SPECIFIC HEAT. 19 



of the receptacle in which the operation has taken place, when- 

 ever the chemical reaction remains the same. 



Hence it is also the same with the ratio between the initial 

 pressure and the developed pressure at constant volume. This 

 is, in fact, what follows from Joule's law, provided that such a 

 law apply to gases so highly compressed as those with which 

 we are concerned. 



4. We will now examine to what degree these various theo- 

 retical data are really known. 



On the one hand, the products which exist at the maximum 

 temperature, and under the conditions of the explosion, are not 

 necessarily identical with those which are found after cooling. 

 At this high temperature, the component elements can be only 

 partially combined, or transformed into simpler compounds. 

 Therefore the heat disengaged at the moment of the explosion 

 will be diminished. On the other hand, the state of combina- 

 tion is the more advanced, and the dissociation less, as the 

 pressure developed is more considerable. In general, the 

 maximum temperature appears to be very much below the 

 theoretical. 



3. SPECIFIC HEAT. 



1. The specific heat of gases, which is the base of all these 

 calculations, requires to be defined. For the sake of greater 

 simplicity, the specific heat really observable, at the ordinary 

 temperature, in products obtained after cooling, is adopted, this 

 specific heat being taken at constant volume, if the reaction 

 take place in a closed receptacle ; or at constant pressure, if we 

 operate at atmospheric pressure. The table of these specific 

 heats will be given on pp. 141-143. 



2. However, these suppositions are not accurate. The specific 

 heat taken at the ordinary temperature, T, does not remain con- 

 stant at higher temperatures for compound bodies, whatever be 

 their state ; it is not even so for simple bodies in the liquid or 

 solid state. In reality the greater number of these specific 

 heats increase rapidly with the temperature. More especially, 

 the specific heat of gases compressed to several thousand atmo- 

 spheres, such as results from the explosion of powder or nitro- 

 glycerin, is unknown, and it doubtless varies extremely with the 

 temperature and pressure. Its variations should be similar to 

 those of liquids, from which the state of gases so compressed is 

 not very remote. Now, the specific heat of certain liquids, such 

 as alcohol, can be doubled between limits of temperature so 

 little separated as and 150, according to the experiments of 

 Eegnault, 1 and those of Him. 2 This is therefore a very uncertain 

 datum. 



1 " Relation des experiences," etc., torn. ii. p. 272. 1862. 



2 " Annales de Chimie et de Physique," 4 e s<Srie, torn. x. p. 86. 1867. 



C 2 



