and the Constitution of Elastic Fluids. 215 
The above velocity will be found equal to produce the atmo- 
spheric pressure, whether the particles strike each other before 
they arrive at the sides of the cubical vessel, whether they strike 
the sides obliquely, and thirdly, into whatever number of par- 
ticles the 36-927 grs. of hydrogen are divided. 
If only one-half the weight of hydrogen, or 18°4635 grs., be 
enclosed in the cubical vessel, and the velocity of the particles 
be as before, 6225 feet per second, the pressure will manifestly 
be only one-half of what it was previously, which shows that the 
law of Boyle and Mariotte flows naturally from the hypothesis. 
The velocity above named is that of hydrogen at the tempera- 
ture of 60°; but we know that the pressure of an elastic fluid 
at 60° is to that at 832° as 519 isto 491. Therefore the velocity 
of the particles at 60° will be to that at 32° as /519: W491, 
which shows that the velocity at the freezing temperature of 
water is 6055 feet per second. 
In the above calculations it is supposed that the particles of 
hydrogen have no sensible magnitude, otherwise the velocity 
corresponding to the same pressure would be lessened. 
Since the pressure of a gas increases with its temperature 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 absolute temperature, pressure, 
and vis viva are proportional to one another, and that the zero 
of temperature is 491° 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 deter- 
mined in the following simple manner :— 
The velocity of the particles of hydrogen, at the temperature 
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 \/ oF, = 6280-98 feet 
per second, which is equivalent to a fall of 603,502 feet. The 
difference between the above falls is 1160 feet, which is there- 
fore the space through which 1 Ib. of pressure must operate upon 
each pound of hydrogen, in order to elevate its temperature one 
degree. But our mechanical equivalent of heat shows that 770 
feet is the altitude representing the force required to raise the 
temperature of water one degree ; consequently the specific heat 
1160 
770 
The specific heats of the 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 
of hydrogen will be =1'506, calling that of water unity. 
