488 POWDERS WITH A NITEATE BASE. 



5. ON THE CHARCOALS EMPLOYED IN THE MANUFACTURE OF 



POWDER. 



1. In equations relative to the combustion of powder, pure 

 carbon is usually considered ; but in reality the charcoal should 

 be taken with its true composition, for the results calculated on 

 the supposition that the oxygen is in the state of water whilst 

 carbon and hydrogen would be free, are not certain, owing to 

 the complex composition of charcoal and the thermal excess 

 which it liberates in its total combustion. 



2. It might be imagined that, in order to take this fact into 

 account in calorimetric calculations, it would be sufficient to 

 calculate the formation of carbonic acid and carbonic oxide 

 from amorphous carbon 



C + 2 = C0 2 liberates + 48'5 Cal., 



instead of -f 47 Cal. for diamond carbon. 



But even this way of reckoning gives figures which are too 

 low, because the charcoal used in the manufacture of powder is 

 not pure carbon, but contains hydrogen and oxygen nearly in 

 the proportions of water. For instance, the charcoal of the 

 powder studied by Bunsen contained in 11*0 parts 



0= 7'6; H = 0-4; = 3-0. 



Now, the combustion of the hydrocarbons yields more heat 

 than that corresponding to the carbon they contain, the hydro- 

 gen and oxygen being supposed in the state of pre-existing 

 water, that is to say, no longer contributing to the production 

 of heat. Thus Favre and Silbermann, 1 burning bakers' embers 

 (which contained to 1 grm. of carbon 0*027 grm. of hydrogen), 

 found 52,440 cal., instead of 47,000 for 6 grms. of carbon burnt, 

 which makes an excess of more than a ninth, or 906 cal. per 

 gramme. 



3. This is intelligible if it be noted that calcined charcoal is 

 derived from a carbohydrate, and that the carbohydrates, as the 

 author pointed out many years ago, yield by their combustion 

 more heat than the carbon which they contain, deduction being 

 made of the oxygen and hydrogen in the form of water. 



The heat- of combustion of a carbohydrate of the formula 

 (C 6 HpOp) is, according to experiment, generally 709 Cal. to 

 726 Cal. for 72 grms. of carbon. 



This would make for the heat of combustion of C = 6 grms. 

 59 CaL to 61*6 Cal., that is, an excess of more than a fourth of 

 the heat of combustion of the real carbon of the substance. 

 When the carbohydrates are dehydrated by heat, a portion of 



1 " Annales de Chimie et de Physique," 3 e serie, torn, xxxiv. p. 420. 1852. 



