MR HAYCRAFT on the Specific Heat of the Gases. 213 



There is another condition under which air is capable of a 

 great variety of specific heats, namely, when it exists in different 

 degrees of density, whether arising from pressure or other causes. 

 The increased capacity of air, when under lesser degrees of atmo- 

 spheric pressure, has been properly made use of to explain the 

 extreme cold which exists in high regions ; and its decreased ca- 

 pacity under mechanical pressure, also satisfactorily accounts for 

 the heat evolved under that condition. This principle, so far as 

 I know, has not been used to explain one cause of the intense 

 heat produced during the combustion of gunpowder and other 

 explosive mixtures. If we reflect a moment, however, we shall 

 perceive that the resistance of the pressure of the atmosphere to 

 the expansion of the nascent gases produced by the combustion, 

 will cause them to exist in a state of greater density than when 

 the resistance of the atmosphere has been finally overcome. It 

 is during this state of potential compression, if I may use teh 

 term, that the intense heat is produced. After the first explo- 

 sion, however, the gaseous products will expand, and then there 

 will necessarily be an absorption of caloric, and consequently 

 comparative coldness, produced. In order to ascertain whether 

 there is a permanent evolution of caloric, occasioned by the com- 

 bustion of gunpowder, I made the following experiment. 



Having a receiver containing 528 cubic inches, filled with wa- 

 ter of a temperature of 52, placed in a pneumatic trough, the sur-r 

 rounding atmosphere being also 52, I introduced 240 inches of 

 the aeriform fluids, produced during the combustion of that com- 

 position of gunpowder which is used for pyrotechnical purposes. 

 After the explosion, the gas in the upper part of the receiver 

 had acquired a temperature of nearly 54, and the water not so 

 much. This experiment shews, that though heat is evolved in 

 the combustion of gunpowder, its quantity is not nearly so great 

 as has been imagined. Again, if we consider that the products 

 pf the combustion of gunpowder have not, by direct experiment^ 



