Now that atmospheric or Bunsen burners for the consumption of acetylene for use in lighting by the incandescent system and in heating have been so much improved that they seem to be within measurable reach of a state of perfection, there appears to be but little use at the present time for a modified or diluted acetylene which formerly seemed likely to be valuable for heating and certain other purposes. Nevertheless, the facts relating to this so-called carburetted acetylene are in no way traversed by its failure to establish itself as an active competitor with simple acetylene for heating purposes, and since it is conceivable that the advantages which from the theoretical standpoint the carburetted gas undoubtedly possesses in certain directions may ultimately lead to its practical utilisation for special purposes, it has been deemed expedient to continue to give in this work an account of the principles underlying the production and application of carburetted acetylene.

It has already been explained that acetylene is comparatively a less efficient heating agent than it is an illuminating material, because, per unit of volume, its calorific power is not so much greater than that of coal-gas as is its illuminating capacity. It has also been shown that the high upper explosive limit of mixtures of acetylene and air--a limit so much higher than the corresponding figure with coal-gas and other gaseous fuels--renders its employment in atmospheric burners (either for lighting or for heating) somewhat troublesome, or dependent upon considerable skill in the design of the apparatus. If, therefore, either the upper explosive limit of acetylene could be reduced, or its calorific value increased (or both), by mixing with it some other gas or vapour which should not seriously affect its price and convenience as a self-luminous illuminant, acetylene would compare more favourably with coal-gas in its ready applicability to the most various purposes. Such a method has been suggested by Heil, and has been found successful on the Continent. It consists in adding to the acetylene a certain proportion of the vapour of a volatile hydrocarbon, so as to prepare what is called "carburetted acetylene." In all respects the method of making carburetted acetylene is identical with that of making "air-gas," which was outlined in Chapter I., viz., the acetylene coming from an ordinary generating plant is led over or through a mass of petroleum spirit, or other similar product, in a vessel which exposes the proper amount of superficial area to the passing gas. In all respects save one the character of the product is similar to that of air-gas, i.e., it is a mixture of a permanent gas with a vapour; the vapour may possibly condense in part within the mains if they are exposed to a falling temperature, and if the product is to be led any considerable distance, deposition of liquid may occur (conceivably followed by blockage of the mains) unless the proportion of vapour added to the gas is kept below a point governed by local climatic and similar conditions. But in one most important respect carburetted acetylene is totally different from air-gas: partial precipitation of spirit from air-gas removes more or less of the solitary useful constituent of the material, reducing its practical value, and causing the residue to approach or overpass its lower explosive limit (cf. Chapter I.); partial removal of spirit from carburetted acetylene only means a partial reconversion of the material into ordinary acetylene, increasing its natural illuminating power, lowering its calorific intensity somewhat, and causing the residue to have almost its primary high upper explosive limit, but essentially leaving its lower explosive limit unchanged. Thus while air-gas may conceivably become inefficient for every purpose if supplied from any distance in very cold weather, and may even pass into a dangerous explosive within the mains; carburetted acetylene can never become explosive, can only lose part of its special heating value, and will actually increase in illuminating power.

It is manifest that, like air-gas, carburetted acetylene is of somewhat indefinite composition, for the proportion of vapour, and the chemical nature of that vapour, may vary. 100 litres of acetylene will take up 40 grammes of petroleum spirit to yield 110 litres of carburetted acetylene evidently containing 9 per cent. of vapour, or 100 litres of acetylene may be made to absorb as much as 250 grammes of spirit yielding 200 litres of carburetted acetylene containing 50 per cent. of vapour; while the petroleum spirit may be replaced, if prices are suitable, by benzol or denatured alcohol.

The illuminating power of acetylene carburetted with petroleum spirit has been examined by Caro, whose average figures, worked out in British units, are:

ILLUMINATING POWER OF CARBURETTED ACETYLENE. HALF-FOOT BURNERS.

Those results may be compared with those referring to air-gas, which emits in incandescent burners from 3.0 to 12.4 candles per cubic foot according to the amount of spirit added to the air and the temperature to which the gas is exposed.

