AND THE CONSTITUTION OP ELASTIC FLUIDS. 113 



ture in arithmetical progression, and since the pressure is 

 proportional to the square of the velocity of the particles, 

 in other words, to their vis viva, it follows that the abso- 

 lute temperature, pressure, and vis viva are proportional to 

 one another, and that the zero of temperature is 49P below 

 the freezing-point of water. Further, the absolute heat of 

 the gas, or, in other words, its capacity, will be represented 

 by the whole amount of vis viva at a given temperature. 

 The specific heat may, therefore, be determined in the 

 following simple manner : — 



The velocity of the particles of hydrogen, at the tem- 

 perature of 60", has been stated to be 6225 feet per second, 

 a velocity equivalent to a fall from the perpendicular height 



of 602,342 feet. The velocity at 61" will be 6225 V^ 



rz 6230*93 feet per second, which is equivalent to a fall of 

 603,502 feet. The difference between the above falls is 

 1 160 feet, which is therefore the space through which 1 lb. of 

 pressure must operate upon each lb. of hydrogen, in order 

 to elevate its temperature one degree. But our mechanical 

 equivalent of heat shows that 770 feet is the altitude repre- 

 senting the force required to raise the temperature of water 

 one degree ; consequently the specific heat of hydrogen will 



be ■- := 1*506, calling that of water unity. 



The specific heats of other gases will be easily deduced 

 from that of hydrogen ; for the whole vis viva and capacity 

 of equal bulks of the various gases will be equal to one 

 another ; and the velocity of the particles will be inversely 

 as the square root of the specific gravity. Hence the spe- 

 cific heat will be inversely proportional to the specific 

 gravitj'^, a law which has been arrived at experimentally by 

 De la Rive and Marcet. 



In the following table I have placed the specific heats of 

 various gases determined in the above manner, in juxta- 



Q 



