated into its elements.

Acetylene readily polymerises when heated, as has been stated in Chapter II., where the meaning of the term "polymerisation" has been explained. The effects of the products of the polymerisation of acetylene on the flame produced when the gas is burnt at the ordinary acetylene burners have been stated in Chapter VIII., where the reasons therefor have been indicated. The chief primary product of the polymerisation of acetylene by heat appears to be benzene. But there are also produced, in some cases by secondary changes, ethylene, methane, naphthalene, styrolene, anthracene, and homologues of several of these hydrocarbons, while carbon and hydrogen are separated. The production of these bodies by the action of heat on acetylene is attended by a reduction of the illuminative value of the gas, while owing to the change in the proportion of air required for combustion (see Chapter VIII.), the burners devised for the consumption of acetylene fail to consume properly the mixture of gases formed by polymerisation from the acetylene. It is difficult to compare the illuminative value of the several bodies, as they cannot all be consumed economically without admixture, but the following table indicates approximately the maximum illuminative value obtainable from them either by combustion alone or in admixture with some non- illuminating or feebly-illuminating gas:

________________________________________________ | | | | | | | Candles per | | | | Cubic Foot | |______________|___________________|_____________| | | | | | | | (say) | | Acetylene | C_2H_2 | 50 | | Hydrogen | H_2 | 0 | | Methane | CH_4 | 1 | | Ethane | C_2H_6 | 7 | | Propane | C_3H_8 | 11 | | Pentane | C_5H_12 (vapour) | 35 | | Hexane | C_6H_14 " | 45 | | Ethylene | C_2H_4 | 20 | | Propylene | C_3H_6 | 25 | | Benzene | C_6H_6 (vapour) | 200 | | Toluene | C_7H_8 " | 250 | | Naphthalene | C_10H_8 " | 400 | |______________|___________________|_____________|

It appears from this table that, with the exception of the three hydrocarbons last named, no substance likely to be formed by the action of heat on acetylene has nearly so high an illuminative value—volume for volume—as acetylene itself. The richly illuminating vapours of benzene and naphthalene (and homologues) cannot practically add to the illuminative value of acetylene, because of the difficulty of consuming them without smoke, unless they are diluted with a large proportion of feebly- or non-illuminating gas, such as methane or hydrogen. The practical effect of carburetting acetylene with hydrocarbon vapours will be shown in Chapter X. to be disastrous so far as the illuminating efficiency of the gas is concerned. Hence it appears that no conceivable products of the polymerisation of acetylene by heat can result in its illuminative value being improved—even presupposing that the burners could consume the polymers properly—while practically a considerable deterioration of its value must ensue.

The heat of combustion of acetylene was found by J. Thomson to be 310.57 large calories per gramme-molecule, and by Berthelot to be 321.00 calories. The latest determination, however, made by Berthelot and Matignon shows it to be 315.7 calories at constant pressure. Taking the heat of formation of carbon dioxide from diamond carbon at constant pressure as 94.3 calories (Berthelot and Matignon), which is equal to 97.3 calories from amorphous carbon, and the heat of formation of liquid water as 69 calories; this value for the heat of combustion of acetylene makes its heat of formation to be 94.3 x 2 + 69 - 315.7 = -58.1 large calories per gramme-molecule (26 grammes) from diamond carbon, or -52.1 from amorphous carbon. It will be noticed that the heat of combustion of acetylene is greater than the combined heats of combustion of its constituents; which proves that heat has been absorbed in the union of the hydrogen and carbon in the molecule, or that acetylene is endothermic, as elsewhere explained. These calculations, and others given in Chapter IX., will perhaps be rendered more intelligible by the following table of thermochemical phenomena:

