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.