266 COMPOUNDS DERIVED FROM NITRIC ACID. 



position ; for the figures are not the same when we are working 

 at constant pressure, as in the open air, as when we are working 

 at constant volume, as in a bomb shell or other closed vessel. 

 The rule for determining corrections of this nature has already 

 been given (p. 15). 



5. Conversely, if we know the heat disengaged by the decom- 

 position of an explosive substance, in a closed vessel, as well 

 as the exact nature of the products, it is easy to deduce the 

 heat of formation of the nitrated compound from its elements. 

 Sarrau and Vieille have followed this method. It furnishes us 

 with a check on the results obtained by the direct method, as 

 the two series of data should at least agree within the limits of 

 error allowed in experiments of this kind. 



6. Sometimes the products of the decomposition are either 

 not well known, or are too complicated, or imperfectly defined 

 as to their physical condition as in the case in which are 

 formed carbonaceous substances still retaining nitrogen, hydrogen, 

 oxygen, etc. This is what happens, for instance, with diazo- 

 benzene nitrate and with the picrates. 



In cases of this kind, the heat developed by the explosion is 

 always a useful quantity to measure, but it cannot be calculated 

 a priori. 



7. For a great number of applications it is necessary to 

 measure the heat of formation of such explosive compounds 

 from their elements. We then have recourse to a general 

 method, which consists in causing the body to explode in an 

 atmosphere of pure oxygen. This converts it entirely into water, 

 nitrogen, and carbonic acid. Calculation then becomes easy. 

 This method was employed for diazobenzene nitrate; Sarrau 

 and Vieille also adopted it for the picrates. 



8. Instead of oxidising the body by free oxygen, we may do 

 so by means of an oxidising compound. This is frequently 

 done in practice, such as when gun-cotton or the picrates are 

 mixed with potassium nitrate, ammonium nitrate, potassium 

 chlorate, or even sometimes certain metallic oxides. 



Under these circumstances it is convenient to calculate the 

 heat of combustion of the hydrocarbon compound, taking into 

 account the heat of formation of the oxidising body, according 

 to the table on page 134. 



The calculation is easy if the oxidising substance be potassium 

 chlorate, each equivalent of oxygen supplied entailing a supple- 

 mentary disengagement of T83 Cal. 



With ammonium nitrate, the additional energy is enormous, 

 amounting to + 25'05 Cal. per equivalent of oxygen. 



With potassium nitrate the calculation is somewhat more 

 complicated, on account of the alkali present, which may change 

 into carbonate or sulphate, according to circumstances. Let 

 us take, for example, a compound containing carbon in sufficient 



