214 Mr. J. P. Joule on Heat, 
in greater space ; and the diminution of temperature during the 
conversion of solids into fluids or gases may be explained on the 
idea of the loss of vibratory motion, in consequence of the revo- 
lution of particles round their axes at the moment when the body 
becomes fluid or aériform, or from the loss of rapidity of vibra- 
tion in consequence of the motion of the particles through greater 
space*.” I have myself endeavoured to prove that a rotary 
motion, such as that described by Sir H. Davy, will account for 
the law of Boyle and Mariotte, and other phenomena presented 
by elastic fluids+; nevertheless, since the hypothesis of Herapath, 
in which it is assumed that the particles of a gas are constantly 
flymg about in every direction with great velocity, the pressure 
of the gas being owing to the impact of the particles against any 
surface presented to them, is somewhat simpler, I shall employ 
it in the following remarks on the constitution of elastic fluids ; 
premising, however, that the hypothesis of a rotatory motion 
accords equally well with the phenomena. 
Let us suppose an envelope of the size and shape of a cubic 
foot to be filled with hydrogen gas, which, at 60° temperature 
and 30 inches barometrical pressure, will weigh 36-937 grs. 
Further, let us suppose the above quantity to be divided into 
three equal and indefinitely small elastic particles, each weigh- 
ing 12°309 grs. ; and further, that each of these particles vibrates 
between opposite sides of the cube, and maintains a uniform 
velocity except at the instant of impact; it is required to find 
the velocity at which each particle must move so as to produce 
the atmospherical pressure of 14,831,712 grs. on each of the 
square sides of the cube. In the first place, it is known that if 
a body moving with the velocity of 324 feet per second be op- 
posed, during one second, by a pressure equal to its weight, its 
motion will be stopped, and that if the pressure be continued 
one second longer, the particle will acquire the velocity of 3824 
feet per second in the contrary direction. At this velocity there 
will be 322 collisions of a particle of 12-309 grs. against each 
side of the cubical vessel in every two seconds of time; and the 
pressure occasioned thereby will be 12°309 x 323=395:938 gys. 
Therefore, since it is manifest that the pressure will be propor- 
tional 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, 
si ASSUME Vin, 
va / (Hr) 321 = 6225 feet per second. 
* Elements of Chemical Philosophy, p. 95. 
+ Mr. Rankine has given a complete mathematical investigation of the 
action of vortices, in his paper on the Mechanical Action of Gases and 
Vapours.—Trans. Roy. Soc. Edinb. vol. xx. part 1—May 1851, J. P. J. 
