142 'Mr. H a t c hett'j Ohfa vations on Bituminous Subjlanch* 



ff«ee accefs ©fair, tlie carbon enters into combination with oxygen 

 and caloric, and forms carbonic acid. 



We have many examples in which carbon is formed or rather 

 liberated from thofe fubftances with which it was combined in 

 vegetables ; and theie are now explained as effects fimilar to thofe 

 of combustion, although fire has not been the caufe. In both 

 cafes the carbon has been freed from the more volatile principles ; 

 and under circumftances not favourable to the union of carbon 

 with oxygen, the former muft neceflarily remain more or lefs un- 

 diminiQted. 



During the combuftion of vegetable matter, the more volatile 

 principles contained in the vegetable fibre (which with carbon alfo 

 form the refmous and other fimilar fubftances) appear to be firft 

 feparated ; and in proportion to this feparation, the other more 

 fixed fubftance, which we call -carbon, is developed. 



Thus, by the progrefs of combuftion, wood becomes brown, and 

 afterwards black ; fo that the ftate of the wood (hews the degree of 

 combuftion to which it has been fubjecled, or, in other words, how 

 far the fepa-ration of the other principles from carbon has been 

 effected. 



Combuftion is therefore a fpecies of analyfis by which the 

 principles of vegetables are feparated, according to their affinities, 

 and according to their degree of volatility. By this operation 

 hydrogen and azote (if it be prefent in the vegetable) are firft dif- 

 engaged and form new combinations, while the carbon is the laft 

 which is acted upon; fo that unlefs a fufneient quantity of oxygen 

 be prefent, it remains fixed and unchanged. 



But the fame feparation of the vegetable principles happens 

 whenever vegetables in the full poneflion of their juices are expofed 

 to circumftances which favour the putrid fermentation. — As in 



combuftion, 



