CARBURETTED HYDROGEN. 75 



being left, many of the dreadful accidents which 

 occur in coal mines are less caused by the violence of 

 the explosion, than by the suffocating effects of the 

 after-damp, as the foul air left in the mines after the 

 explosion is termed. 



134. There are also many other compounds of car- 

 bon and hydrogen, in which different proportions of 

 the two elements give rise to a great variety of differ- 

 ent substances: one of the most important of these is 

 the common coal gas, obtained by distilling or roast- 

 ing coals in close iron vessels, and which is used for 

 lighting the streets; it differs from the fire-damp of 

 mines, in containing rather more carbon. India-rub- 

 ber, gutta-percha, coal-tar naphtha, oil of turpentine, 

 &c., are also compounds of carbon and hydrogen. 



135. In expressing the composition of any sub- 

 stance, chemists are in the habit of saying that it con- 

 sists of such and such proportions of its elements; 

 whatever quantity they may have taken for analysis, 

 they generally calculate the proportion which a hun- 

 dred or a thousand parts would consist of. Thus, for 

 example, 550 grains Of pure carbonate of lime contain 

 308 grains of lime, and 222 grains of carbonic acid; 

 hence 1000 grains must contain 560 grains of lime, 

 and 440 grains of carbonic acid ; and 100 grains of 

 carbonate of lime contain b6 grains of lime, and 44 

 grains of carbonic acid. 



136. This is a very simple example, but it con- 

 stantly happens that the composition of substances is 

 not so easily expressed. 1000 grains of dry gypsum, 

 or sulphate of lime, consist of 412 grains of lime, and 



