HEAT OF FORMATION AND TOTAL COMBUSTION. 265 



3. We may, then, arrive at the heat disengaged in the forma- 

 tion of nitrated compounds from pure nitric acid and organic 

 principles, such as alcohol, benzene, phenol, glycerin, mannite, 

 cellulose, etc. But this quantity does not enable us to calculate 

 the heat given off by their explosive decomposition, even if we 

 know exactly the products of this decomposition. It is neces- 

 sary, in addition, to have the heat of formation of these 

 products from their elements, together with that of nitric acid, 

 water, and the original compound that gave rise to the nitrated 

 body. 



The products of the explosive decomposition of nitrated com- 

 pounds are generally simple, e.g. water, carbonic acid, and 

 nitrogen ; these three being the only substances produced in a 

 complete combustion, such as that of nitroglycerin or nitro- 

 mannite. But in incomplete combustion, where the oxygen is 

 deficient, as in that of gun-cotton, we get also carbonic oxide, 

 hydrocyanic acid, hydrogen, marsh gas, occasionally oxides of 

 nitrogen, etc. The heat of formation of all these substances 

 should be known beforehand. 



In fact, the heat of formation of all these compounds has 

 already been given (pp. 128, et seq.), together with that of nitric 

 acid from its elements. With regard to the original generator 

 of the nitrogenous body, its heat of formation may be determined 

 by its total combustion in oxygen, or by various other processes. 

 For all the substances enumerated above, the heat of formation 

 will be found in the thermo-chemical tables (pp. 136, 137). We 

 will give an example, in order to make this clear. Let us 

 estimate the heat disengaged in the combination of the elements 

 of nitric ether. For this purpose we add the heat disengaged in 

 the formation of alcohol, C 2 H 6 (+70 Cal.), to that disengaged 

 in the formation of nitric acid, HN0 3 -f 41*6 Cal., and then to 

 the sum we add that disengaged in the reciprocal reaction of 

 these two bodies (+ 6*2 Cal.), which reaction produces nitric 

 ether. The sum of these three quantities, minus the heat of 

 formation of the water eliminated in the reaction (H 2 0), i.e. 

 69 Cal., gives the quantity, + 49*& Cal., which represents the 

 heat disengaged by the combination of the elements of nitric 

 ether. On subtracting this quantity from the heat disengaged 

 by the pure and simple combustion of the said elements by 

 means of free oxygen, we get the heat of total combustion of 

 nitric ether in free oxygen, or + 311'2 Cal. 



4. In this way were calculated both the heat of formation 

 from the elements, and the heat of total combustion, of nitric 

 ether, nitroglycerin, gun-cotton, and nitrobenzene. 



The heat liberated "by their explosive decomposition can be at 

 once deduced, provided that we know exactly the real equation 

 representing this decomposition. It is also necessary to take 

 into account, in the calculations, the conditions of the decom- 



