[ i66 ] 



though it contains but 58 per cent, of carbon, Coaks therefore 

 always retain fome remains of bitumen, and confequently of 

 hydrogen ; and in this cafe ~-j^ or 39 parts of hydrogen would 

 compenfate for the defalcation of 1 1 9 of carbon. 



On the other hand, if we compare the quantities of pit coal 

 and charcoal we fhall find that 100 parts of fuch pit coal as is 

 commonly chared, containing at a medium 60 per cent, or 

 more of mere carbon, 6oolbs. of it fliould contain 360 of car- 

 bon, or perhaps more ; and as it alfo contains much hydrogen, 

 it might be expedlcd to give out a much greater proportion of 

 heat than 391 of coak or 510 of charcoal ; but here we muft 

 take into the account the vaft quantities of fmoke and foot it 

 throws out, which abforb a large proportion of its heat, and 

 moreover the bottom of the veflels heated by it are moftly co- 

 vered with foot, which confiderably obftruds the communication 

 of heat. 



The fame obfervations are in great meafure applicable to 

 woods. .According to Dodor Watfon oak yields but 22 per 

 cent, of charcoal by diflillation, and ftill lefs when chared ; and 

 beech, according to Wiegleb, only 20 per cent, confequently 

 1089 parts of oak yield but 239 of carbon, the remainder of 

 the heat then proceeds from the hydrogen contained in the oils 

 of the wood. 



Mr. 



I 



