ACTUAL AND CALCULATED TEMPERATURE. 11 



portion does not contribute any heat and only operates by the 

 difference between the specific heat of the compounds and the 

 sum of that of the components, a difference which is nil 

 according to the hypothesis of Clausius. 



6. Without carrying this discussion any further, the following 

 general proposition may be deduced from it relative to chemical 

 combination. 



When the heat disengaged in a reaction taking place between 

 gaseous bodies, and with the exclusive formation of gaseous pro- 

 ducts, is entirely applied to heating the products, then there is 

 always increase of pressure, the volume being constant. 



This proposition has very important applications in the study 

 of explosive substances ; but, of course, it is applicable only to 

 gases forming gaseous products, for it is evident that the forma- 

 tion of a solid compound from gaseous components would cause 

 a reduction of pressure. 



The influence of dissociation being thus marked by a lowering 

 in the pressure of the gaseous systems, it must also be observed 

 that its existence and effect should not be unduly exaggerated ; 

 the latter must be less than one would at first suppose on 

 account of certain compensations. 



We will dwell a little on this matter on account of its great 

 importance. 



7. The actual temperature which is developed in an explosive 

 reaction is in general less than the temperature calculated in 

 accordance with the specific heats of the gas, estimated at about 

 the normal pressure and ordinary temperature, since the specific 

 heat of greatly compressed gases is not constant. In fact, the 

 specific heat of gases formed with condensation increases with 

 the temperature, according to the facts observed by Eegnault 

 and M. E. Wiedemann on gaseous carbonic acid and other 

 compound gases. It must also increase with the pressure, at 

 one and the same temperature, in proportion as the gas 

 approaches the liquid state, the specific heat of a liquid being 

 nearly always greater than that of the same body in its gaseous 

 form, at the same temperature. An equal quantity of heat 

 applied to compressed gases, such as those which are produced 

 in explosive phenomena, will therefore produce less rise in 

 temperature than if their specific heat were constant, and equal 

 to that of the same gases at the normal pressure, as is generally 

 assumed in these calculations. 



Hence a smaller increase in dissociation, which depends 

 chiefly on the temperature. It is further limited by another 

 circumstance, relative to the pressure developed. 



8. Now, the actual pressure is not so much diminished as 

 one might judge from a calculation founded on the ordinary 

 laws of gases, and on the lowering of the theoretical temperature. 

 The laws of Mariotte and of Gay-Lussac are hardly applicable 



