MANUFACTURE OF COAL-GAS. 33 



forming salts which sublime at a comparatively low temperature : it also unites 

 with sulphuretted hydrogen, producing a highly offensive volatile substance. 



The principal part of the ammonia is found in the liquor which floats on 

 the surface of the bituminous substances in the tar-well, formed during the 

 production of gas ; it is collected, and sold to the manufacturers of ammoni- 

 acal salts, or otherwise disposed of. In the account I shall give of secondary 

 products will be found a description of the best methods pursued by these 

 manufacturers. 



The presence of CYANOGEN is frequently detected in coal-gas before puri- 

 fication ; it contains its own bulk of nitrogen, and twice its volume of the 

 vapour of carbon. It is, however, a compound of little interest to the gas 

 manufacturer. 



CHLORINE is one of the elementary substances, supporters of combustion. 

 It is possessed of striking properties, but that which alone is of interest at 

 present is its action on olefiant gas. On mixing chlorine with a gas in which 

 olefiant is contained, a diminution of volume is observed, and drops of oil are 

 seen to fall on the surface of the water over which the operation is conducted. 

 The drops consist of an oily substance, which is produced by the direct union 

 of the two gases. This fact enables us to estimate correctly the bulk of de- 

 fiant in any given portion of mixed gases, 