The calorific values of carburetted acetylene (Caro), and those of other gaseous fuels are:

Besides its relatively low upper explosive limit, carburetted acetylene exhibits a higher temperature of ignition than ordinary acetylene, which makes it appreciably safer in presence of a naked light. It also possesses a somewhat lower flame temperature and a slower speed of propagation of the explosive wave when mixed with air. These data are:

In making carburetted acetylene, the pressure given by the ordinary acetylene generator will be sufficient to drive the gas through the carburettor, and therefore there will be no expense involved beyond the cost of the spirit vaporised. Thus comparisons may fairly be made between ordinary and carburetted acetylene on the basis of material only, the expense of generating the original acetylene being also ignored. In Great Britain the prices of calcium carbide, petroleum spirit, and 90s benzol delivered in bulk in country places may be taken at 15£ per ton, and 1s. per gallon respectively, petroleum spirit having a specific gravity of 0.700 and benzol of 0.88. On this basis, a unit volume (100 cubic metres) of plain acetylene costs 1135d., of "petrolised" acetylene containing 66 per cent. of acetylene costs 1277d., and of "benzolised" acetylene costs 1180d. In other words, 100 volumes of plain acetylene, 90 volumes of petrolised acetylene, and 96 volumes of benzolised acetylene are of equal pecuniary value. Employing the data given in previous tables, it appears that 38.5 candles can be won from plain acetylene in a self-luminous burner, and 103 candles therefrom in an incandescent burner at the same price as 25.5-29.1 and 78-87 candles can be obtained from carburetted acetylene; whence it follows that at English prices petrolised acetylene is more expensive as an illuminant in either system of combustion than the simple gas, while benzolised acetylene, burnt under the mantle only, is more nearly equal to the simple gas from a pecuniary aspect. But considering the calorific value, it appears that for a given sum of money only 363 calories can be obtained from plain acetylene, while petrolised acetylene yields 516, and benzolised acetylene 658; so that for all heating or cooking purposes (and also for driving small motors) carburetted acetylene exhibits a notable economy. Inasmuch as the partial saturation of acetylene with any combustible vapour is an operation of extreme simplicity, requiring no power or supervision beyond the occasional recharging of the carburettor, it is manifest that the original main coming from the generator supplying any large establishment where much warming, cooking (or motor driving) might conveniently be done with the gas could be divided within the plant-house, one branch supplying all, or nearly all, the lighting burners with plain acetylene, and the other branch communicating with a carburettor, so that all, or nearly all, the warming and cooking stoves (and the motor) should be supplied with the more economical carburetted acetylene. Since any water pump or similar apparatus would be in an outhouse or basement, and the most important heating stove (the cooker) be in the kitchen, such an arrangement would be neither complicated nor involve a costly duplication of pipes.

It follows from the fact that even a trifling proportion of vapour reduces the upper limit of explosibility of mixtures of acetylene with air, that the gas may be so lightly carburetted as not appreciably to suffer in illuminating power when consumed in self-luminous jets, and yet to burn satisfactorily in incandescent burners, even if it has been generated in an apparatus which introduces some air every time the operation of recharging is performed. To carry out this idea, Caro has suggested that 5 kilos. of petroleum spirit should be added to the generator water for every 50 cubic metres of gas evolved, i.e., 1 lb. per 160 cubic feet, or, say, 1 gallon per 1000 cubic feet, or per 200 lb. of carbide decomposed. Caro proposed this addition in the case of central installations supplying a district where the majority of the consumers burnt the gas in self-luminous jets, but where a few preferred the incandescent system; but it is clearly equally suitable for employment in all private plants of sufficient magnitude.

A lowering of the upper limit of explosibility is also produced by the presence of the acetone which remains in acetylene when obtained from a cylinder holding the compressed gas (cf. Chapter XI.). According to Wolff and Caro such gas usually carries with it from 30 to 60 grammes of acetone vapour per cubic metre, i.e., 1.27 grammes per cubic foot on an average; and this amount reduces the upper limit of explosibility by about 16 per cent., so that to this extent the gas behaves more smoothly in an incandescent burner of imperfect design.

LÉpinay has described some experiments on the comparative technical value of ordinary acetylene, carburetted acetylene, denatured alcohol and petroleum spirit as fuels for small explosion engines. One particular motor of 3 (French) h.p. consumed 1150 grammes of petroleum spirit per hour at full load; but when it was supplied with carburetted acetylene its consumption fell to 150 litres of acetylene and 700 grammes of spirit (specific gravity 0.680). A 1-1/4 h.p. engine running light required 48 grammes of 90 per cent. alcohol per horse-power-hour and 66 litres of acetylene; at full load it took 220 grammes of alcohol and 110 litres of acetylene. A 6 h.p. engine at full load required 62 litres of acetylene carburetted with 197 grammes of petroleum spirit per horse-power-hour (uncorrected); while a similar motor fed with low-grade Taylor fuel-gas took 1260 litres per horse-power-hour, but on an average developed the same amount of power from 73 litres when 10 per cent. of acetylene was added to the gas. LÉpinay found that with pure acetylene ignition of the charge was apt to be premature; and that while the consumption of carburetted acetylene in small motors still materially exceeded the theoretical, further economics could be attained, which, coupled with the smooth and regular running of an engine fed with the carburetted gas, made carburetted acetylene distinctly the better power-gas of the two.