570 TRANSACTIONS OP THE AMERICAN INSTITUTE. 



centagG of oxygen in proportion to the hydrogen in them, cannot be re- 

 garded as available. We therefore turn our attention to coal as the cheap- 

 est compound from which to obtain the gases essential to illumination. 



The destructive distillation of coal is accomplished by placing it in re- 

 torts made of iron or clay and exposing them to a constant and even heat. 

 These retorts are generally recharged everj' twelve hours. The most rapid 

 process for extracting the gas is to reduce the coal to pieces about the size 

 of corn, and pass them, by means of a hopper and endless web, over a flat 

 surface of wrought iron plate, which is heated from the fires below. By 

 this plan the whole gas is expelled from the coal during its passage across 

 the plate which is accomplished in about fifteen minutes. The complica- 

 tions attending this process are such that it has not come into general use. 

 The products of distillation by means of the ordinary retort exceed thirty 

 in number, and consist of gases, liquids and solids. They are not, how- 

 ever, separated during the process. All the liquids and some of the solids 

 are dissolved in the gaseous products, yet it would greatly facilitate a clear 

 comprehension of the subject, if we would turn our attention to the several 

 distinct compounds of which the gaseous mixture is composed. 



First, and most important among the gaseous products are the hydro- 

 carbons, possessing illuminating power, viz: Olefiant gas, C4 H4 ; propy- 

 lene gas, Ce He ; butylene gas, Cs Hs. It will be perceived that in these 

 compounds the atoms or equivalents of carbon are in number the same as 

 those of hydrogen. In addition to these gases are the hydro-carbon va- 

 pors, having the formula, Cn Hn, Cn Hn-e, Cn Hn-12. The gaseous products, 

 which may be considered as diluents, are hydrogen, light carburetted 

 hydrogen C2 H4, and carbonic oxide, G 0. The gaseous products which 

 must be regarded as impurities are carbonic acid C O2, sulphuretted hy- 

 drogen, S H, oxygen and nitrogen. The vapors regarded as impurities are 

 bisulphide of carbon and aqueous vapor, H 0. The solid impurities are 

 carbonate of ammonia, N H4 C O3, sulphite of ammonia, N H4 S O3, chloride 

 of ammonium, N H4 CI, and the hydrosulphate of the sulphide of ammonium 

 N H4 S -j- H S. Other substances, such as picoliue and aniline, C12 H7 N; 

 leucoline, Cis Ha N, carbolic acid. C12 He O2, are found in the coal-tar resi- 

 duum. It should be borne in mind that the luminiferous power is stored in 

 the carbon, which, however, requires with' it a certain proportion of hydro- 

 gen to give it the gaseous form, so that, when the hydro-carbon compound 

 unites with oxygen, in the process of burning, the all-pervading ether or 

 (Eth, will be thrown into undulations, thereby producing an efiect which 

 we perceive through the visual organs, and distinguish by the name of 

 light. If the hydrogen is in excess as in carburetted hydrogen gas, C2 H4, 

 the colorific power is not exhibited; so, too, when carbonic oxide gas C 

 is burned and becomes C O2, the light generated is scarcely perceptible. 

 On the other hand, if in the hydro-carbon compounds the carbon is in ex- 

 cess, the illuminating power produced by combustion is increased. For 

 this reason small portions of the liquids benzol C12 He, toluol C14 Hs, cu- 

 mol C18 Hij, and cymol C20 Hu, add greatly to the brilliancy of the liame. 

 When, however, the relative amount of carbon is still farther increased, 

 as the compound, upon becoming cold, has a solid form, a detrimental ef- 

 fect is produced, by the tendency of such hydro-carbons to condense oa 



