Mr. Haycraft on the Specific Heat of the Gases. 267 



lumn of the volatile spirit of turpentine. Upon any rise of 

 the temperature of the atmosphere, the column immediately 

 rose at the side of the dry ball. After some time, however, 

 the instrument seemed to have lost its power ; and after a still 

 longer period, the ball containing dry air had the greater ex- 

 pansive force. This I accounted lor by supposing, that the va- 

 pour of turpentine had in process of time combined with the 

 dry air, and had given it its greater expansive power. This 

 thermometer is now a remarkably delicate one, though its de- 

 grees are of very unequal length, and appear to vary by lapse 

 of time. Probably hydrogen gas contained in two platina 

 balls, in one of which a little mercury might be placed, con- 

 nected together in the same way, would make an accurate py- 

 rometer, indicating temperatures as high as the melting point 

 of platina. 



There is another condition under which air is capable of a 

 great variety of specific heats, namely, when it exists in dif- 

 ferent degrees of density, whether arising from pressure or 

 other causes. The increased capacity of air, when under lesser 

 degrees of atmospheric pressure, has been properly made use 

 of to explain the extreme cold which exists in high regions ; 

 and its decreased capacity under mechanical pressure, also 

 satisfactorily accounts for the heat evolved under that condi- 

 tion. 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 re- 

 sistance 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 re- 

 sistance of the atmosphere has been finally overcome. It is 

 during this state of potential compression, if I may use the 

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

 plosion, however, the gaseous products will expand, and then 

 there will necessarily be an absorption of caloric, and conse- 

 quently comparative coldness, produced. In order to ascer- 

 tain whether there is a permanent evolution of caloric, occa- 

 sioned by the combustion of gunpowder, I made the following 

 experiment. 



Having a receiver containing 528 cubic inches, filled with 

 water of a temperature of 52°, placed in a pneumatic trough, 

 the surrounding atmosphere being also 52°, I introduced 240 

 inches of the aeriform fluids, produced during the combustion 

 of that composition of gunpowder which is used for pyrotcch- 

 nital purposes. After the explosion, the gas in the upper part 

 L 1 2 of 