_______________________________________________________________ | | | | | | Reaction. | Diamond | Amorphous | | | | Carbon. | Carbon. | | |________________________________|_________|___________|________| | | | | | | (1) C (solid) + O . . . | 26.1 | 29.1 | … | | (2) C (solid) + O_2 . . . | 94.3 | 97.3 | … | | (3) CO + O (2 - 1) . . . | … | … | 68.2 | | (4) Conversion of solid carbon | | | | | into gas (3 - 1) . . . | 42.1 | 39.1 | … | | (5) C (gas) + O (1 + 4) . . | … | … | 68.2 | | (6) Conversion of amorphous | | | | | carbon to diamond . . | … | … | 3.0 | | (7) C_2 + H_2 . . . . | -58.1 | -52.1 | … | | (8) C_2H_2 + 2-1/2O_2 . . | … | … | 315.7 | |________________________________|_________|___________|________|

W. G. Mixter has determined the heat of combustion of acetylene to be 312.9 calories at constant volume, and 313.8 at constant pressure. Using Berthelot and Matignon's data given above for amorphous carbon, this represents the heat of formation to be -50.2 (Mixter himself calculates it as -51.4) calories. By causing compressed acetylene to dissociate under the influence of an electric spark, Mixter measured its heat of formation as -53.3 calories. His corresponding heats of combustion of ethylene are 344.6 calories (constant volume) and 345.8 (constant pressure); for its heat of formation he deduces a value -7.8, and experimentally found one of about -10.6 (constant pressure).

THE ACETYLENE FLAME.—It has been stated in Chapter I. that acetylene burnt in self-luminous burners gives a whiter light than that afforded by any other artificial illuminant, because the proportion of the various spectrum colours in the light most nearly resembles the corresponding proportion found in the direct rays of the sun. Calling the amount of monochromatic light belonging to each of the five main spectrum colours present in the sun's rays unity in succession, and comparing the amount with that present in the light obtained from electricity, coal-gas, and acetylene, Münsterberg has given the following table for the composition of the several lights mentioned:

______________________________________________________________________ | | | | | | | | Electricity | Coal-Gas | Acetylene | | | |________________|__________________|_______________|_______| | Colour | | | | | | | | | in | | | | | | With | | | Spectrum.| Arc. | Incan- | Lumin- | Incan- | Alone.| 3 per | Sun- | | | | descent.| ous. | descent.| | Cent. | light.| | | | | | | | Air. | | |__________|______|_________|________|_________|_______|_______|_______| | | | | | | | | | | Red | 2.09 | 1.48 | 4.07 | 0.37 | 1.83 | 1.03 | 1 | | Yellow | 1.00 | 1.00 | 1.00 | 0.90 | 1.02 | 1.02 | 1 | | Green | 0.99 | 0.62 | 0.47 | 4.30 | 0.76 | 0.71 | 1 | | Blue | 0.87 | 0.91 | 1.27 | 0.74 | 1.94 | 1.46 | 1 | | Violet | 1.08 | 0.17 | 0.15 | 0.83 | 1.07 | 1.07 | 1 | | Ultra- | | | | | | | | | Violet | 1.21 | … | … | … | … | … | 1 | |__________|______|_________|________|_________|_______|_______|_______|

These figures lack something in explicitness; but they indicate the greater uniformity of the acetylene light in its proportion of rays of different wave-lengths. It does not possess the high proportion of green of the Welsbach flame, or the high proportion of red of the luminous gas- flame. It is interesting to note the large amount of blue and violet light in the acetylene flame, for these are the colours which are chiefly concerned in photography; and it is to their prominence that acetylene has been found to be so very actinic. It is also interesting to note that an addition of air to acetylene tends to make the light even more like that of the sun by reducing the proportion of red and blue rays to nearer the normal figure.

