THEORT of the EARTH. 239 



WK come now to the fecond fpecies of inflammable bodies 

 called oily or bituminous. Thefe fubftances are alfo found va- 

 rioufly mixed with mineral bodies, as well as forming ftrata of 

 themfelves ; they are, therefore, a proper fubjecl for a particu- 

 lar examination. 



IN the procefs of vegetation, there are prodxiced oily and re- 

 finous fubftances ; and from the collection of thefe fubftances 

 at the bottom of the ocean, there are formed ftrata, which have 

 afterwards undergone various degrees of heat, and have been 

 varioufly changed, in confequence of the effects of that heat, 

 according as the diftillation of the more volatile parts of thofe 

 bodies has been fuffered to proceed. 



IN order to underftand this, it muft be confidered, that, 

 while immerfed in water, and under infuperable compreffion, 

 the vegetable, oily, and refinous fubftances, would appear to be 

 unalterable by heat ; and it is only in proportion as certain che- 

 mical feparations take place, that thefe inflammable bodies are 

 changed in their fubftance by the application of heat. Now, 

 the moft general change of this kind is in confequence of eva- 

 poration, or the diftillation of their more volatile parts, by 

 which oily fvibftances become bituminous, and bituminous fub- 

 ftances become coaly. 



THERE is here a gradation which may be beft underftood by 

 comparing the extremes. 



ON 



lots fepares. Dans la mine de manganefe native, elle n'eft point en une feule made j 

 elle eft difpofee egalement en plufieurs culots fepare's, et un peu applatis, comme ceux 

 que 1'art produit j beaucoup plus gros, a la verite, parce que les agens cle la nature doi- 

 vent avoir une autre nergie, que ceux de nos laboratoires } et cette reflemblance fi ex- 

 afte, femble devoir vous faire penfer que la mine native a etc produite par le feu, tout 

 comme (on regule. La prefence de la chaux argentee de la manganefe, me permettroit 

 de croire que la nature n'a fait que reduire cette chaux. Du refte, cette mine native eft 

 tres-pure, et ne contient aucune partie attirable a 1'aimant. Cette mine, unique jufqu'il 

 ce moment, vient, tout comme les autres manganefe que j'ai decrites, des mines de fer 

 de Sem, dans la vall&e de Viedsrfos, eu Comte dc Foix." Journal de Pbv/tque, Janvier 

 1786- 



