370 SCIENTIFIC RECREATIONS. 



resulted in their disappearance, and when the residue was examined it was 

 found that the diamonds had been burned, that they had combined with 

 oxygen and formed carbonic acid, just as when coal burns. The diamond is 

 the hardest of all substances, the most valuable of gems, and the purest 

 condition in which carbon appears. 



GRAPHITE (Plumbago) is termed " black-lead," and is the next purest 

 form of carbon. It crystallizes and belongs to the primitive formations. 

 In Cumberland it is dug up and used to make pencils ; the operations can 

 be seen at Keswick. It has other uses of a domestic character. 



Charcoal is the third form of carbon, and as it possesses no definite 

 form, is said to be amorphous. Charcoal is prepared in air-tight ovens, so 



Fig. 366. Coke ovens. 



that no oxygen can enter and burn the wood thus treated. Coke is the 

 result of the same process applied to coal. The gas manufactories are the 

 chief depots for this article, and it is used in locomotive engines. The 

 various smokeless coals and prepared fuels, however, are frequently substi- 

 tuted. 



Coke ovens were formerly much resorted to by the railway companies, 

 who found the ordinary coal too smoky for locomotive purposes, and apt to 

 give rise to complaints by passengers and residents near the line. 



The origin of wood charcoal we have seen. All vegetable substances 

 contain carbon. When we burn wood, in the absence of air as far as 

 possible, oxygen and hydrogen are expelled. The wood is piled in layers as 

 in the illustration (fig. 368), covered over with turf and mould, with occasional 

 apertures for air. This mass is ignited, the oxygen and hydrogen are driven 



