270 MR. JOHN LEIGH ON THE CHEMICAL CHANGES 



leakage in the apparatus, and is not a product of the de- 

 composition of the coal at all ; it need not, therefore, enter 

 into our consideration. 



The quahty and illuminating power of the gas will be 

 affected, not only by the quality (composition) and cojidi- 

 tion (wet or dry, old or recently got) of the coal or cannel, 

 but by the degree of heat employed in its preparation, and 

 the mode in which the operation is conducted. The chief 

 products of the distillation are compounds of carbon and 

 hydrogen, and these alone yield light; but of these we find 

 that some are solids, some liquids, and some gaseous — ^the 

 two first are valueless for the purpose of illumination, be- 

 cause their physical condition (solid and liquid) precludes 

 their use. The gaseous are three — one containing very 

 little carbon (light carburetted hydrogen), and, therefore, 

 giving very poor light ; the other two very rich in carbon 

 (olefiant gas — carbon 4, hydrogen 4 ; and volatile hydro-car- 

 bon — carbon 6, hydrogen 6, or carbon 8, hydrogen 8, &c.), 

 and giving great light, though in small quantity. We find 

 mixed with these, besides the necessary impurities (sul- 

 phuretted hydrogen, ammonia, carbonic acid, &c.), two 

 gases constituting the chief bulk of the mixed coal gas, 

 which we have also seen are never given off in natural 

 operations, viz., hydrogen and carbonic oxide. They are 

 also not necessary products of the distillation, but result 

 from the mode of distillation. 



The carburets of hydrogen may be conveniently divided 

 into three classes ; in the first, the number of atoms exceeds 

 that of the hydrogen — they are very rich in carbon, as 

 benzole (carbon 12, hydrogen 6); these are either liquid or 

 solid, and would give great light could they be burnt, but 

 give great smoke; — in the second, the atoms of carbon and 

 hydrogen are equal in the compounds, as olefiant gas (car- 

 bon 4, hydrogen 4), volatile hydro-carbon (carbon 4, 

 hydrogen 6, and carbon 8, hydrogen 8) ; these are gaseous, 



