FUEL 



301 



smokeless fuel is required. It was orif>inally a by-product 

 in the manufacture of illuminating gas. Now it is manu- 

 factured expressly for metallurgical purposes, the ovens 

 being so constructed that the inflammable gases driven off 

 are made to serve largely as a source of heat in the process. 



Fig. 278. — Fos.sil remains of a giant club-moss {Lcpidodendrun up., Scale- 

 tree Family, Lcpidodendraceoe) . From the coal period. (Baillon.) 



Illuminating-gas is made by subjecting coal or wood to a 

 high temperature in a retort, and collecting and purifying the 

 gas given off. For obvious reasons coal-gas has proved to be 

 a most convenient fuel especially adapted for household use 

 in large cities. 



The study of fuels leads one to think not only of the forests 



