2j6 Elaftic 'Bitumen. . fBook VL 



elaftic gum, and which is found near Caftletown in 



Derbyfhire. Its colour is brown, partly Toft, flexible 



and elaftic, and this when cut appears internally of a 



greenifh yellow colour ; partly brittle, with a conchoi- 



dal fracture. Both varieties are frequently united in 



, the fame fpecimen. ^oth are infoluble in fpirit of 



. wine, ether, and oil of turpentine, but yield to that of 



olives. 



The production of all thefe bitumens is attributed 

 ro the action of fubterraneous fire on ftrata of pit- coal, 

 by which the oily parts are feparated and fublimed in 

 the fame manner as by artificial heat. . 



A curious experiment is mentioned by Bifhop Wat- 

 fon, which illiiftrates the relation of thefe four bitumens 

 to each other. The moft transparent oil of turpentine, 

 refembling n^phta, may be changed into an oil refem^ 

 bling petroleum, by mixing it \vith a fmall portion of 

 vitriolic acid ;; with a larger proportion of the acid the 

 mixture becomes black and tenacious like Barbadoes 

 tar, and the proporti6ns of the ingredients may be fo 

 adjufted, that' the mixture will even acquire a folicj 

 confidence like afp-halmm. 



All thefe fubftances become more tenacious, and 

 acquire a darker colour by'expofure to air. This 

 feems to be owing to the abforptioh of oxygen, which, 

 like oils, they pofiefs the property of attracting from 

 the atmofphere. 



It appears that on-diiiillatioa bitumens leave fcarcely 

 any refiduum, the earthy part being merely accidental, 

 and that the carbonic fubftance is by much the fmalleft 

 in their compofition^ as their concretion feems wholly 

 owing to the abforption of air, and not to the preva- 

 lence of the carbonaceous .principle, fmce no coal ap- 

 pears, when they are burned on a hot iron. 



