2o8 the Development of Car hit In vegetable Matter. 



to reft on the volcanic nature bf Iceland ; but it furely would be going too far were we t« 

 afcribe to fire all the phenomena which are obferved in volcanic countries. 



Bovey coal, lilce the furturbrand, refembles half-charred wood ; and I will allow, and in- 

 deed am difpofed to believe, that it is in a ftate nearly fimilar j but from this it does not follow 

 that fire has been the caufe. 



Carbon is known to be one of the grand principles of vegetables, and alfo as that which is 

 the mod fixed, excepting the fmali portion of the earths contained in them. As a fixed prin- 

 ciple, carbon appears to form, in great meafure, the vegetable fibre ; and after a certain de- 

 gree of combuftion,. (by which the other principles have been diflipated,) it remains, and the 

 particles of it keep the fame arrangement which they poflcfl'ed when the vegetable was com- 

 plete. If, however, the combuftion has been carried on with the free accefs of air, the 

 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 fub- 

 ftances with which it was combined in vegetables ; and thefe are now explained as efFe£ts 

 fimilar to thofe of combuftion, although fire has not been the caufe. In both cafes the car- 

 bon has been freed from the more volalile principles ; and under circumftances not 

 favourable to the union of carbon with oxygen, the former muft neceflkrily remain more or 

 lefsundiminiflied. 



During the combuftion of vegetable matter, the more volatile principles contained in the 

 vegetable fibre (which with carbon alfo form the refinous and other fimilar fubftances) ap- 

 pear to be firft feparated ; and in proportion to this feparation, the other more fixed fub- 

 llance, 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 fhews the degree of combuftion to which it has beenfubjeded, or, 

 in other words, how far the feparation of the other principles from carbon has been 

 ciFe<aed. 



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 difengaged and form 

 new combinations, while the carbon is the laft which is a£led upon j fo that unlefs a fufficient 

 ijuantity of oxygen be prefent, it remains fixed and unchanged. 



But the fame feparation of the vegetable principles happens whenever vegetables in the 



full pofleffion of their juices areexpofed to circumftances which favour the putrid fermenta- 



. tion. — As in combuftion, fo by the progrefs of putrefa(fl:ion does the vegetable lofe its colour, 



become brown, and afterwards black ; at the fame time a gas is difcharged, which is com- 



pofed of hydrogen, azote, and carbonic acid. 



When combuftion is long continued with the free accefs of air, the whole of the carbon is 

 difiipated in the ftate of carbonic acidj but in the procefs of putrefacftion a confiderable 

 portion of carbon commonly remains even long after the putrid fermentation has ceafed. 

 Although, therefore, it is as readily developed by putrefaction as by combuftion, it is not, 

 however, when liberated from the other principles, fo fpeedily diflipated by the former as by 

 the latter procefs. 



According to the degree of combuftion within certain limits the carbon is more or lefs 

 apparent, and the like prevails according to the degree of putrefadion j fo that whenever 



the 



