704 THE WONDERS OE GEOLOGY. Lect. VII. 



succinite, and other similar bodies which occur in the strata. 

 The springs of petroleum do not seem to depend on com- 

 bustion, as has been supposed, but to be simply the effect 

 of subterranean heat. According to the information we 

 now possess, it is not necessary that strata should be at 

 a very great depth beneath the surface to acquire a tem- 

 perature equal to the boiling point of water, or mineral 

 oil. In such a position the oil must have suffered a slow 

 distillation, and have found its way to the surface ; or 

 have so impregnated a portion of the earth, as to form 

 springs or wells, as in various parts of Persia and India.* 



Bitumen is an inspissated mineral oil, of a dark brown 

 colour, with a strong odour of tar. In the Odin mine of 

 Derbyshire, a species occurs which is elastic, being of 

 the consistence of thick jelly, and bearing some resemblance 

 to soft India-rubber; as it will remove the traces of a 

 pencil, it has been named mineral caoutchouc. Some 

 bitumens possess the colour and transparency of amber: 

 the soft varieties may be rendered solid by heat. 



From these bituminous substances, we pass by an easy 

 transition to Amber, of which we have already spoken 

 (p. 242), for black amber, both in its appearance and com- 

 position, closely resembles the solid bitumens. 



A mineral called mellite, or honey-stone, from its colour, 

 is found among the bituminous wood of Thuringia, which 

 in its chemical composition, and electrical properties, bears 

 a great analogy to amber ; it is usually crystallized in 

 small octahedrons. t 



28. The Diamond. — The chemical constituents of the 

 substances above described are chiefly carbon and hydrogen, 

 with a small proportion of oxygen, the essential elements 

 of vegetable matter. But the Diamond is pure carbon; 

 at a heat less than the melting point of silver, it burns, and 

 is volatilized, yielding the same elementary products as 



* Dr. Reichenbach. 



f Organic Remains of a former World, vol. i. pi. i. fig. 2. 