H. Erdmann has made somewhat similar calculation, comparing the light of acetylene with that of the Hefner (amyl acetate) lamp, and with coal-gas consumed in an Argand and an incandescent burner. Consecutively taking the radiation of the acetylene flame as unity for each of the spectrum colours, his results are:

__________________________________________________________________ | | | | | | | | | Coal-Gas | | Colour in | Wave-Lengths, | |_______________________| | Spectrum | uu | Hefner Light | | | | | | | Argand | Incandescent | |___________|_______________|______________|________|______________| | | | | | | | Red | 650 | 1.45 | 1.34 | 1.03 | | Orange | 610 | 1.22 | 1.13 | 1.00 | | Yellow | 590 | 1.00 | 1.00 | 1.00 | | Green | 550 | 0.87 | 0.93 | 0.86 | | Blue | 490 | 0.72 | 1.27 | 0.92 | | Violet | 470 | 0.77 | 1.35 | 1.73 | |___________|_______________|______________|________|______________|

B. Heise has investigated the light of different flames, including acetylene, by a heterochromatic photometric method; but his results varied greatly according to the pressure at which the acetylene was supplied to the burner and the type of burner used. Petroleum affords light closely resembling in colour the Argand coal-gas flame; and electric glow-lamps, unless overrun and thereby quickly worn out, give very similar light, though with a somewhat greater preponderance of radiation in the red and yellow.

____________________________________________________________________ | | | | | | Percent of Total | | | Light. | Energy manifested | Observer. | | | as Light. | | |____________________________|___________________|___________________| | | | | | Candle, spermaceti . . | 2.1 | Thomsen | | " paraffin . . . | 1.53 | Rogers | | Moderator lamp . . . | 2.6 | Thomsen | | Coal-gas . . . . . | 1.97 | Thomsen | | " . . . . . | 2.40 | Langley | | " batswing . . . | 1.28 | Rogers | | " Argand . . . | 1.61 | Rogers | | " incandesce . . | 2 to 7 | Stebbins | | Electric glow-lamp . . | about 6 | Merritt | | " " . . | 5.5 | Abney and Festing | | Lime light (new) . . . | 14 | Orehore | | " (old) . . . | 8.4 | Orehore | | Electric arc . . . . | 10.4 | Tyndall; Nakano | | " . . . . | 8 to 13 | Marks | | Magnesium light . . . | 12.5 | Rogers | | Acetylene . . . . | 10.5 | Stewart and Hoxie | | " (No. 0 slit burner | 11.35 | Neuberg | | " (No. 00000 . . | | | | Bray fishtail) | 13.8 | Neuberg | | " (No. 3 duplex) . | 14.7 | Neuberg | | Geissler tube . . . | 32.0 | Staub | |____________________________|___________________|___________________|

Violle and Féry, also Erdmann, have proposed the use of acetylene as a standard of light. As a standard burner Féry employed a piece of thermometer tube, cut off smoothly at the end and having a diameter of 0.5 millimetre, a variation in the diameter up to 10 per cent. being of no consequence. When the height of the flame ranged from 10 to 25 millimetres the burner passed from 2.02 to 4.28 litres per hour, and the illuminating power of the light remained sensibly proportional to the height of the jet, with maximum variations from the calculated value of ±0.008. It is clear that for such a purpose as this the acetylene must be prepared from very pure carbide and at the lowest possible temperature in the generator. Further investigations in this direction should be welcome, because it is now fairly easy to obtain a carbide of standard quality and to purify the gas until it is essentially pure acetylene from a chemical point of view.

L. W. Hartmann has studied the flame of a mixture of acetylene with hydrogen. He finds that the flame of the mixture is richer in light of short wave-lengths than that of pure acetylene, but that the colour of the light does not appear to vary with the proportion of hydrogen present.

Numerous investigators have studied the optical or radiant efficiency of artificial lights, i.e., the proportion of the total heat plus light energy emitted by the flame which is produced in the form of visible light. Some results are shown in the table on the previous page.

