302 CHEMICAL DISCOVERY AND INVENTION 



valuable compounds, their chief constituents resembling the 

 oils obtained by moderate distillation of shales and consist 

 chiefly of paraffins. 



The Mond gas system has been adopted on a large scale in 

 Staffordshire, a company having been formed a few years ago 

 which generates the gas and supplies it through a system of 

 mains to the surrounding district. Here it finds extensive em- 

 ployment in the gas engines which now assume such enormous 

 dimensions, and in metallurgical furnaces as well as for raising 

 steam and other purposes. A view is shown facing page 29 of a 

 small Mond gas plant installed for experimental purposes at 

 the Imperial College at South Kensington. 



The tar obtained from gas works is black, heavier than water, 

 and the amount produced per ton of coal varies considerably 

 according to the character of the coal. When the distillation 

 of the coal is so conducted as to yield 10,000 to 11,000 cubic feet 

 of gas, which is the usual practice, the tar produced amounts to 

 10 to 12 gallons of specific gravity about 1-2. Roughly speaking 

 then coal yields at a red heat about 10 per cent of its weight of tar. 



From whatever source it is obtained, the distillation of coal- 

 tar forms a separate industry, some of the features of which, on 

 account of its great practical and scientific importance, it is 

 desirable to survey. 



Before proceeding further, however, it will be interesting to 

 glance at the following tables, which will give a rough idea of 

 the enormous quantities of tar available and its connection with 

 other valuable products. 



The tendency is to save more and more of the by-products 

 obtained by heating coal, and as the demand for the innumerable 

 dyes, drugs, antiseptics, and other chemical compounds in- 

 creases, so will the utilisation of the hydrocarbons from coal-tar 

 continue to extend. At the time of writing these lines the chief 

 nations of Europe are engaged in a war upon which it is useless 

 to waste epithets. But on both sides there is an unlimited 

 demand for what are called high explosives, such as picric acid 

 and trinitrotoluene, which are obtained by the chemical treat- 

 ment of certain distillation products of coal. 



It is obvious, therefore, that there is every inducement to 

 practise economy in the treatment of coal whether considered 

 from the domestic or industrial point of view. The national im- 

 portance of this economy has not as yet attracted the serious 



