100 Comparisons of the quality of gas are also made by the ' jet photometer, an apparatus which depends on the prin- ' ciple that gas of uniform quality burned at invariable pressure, through a small orifice, yields a flame of uniform height. If the flame is to be maintained at a uniform height the pressure in the pipes must increase as the quality of the gas decreases. The jet photometer forms a ready and convenient means of ascertaining any variations in the quality of gas supply 5 but it is not available for purposes of comparison. Analysis of gas does not yield so satisfactory evidence of its illuminating value as photometric comparisons, but various methods of ascertaining the proportion of lumini- ferous olefines contained in any gas are occasionally prac- tised. The absorption of the heavy hydrocarbons by chlorine or by bromine, and Dr Fyfe’s durability test, are of theoretical rather than practical importance. Residual Prod uc(s.—Under this term are embraced coke, ammoniacal liquor, and gas-tar, all of which are sources of income in the gas manufacture. Indeed the value of these products has increased so rapidly of late years, and they now form the basis of manufactures of such consequence, that the residual products can scarcely be regarded as of secondary importance, and they will certainly play no small part in determining the future maintenance of gas-lighting in the face of other competing systems. The change in the valuation of ammonia and tar liquors is well illustrated by the circumstance that, during the year 1878, the corpora- tion of Bradford was offered £10,000 per annum for these products, which about eight years previously had been disposed of for a yearly payment of £800. Coke is asubstance which varies much in value, according to local circumstances, and the nature of the coal distilled. Vhen shale is used, there remains in the retorts an ashy residue which is absolutely worthless; and the coke of cannel coal is also comparatively of little value, owing to the amount of ash it yields. Indeed, in Scotch works where ashy cannel alone is distilled, the retorts have to be partly fired with common coal. The coke obtained from the distillation of caking coal, on the other hand, is of high value, and after a supply is set aside for heating the retorts there generally remains from 65 to 85 per cent. of the whole amount to be disposed of by sale. Ammoniacal liquor is more abundantly produced by the distillation of cannel than by common coal, from 18 to 22 lb of ammonia, as sulphate, being obtained from each ton of cannel distilled, as against about 16 lb derived from ordinary coal. Gas liquor is now almost the sole source of ammonia, which, among other purposes, is very largely employed as an agricultural fertilizer. Tar liquor yields by destructive distillation a wide range of products possessing a great and increasing industrial value. The cannel coals, and other varieties rich in volatile matter, are also the kinds which yield the largest propor- tion of tar. In the distillation of coa1—tar, after some am- GAS septic, is the basis of many valuable dyes; anthracene forms the source of the now most important dye, artificial alizarin ; and most of the substances have other applica- tions of minor importance. The relative position and value of the various products of the gas manufacture is exhibited by the following con- densed statement of the position and operations of the various London gas companies during the year 1875 :— Total capital of the companies ............................ .. £12,516,009 Capital called up ............................................. .. 11,005,589 Total gas rental ............................................... .. 2,606,818 Cost of coal .................................................... .. 1,455,407 Receipts for coke and breeze ............................... .. 492,927 ,, for tar ............................................. .. 162,151 ,, for ammonia ...................................... .. 111,951 Gas produced ................................ ..1-1,888,133 thousand feet. Gas sold ...................................... ..13,622,639 ,, ,, Coal carbonized (4 per cent. cannel) 1,505,000 tons. Coke produced, 34 bushels per ton ..... .. 1,417,654 chaldrons. Coke used as fuel in retorts, 31 per ecnt., 440,685 ,, Coke sold, 69 per cent. .................... .. 976,969 ,, Average yield of gas per ton of coal .. 9,892 cubic feet. Gas rnom Socncns OTHER THAN ('0.-L. I’e(7°olemn—G'as.—I’etroleum being a substance obtained in great abundance, notably in America, is used, not only directly as an illuminating agent, but also for the production of gas; and as an enricher of common coal-gas it is applied at several works in New York and Brooklyn. Its prepara- tion is efi'ected by distilling it first at a low temperature into a rich vapour, which, when passed into highly heated retorts, is converted into permanent gas of an illuminating power about five times greater than common gas, and which is, moreover, absolutely free from ammonia, sulphur C0111- pounds, and carbonic acid. On account of its great riel1— ness, petroleum-gas must be consumed in special burners of very fine aperture, at a rate varying from 5 to 2 feet per hour. Oil-G'as.-—In the early stages of gas manufacture many attempts were made to substitute gas distilled from inferior oils for coal-gas. The oil was distilled by allowing it to percolate into highly heated retorts, in which a quantity of coke or a like porous solid was placed, and the distillate was a richly luminiferous gas free from hurtful impurities. Although oil in this form yields a convenient and powerful illuminant, its direct combustion is much more economical: and as all oils and fats are highly valuable for many purposes besides illumination, they cannot compete with gas coal as a source of gas. Nevertheless the New York Gas Light Company manufactured oil-gas exclusively from 1824 till 1828, and sold their product at $10 per 1000 feet. The distillation of suint from wool washing, and of re- covered spent soap, are examples of the application of oleaginous substances for gas-making. Resin-G'as.—In its treatment and results resin, as a source of gas, is very similar to oil. It yields a pure gas moniacal and watery vapours have been given off, there is of great illuminating power, and for twenty years (1828-48) distilled over a proportion of highly volatile fluid hydro- I it was supplied in New York at $7 per 1000 feet. Previous carbons which consist principally of benzol ; and afterwards to tl1e.civil war of 1861—65 it was a good deal used on a large amount of a light oil, known as coal naphtha (also the European continent. a mixture of various hydrocarbons), is obtained. At this point the residue in the retort is called artificial asphalt, Wood-G'as.—-The original experiments of Lebon, it will be remembered, were made with wood-gas, but he failed to andas such is a commercial article; but if the heat is forced, I obtain from his product an illuminating power that would and the distillation continued, a large amount of “heavy” compare with that of coal-gas. Lebon’s failure was in later or “dead oils ” is obtained, and the mass left in the still is years shown to arise from distilling at a temperature which “hard pitch.” The heavy oils are a mixture of naphthalin, phenol (carbolicacid), cresol (cresylic acid), and anthracene, &c. The benzol obtained in the first stage of the distilla- tion is the basis of aniline and its various dyes; naphtha is used as a solvent, and for lighting and other purposes; carbolic acid, in addition to its employment as an anti- ' gave ofi' chiefly carbonic acid with non-luminous carbonic oxide and light carburetted hydrogen, leaving in the retort a tar which the application of a higher heat would have resolved into highly luminiferous gases and vapours. Pettenkofer, who pointed out the fact, devised a system of
wood-gas making in which the products of the low-heat