Figures showing the ratio of the visible light emitted by various illuminants to the amount of energy expended in producing the light and also the energy equivalent of each spherical Hefner unit evolved have been published by H. Lux, whose results follow:

_______________________________________________________________________ | | | | | | | | Ratio of | Ratio of | Mean | Energy | | | Light | Light | Spherical | Equiva- | | Light. | emitted to | emitted to | Illuminat- | lent to 1 | | | Total | Energy | ing Power. | Spherical | | | Radiation. | Impressed. | Hefners. | Hefner in | | | | | | Watts. | |____________________|____________|____________|____________|___________| | | | | | | | | Per Cent. | Per Cent. | | | | Hefner lamp | 0.89 | 0.103 | 0.825 | 0.108 | | Paraffin lamp, 14" | 1.23 | 0.25 | 12.0 | 0.105 | | ACETYLENE, 7.2 | | | | | | litre burner | 6.36 | 0.65 | 6.04 | 0.103 | | Coal-gas incandes- | | | | | | cent, upturned | 2.26-2.92 | 0.46 | 89.6 | 0.037 | | " incandes- | | | | | | cent, inverted | 2.03-2.97 | 0.51 | 82.3 | 0.035 | | Carbon filament | | | | | | glow-lamp | 3.2-2.7 | 2.07 | 24.5 | 0.085 | | Nernst lamp | 5.7 | 4.21-3.85 | 91.9 | 0.073 | | Tantalum lamp | 8.5 | 4.87 | 26.7 | 0.080 | | Osram lamp | 9.1 | 5.36 | 27.4 | 0.075 | | Direct-current arc | 8.1 | 5.60 | 524 | 0.047 | | " " enclosed | 2.0 | 1.16 | 295 | 0.021 | | Flame arc, yellow | 15.7 | 13.20 | 1145 | 0.041 | | " " white | 7.6 | 6.66 | 760 | 0.031 | | Alternating- | | | | | | current arc | 3.7 | 1.90 | 89 | 0.038 | | Uviol mercury | | | | | | vapour lamp | 5.8 | 2.24 | 344 | 0.015 | | Quartz lamp | 17.6 | 6.00 | 2960 | 0.014 | |____________________|____________|____________|____________|___________|

CHEMICAL PROPERTIES.—It is unnecessary for the purpose of this work to give an exhaustive account of the general chemical reactions of acetylene with other bodies, but a few of the more important must be referred to. Since the gases are liable to unite spontaneously when brought into contact, the reactions between, acetylene and chlorine require attention, first, because of the accidents that have occurred when using bleaching- powder (see Chapter V.) as a purifying material for the crude gas; secondly, because it has been proposed to manufacture one of the products of the combination, viz., acetylene tetrachloride, on a large scale, and to employ it as a detergent in place of carbon tetrachloride or carbon disulphide. Acetylene forms two addition products with chlorine, C_2H_2Cl_2, and C_2H_2Cl_4. These are known as acetylene dichloride and tetrachloride respectively, or more systematically as dichlorethylene and tetrachlorethane. One or both of the chlorides is apt to be produced when acetylene comes into contact with free chlorine, and the reaction sometimes proceeds with explosive violence. The earliest writers, such as E. Davy, Wöhler, and Berthelot, stated that an addition of chlorine to acetylene was invariably followed by an explosion, unless the mixture was protected from light; whilst later investigators thought the two gases could be safely mixed if they were both pure, or if air was absent. Owing to the conflicting nature of the statements made, Nieuwland determined in 1905 to study the problem afresh; and the annexed account is chiefly based on his experiments, which, however, still fail satisfactorily to elucidate all the phenomena observed. According to Nieuwland's results, the behaviour of mixtures of acetylene and chlorine appears capricious, for sometimes the gases unite quietly, although sometimes they explode. Acetylene and chlorine react quite quietly in the dark and at low temperatures; and neither a moderate increase in temperature, nor the admission of diffused daylight, nor the introduction of small volumes of air, is necessarily followed by an explosion. Doubtless the presence of either light, air, or warmth increases the probability of an explosive reaction, while it becomes more probable still in their joint presence; but in given conditions the reaction may suddenly change from a gentle formation of addition products to a violent formation of substitution products without any warning or manifest cause. When the gases merely unite quietly, tetrachlorethane, or acetylene tetrachloride, is produced thus:

C_2H_2 + 2Cl_2 = C_2H_2Cl_4;

but when the reaction is violent some hexachlorethane is formed, presumably thus:

2C_2H_2 + 5Cl_2 = 4HCl + C_2 + C_2Cl_6.

