MANUFACTURE OF COAL-GAS. 27 







The effects of caloric should be well understood by the chemist : in his 

 hands it is an important power, by which he not only brings bodies into a 

 proper state for combining, as when he converts a solid into a liquid by heat, 

 but also is enabled to destroy previously existing combinations ; for caloric not 

 only counteracts the effects of cohesion, but also those of affinity, when the 

 heating process is carried far enough. Thus we form quicklime by heating 

 the natural compound carbonate of lime, or limestone, to redness; the affinity 

 which unites the carbonic acid to the lime becomes neutralized or destroyed 

 by the caloric, and in consequence the gaseous acid escapes, leaving the pure 

 lime in the kiln. 



Frequently, however, the action is not so simple ; the compound perhaps 

 consists of numerous elements, and these, when freed by the heat from their 

 former union, combine together in new forms. This is the case in the destruc- 

 tive distillation of coal, for the purpose of forming gas. Coal consists of the 

 elementary substances, carbon, hydrogen, oxygen, and nitrogen : by the 

 action of the heat these are separated, but at the same instant they recombine, 

 forming new compounds, which escape. Thus a part of the hydrogen unites 

 with the carbon, and forms gas : another portion unites with oxygen to form 

 water, while a third takes up nitrogen to form ammonia. All these products 

 being volatile at the temperature at which we operate, escape, leaving a large 

 portion of uncombined carbon, in the form of coke, in the retort. But these 

 complicated changes will be best understood when we have entered into a 

 short description of the elements concerned in the production and purification 

 of gas. 



OXYGEN was discovered by Priestley in 1774, and termed by him " dephlogis- 

 ticated air," a name which was changed for " vital air," and subsequently for 

 the name it at present bears. 



This gas has resisted all attempts to convert it into a liquid ; it is colour- 

 less and inodorous, and heavier than atmospheric air, 100 cubic inches 

 weighing 34*193 grains, while 100 cubic inches of the latter only weigh 

 3r0117. As atmospheric air is considered as unity when comparing the 

 density of gases, the specific gravity of oxygen is T1026. It eminently sup- 

 ports combustion, all combustible bodies when introduced into it burning much 

 more vividly than in common air ; indeed it is to the presence of this gas 



F,2 



