the Propagation of Sound. 443 



happen to be considerably different from the mean velocity 

 of the six moving in the opposite direction ; but since it is 

 a case of millions of molecules, all inequalities are equalized. 

 It is evident, therefore, that in order that equilibrium of 

 pressure may exist in a gas, or in order that the molecules 

 in their mutual collisions may balance each other's effects, 

 as many molecules must be moving in one direction as in 

 the opposite, so that the vis viva in two opposite directions is 

 equal. If, for example, in any portion of a gas, such as a 

 cubic foot, the number of molecules moving towards one ima- 

 ginary bounding plane of this cubic foot were greater than 

 the number moving towards the opposite bounding plane, the 

 whole cubic foot of gas would tend to be propelled bodily 

 in that direction towards which the greater number of its 

 molecules was moving, thus producing a current, whereas 

 no such currents in gases are observed. Indeed, according to 

 actual observation, each portion of a gas however small, ap- 

 pears to be at rest. The portion of the gas could only be at 

 rest under the condition that it exerted a uniform pressure in 

 all directions : for if it exerted a less pressure in any given 

 direction, it would be reacted upon by the surrounding gas 

 and propelled from that side towards which it exerted the least 

 pressure. Since, however, any portion of the gas, however 

 small, appears to be at rest, it follows that this portion of the 

 gas must exert a uniform pressure in all directions, and there- 

 fore that the motion of the molecules composing this portion 

 of the gas must take place uniformly towards all directions, 

 L e. as many molecules must be moving in any one direction 

 as in the opposite. 



3. As a direct corollary to this, it may be shown that there 

 is a self-acting tendency for this form of motion to be kept 

 up; or, in other words, a mechanical self-adjustment is conti- 

 nually going on among the molecules of a gas to produce a 

 special character of motion, viz. that the motion of the molecules 

 takes place uniformly towards all directions, or the numbers of 

 molecules moving in any two opposite directions are equal — 

 that, therefore, if by any artificial means the motion of the 

 molecules of a gas could be interfered with or changed, they 

 would, when left to themselves, automatically return back to 

 the above regular form of motion. 



This evidently follows from the consideration that, since the 

 equilibrium or uniformity of pressure requires that the motion 

 of the molecules should take place uniformly towards all di- 

 rections, and since any disturbance of this form of motion 

 would disturb the equilibrium of pressure, and since the equi- 

 librium of pressure is self-adjusting, the uniformity of motion 