The heat evolved by the decomposition of the acetylene by the formation of the hydrochloric acid in the last equation is then propagated amongst the rest of the gaseous mixture, accelerating the action, and causing the acetylene to react with the chlorine to form more hydrochloric acid and free carbon thus;

C_2H_2 + Cl_2 = 2HCl + C_2.

It is evident that these results do not altogether explain the mechanism of the reactions involved. Possibly the formation of substitution products and the consequent occurrence of an explosion is brought about by some foreign substance which acts as a catalytic agent. Such substance may conceivably be one of the impurities in crude acetylene, or the solid matter of a bleaching-powder purifying material. The experiments at least indicate the direction in which safety may be sought when bleaching- powder is employed to purify the crude gas, viz., dilution of the powder with an inert material, absence of air from the gas, and avoidance of bright sunlight in the place where a spent purifier is being emptied. Unfortunately Nieuwland did not investigate the action on acetylene of hypochlorites, which are presumably the active ingredients in bleaching- powder. As will appear in due course, processes have been devised and patented to eliminate all danger from the reaction between acetylene and chlorine for the purpose of making tetrachlorethane in quantity.

Acetylene combines with hydrogen in the presence of platinum black, and ethylene and then ethane result. It was hoped at one time that this reaction would lead to the manufacture of alcohol from acetylene being achieved on a commercial basis; but it was found that it did not proceed with sufficient smoothness for the process to succeed, and a number of higher or condensation products were formed at the same time. It has been shown by Erdmann that the cost of production of alcohol from acetylene through this reaction must prove prohibitive, and he has indicated another reaction which he considered more promising. This is the conversion of acetylene by means of dilute sulphuric acid (3 volumes of concentrated acid to 7 volumes of water), preferably in the presence of mercuric oxide, to acetaldehyde. The yield, however, was not satisfactory, and the process does not appear to have passed beyond the laboratory stage.

It has also been proposed to utilise the readiness with which acetylene polymerises on heating to form benzene, for the production of benzene commercially; but the relative prices of acetylene and benzene would have to be greatly changed from those now obtaining to make such a scheme successful. Acetylene also lends itself to the synthesis of phenol or carbolic acid. If the dry gas is passed slowly into fuming sulphuric acid, a sulpho-derivative results, of which the potash salt may be thrown down by means of alcohol. This salt has the formula C_2H_4O_2,S_2O_6K_2, and on heating it with caustic potash in an atmosphere of hydrogen, decomposing with excess of sulphuric acid, and distilling, phenol results and may be isolated. The product is, however, generally much contaminated with carbon, and the process, which was devised by Berthelot, does not appear to have been pursued commercially. Berthelot has also investigated the action of ordinary concentrated sulphuric acid on acetylene, and obtained various sulphonic derivatives. Schröter has made similar investigations on the action of strongly fuming sulphuric acid on acetylene. These investigations have not yet acquired any commercial significance.

If a mixture of acetylene with either of the oxides of carbon is led through a red-hot tube, or if a similar mixture is submitted to the action of electric sparks when confined within a closed vessel at some pressure, a decomposition occurs, the whole of the carbon is liberated in the free state, while the hydrogen and oxygen combine to form water. Analogous reactions take place when either oxide of carbon is led over calcium carbide heated to a temperature of 200° or 250° C., the second product in this case being calcium oxide. The equations representing these actions are:

C_2H_2 + CO = H_2O + 3C

2C_2H_2 + CO_2 = 2H_2O + 5C

CaC_2 + CO = CaO + 3C

2CaC_2 + CO_2 = 2CaO + 5C

By urging the temperature, or by increasing the pressure at which the gases are led over the carbide, the free carbon appears in the graphitic condition; at lower temperatures and pressures, it is separated in the amorphous state. These reactions are utilised in Frank's process for preparing a carbon pigment or an artificial graphite (cf. Chapter XII.).

