112 ME. J. P. JOULE ON HEAT, 



the velocity of 32^ feet per second in the contrary direc- 

 tion. At this velocity there will be 32^ collisions of a 

 particle of 1 2*309 grs. against each side of the cubical 

 vessel in every two seconds of time ; and the pressure 

 occasioned thereby will be 12*309 X 32^ — 395*938 grs. 

 Therefore, since it is manifest that the pressure will be 

 proportional to the square of the velocity of the particles, 

 we shall have for the velocity of the particles requisite to 

 produce the pressure of 14,831,712 grs. on each side of the 

 cubical vessel, 



i' = V (}^^~~) ^H = 6225 feet per second. 



The above velocity will be found equal to produce the 

 atmospheric 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 particles 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 Mariotle flows 

 naturally from the hypothesis. 



The velocity above named is that of hydrogen at the 

 temperature of 60" ; but we know that the pressure of an 

 elastic fluid at 60", is to that at 32° as 519 is to 491. There- 

 fore, the velocity of the particles at 60" will be to that at 

 32° asv'519 : ^4*91; 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 less- 

 ened. 



Since the pressure of a gas increases with its tempera- 



