16 HEAT DISENGAGED. 



This figure is deduced from the heat of formation of water, 

 carbonic acid and nitroglycerin, the latter being taken from the 

 three following data : the heat of combustion of glycerin, 

 which leads to its heat of formation ; the heat of formation of 

 nitric acid ; and lastly, the heat disengaged by the action of this 

 acid on the glycerin. 



Sarrau and Vieille have measured directly the heat disengaged 

 by the explosion of nitroglycerin in a closed vessel, and have 

 found 1600 cal. for one grm. 



The figures 1590 and 1600 have thus been obtained by the 

 two inverse methods just indicated, and they are as concordant 

 as can be expected, taking into consideration the small errors 

 inseparable from all experiments. 



7. It should be remarked here, that the quantity of heat dis- 

 engaged by an explosive is only a fixed quantity, which can be 

 calculated beforehand, when the material undergoes total com- 

 bustion, otherwise the heat cannot be calculated for lack of 

 knowledge of the products of combustion, for these can vary 

 with the pressure, the manner of ignition, and many other 

 circumstances (pp. 6, 7). 



Further, when working with closed vessels, the oxygen of the 

 air contained in the space plays a part when the combustion is 

 incomplete its effect is greater the smaller the density of charge. 

 Thus, in calorimetric experiments it is advisable to operate in 

 an atmosphere of nitrogen when there is not total combustion. 

 The walls of the vessel, especially when of iron or copper, bear 

 a part in the chemical reaction which has often been overlooked. 

 These metals are oxidised at the expense of the air or of the 

 nitrates, or attacked by the sulphur, etc. Hence there are 

 subsidiary disengagements of heat which affect the determina- 

 tions. To avoid these troubles the author conducts all his 

 determinations in vessels lined with platinum. 



8. In what has preceded it has been supposed that the 

 chemical reaction was not accompanied by any special mechanical 

 effect. But in general the object of explosions is to do certain 

 work ; the measure and valuation of this work ought to be made 

 in each particular case. Hence result most important but 

 complicated calculations for the theory of firearms, in which the 

 expansion of the gases plays an important part. Details of 

 these will be found in the memoirs of Sarrau, De Saint Eobert, 

 Noble and Abel, Sebert and Hugoniot, and other authorities 

 who have devoted special attention to ballistics. 



9. Without any theory the sum of this work might be arrived 

 at by an inverse process, namely, by effecting the explosive 

 reaction in a calorimeter, and measuring the heat disengaged at 

 the instant in which the work is accomplished. The difference 

 between the quantity of heat disengaged in a reaction effected 

 without mechanical effects, and the same reaction with 