Parallel decompositions occur between carbon bisulphide and either acetylene or calcium carbide, all the carbon of both substances being eliminated, while the by-product is either sulphuretted hydrogen or calcium (penta) sulphide. Other organic bodies containing sulphur are decomposed in the same fashion, and it has been suggested by Ditz that if carbide could be obtained at a suitable price, the process might be made useful in removing sulphur (i.e., carbon bisulphide and thiophen) from crude benzol, in purifying the natural petroleum oil which contains sulphur, and possibly in removing "sulphur compounds" from coal-gas.

COMPOUNDS WITH COPPER. By far the most important chemical reactions of acetylene in connexion with its use as an illuminant or fuel are those which it undergoes with certain metals, notably copper. It is known that if acetylene comes in contact with copper or with one of its salts, in certain conditions a compound is produced which, at least when dry, is highly explosive, and will detonate either when warmed or when struck or gently rubbed. The precise mechanism of the reaction, or reactions, between acetylene and copper (or its compounds), and also the character of the product, or products, obtained have been studied by numerous investigators; but their results have been inconclusive and sometimes rather contradictory, so that it can hardly be said that the conditions which determine or preclude the formation of an explosive compound and the composition of the explosive compound are yet known with certainty. Copper is a metal which yields two series of compounds, cuprous and cupric salts, the latter of which contain half the quantity of metal per unit of acid constituent that is found in the former. It should follow, therefore, that there are two compounds of copper with carbon, or copper carbides: cuprous carbide, Cu_2C_2, and cupric carbide, CuC_2. Acetylene reacts at ordinary temperatures with an ammoniacal solution of any cupric salt, forming a black cupric compound of uncertain constitution which explodes between 50° and 70° C. It is decomposed by dilute acids, yielding some polymerised substances. At more elevated temperatures other cupric compounds are produced which also give evidence of polymerisation. Cuprous carbide or acetylide is the reddish brown amorphous precipitate which is the ultimate product obtained when acetylene is led into an ammoniacal solution of cuprous chloride. This body is decomposed by hydrochloric acid, yielding acetylene; but of itself it is, in all probability, not explosive. Cuprous carbide, however, is very unstable and prone to oxidation; so that, given the opportunity, it combines with oxygen or hydrogen, or both, until it produces the copper acetylide, or acetylene-copper, which is explosive—a body to which Blochmann's formula C_2H_2Cu_2O is generally ascribed. Thus it should happen that the exact nature of the copper acetylene compound may vary according to the conditions in which it has been formed, from a substance that is not explosive at all at first, to one that is violently explosive; and the degree of explosiveness should depend on the greater exposure of the compound to air and moisture, or the larger amount of oxygen and moisture in the acetylene during its contact with the copper or copper salt. For instance, Mai has found that freshly made copper acetylide can be heated to 60° C. or higher without explosion; but that if the compound is exposed to air for a few hours it explodes on warming, while if warmed with oxygen it explodes on contact with acetylene. It is said by Mai and by Caro to absorb acetylene when both substances are dry, becoming so hot as to explode spontaneously. Freund and Mai have also observed that when copper acetylide which has been dried in contact with air for four or five hours at a temperature of 50° or 60° C. is allowed to explode in the presence of a current of acetylene, an explosion accompanied by light takes place; but it is always local and is not communicated to the gas, whether the latter is crude or pure. In contact with neutral or acid solutions of cuprous salts acetylene yields various double compounds differing in colour and crystallising power; but according to Chavastelon and to Caro they are all devoid of explosive properties. Sometimes a yellowish red precipitate is produced in solutions of copper salts containing free acid, but the deposit is not copper acetylide, and is more likely to be, at least in part, a copper phosphide—especially if the gas is crude. Hence acid solutions or preparations of copper salts may safely be used for the purification of acetylene, as is done in the case of frankoline, mentioned in Chapter V. It is clear that the amount of free acid in such a material is much more than sufficient to neutralise all the ammonia which may accompany the crude acetylene into the purifier until the material is exhausted in other respects; and moreover, in the best practice, the gas would have been washed quite or nearly free from ammonia before entering the purifier.

From a practical aspect the possible interaction of acetylene and metallic copper has been investigated by Gerdes and by Grittner, whose results, again, are somewhat contradictory. Gerdes exposed neat acetylene and mixtures of acetylene with oil-gas and coal-gas to a pressure of nine or ten atmospheres for ten months at ordinary summer and winter temperatures in vessels made of copper and various alloys. Those metals and alloys which resisted oxidation in air resisted the attack of the gases, but the more corrodible substances were attacked superficially; although in no instance could an explosive body be detected, nor could an explosion be produced by heating or hammering. In further experiments the acetylene contained ammonia and moisture and Gerdes found that where corrosion took place it was due exclusively to the ammonia, no explosive compounds being produced even then. Grittner investigated the question by leading acetylene for months through pipes containing copper gauze. His conclusions are that a copper acetylide is always produced if impure acetylene is allowed to pass through neutral or ammoniacal solutions of copper; that dry acetylene containing all its natural impurities except ammonia acts to an equal extent on copper and its alloys, yielding the explosive compound; that pure and dry gas does not act upon copper or its alloys, although it is possible that an explosive compound may be produced after a great length of time. Grittner has asserted that an explosive compound may be produced when acetylene is brought into contact with such alloys of copper as ordinary brass containing 64.66 per cent. of copper, or red brass containing 74.46 per cent. of copper, 20.67 per cent. of zinc, and 4.64 per cent. of tin; whereas none is obtained when the metal is either "alpaca" containing 64.44 per cent. of copper, 18.79 per cent. of nickel, and 16.33 per cent. of zinc, or britannia metal composed of 91.7 per cent. of copper and 8.3 per cent. of tin. Caro has found that when pure dry acetylene is led for nine months over sheets or filings of copper, brass containing 63.2 per cent. of copper, red brass containing 73.8 per cent., so-called "alpaca-metal" containing 65.3 per cent., and britannia metal containing 90.2 per cent. of copper, no action whatever takes place at ordinary temperatures; if the gas is moist very small quantities of copper acetylide are produced in six months, whatever metal is tested, but the yield does not increase appreciably afterwards. At high temperatures condensation occurs between acetylene and copper or its alloys, but explosive bodies are not formed.

Grittner's statement that crude acetylene, with or without ammonia, acts upon alloys of copper as well as upon copper itself, has thus been corroborated by Caro; but experience renders it tolerably certain that brass (and presumably gun-metal) is not appreciably attacked in practical conditions. Gerdes' failure to obtain an explosive compound in any circumstances may very possibly be explained by the entire absence of any oxygen from his cylinders and gases, so that any copper carbide produced remained unoxidised. Grittner's gas was derived, at least partially, from a public acetylene supply, and is quite likely to have been contaminated with air in sufficient quantity to oxidise the original copper compound, and to convert it into the explosive modification.

For the foregoing reasons the use of unalloyed copper in the construction of acetylene generators or in the subsidiary items of the plant, as well as in burner fittings, is forbidden by statute or some quasi-legal enactment in most countries, and in others the metal has been abandoned for one of its alloys, or for iron or steel, as the case may be. Grittner's experiments mentioned above, however, probably explain why even alloys of copper are forbidden in Hungary. (Cf. Chapter IV., page 127.)

When acetylene is passed over finely divided copper or iron (obtained by reduction of the oxide by hydrogen) heated to from 130° C. to 250° C., the gas is more or less completely decomposed, and various products, among which hydrogen predominates, result. Ethane and ethylene are undoubtedly formed, and certain homologues of them and of acetylene, as well as benzene and a high molecular hydrocarbon (C_7H_6)_n termed "cuprene," have been found by different investigators. Nearly the same hydrocarbons, and others constituting a mixture approximating in composition to some natural petroleums, are produced when acetylene is passed over heated nickel (or certain other metals) obtained by the reduction of the finely divided oxide. These observations are at present of no technical importance, but are interesting scientifically because they have led up to the promulgation of a new theory of the origin of petroleum, which, however, has not yet found universal acceptance.

THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE