442 Mr. S. T. Preston on the Mode of 



in agreement with experiment, it becomes a point of interest 

 to inquire how the propagation of sound (or the propagation 

 of waves in gases generally) would be explained by the aid of 

 the kinetic theory. 



Since; in accordance with this theory, the molecules of 

 gases are in motion among each other. in straight lines, colli- 

 ding among themselves, it would appear somewhat difficult to 

 form a distinct idea as to the mode of propagation of a wave 

 in a gas and the condition determining the rate of propagation, 

 unless some law or guiding principle could be conceived of 

 according to which the molecules moved. Now I think it 

 will be found, on considering the subject, that there is a 

 guiding principle governing the motions of the molecules of 

 a gas among each other. I propose to show that the mo- 

 lecules of a gas in a fixed vessel under the influence of their 

 mutual collisions tend to arrange their motions in such a 

 way that an equal number of molecules move at any instant 

 in any two opposite directions ; or a self-acting adjustment 

 goes on among the molecules of a gas in such a way that 

 when an imaginary plane is placed in any position outside 

 the vessel, the number of molecules which at any instant 

 are approaching the plane is equal to the number which at 

 the same instant are receding from it. 



2. This will be found to be a simple condition following 

 necessarily from the conditions of equilibrium of pressure of 

 a gas ; for if a preponderating number of molecules were 

 moving in any special direction in a gas, this would be fol- 

 lowed by an increased pressure in that direction, whereas ob- 

 servation shows that this is not the case, or the pressure of a 

 gas is uniform in all directions. This therefore proves that 

 the motion of the molecules which produces this pressure is 

 uniform in all directions (and does not take place in one di- 

 rection in preference to another), and therefore that the num- 

 ber of molecules moving in any direction at a given instant is 

 equal to the number moving in the opposite direction. It 

 might be said that some of the molecules moving in one direc- 

 tion might happen to possess a less velocity than some of those 

 moving in the opposite direction, and therefore an increased 

 number of molecules would be required in that direction in 

 order to produce an equilibrium of pressure ; but it is to be 

 observed that a space of any perceptible capacity encloses a 

 vast number of molecules, so that every conceivable velocity 

 of motion is encountered as much in one direction as in the 

 opposite direction, and all inequality is thus equalized. It is 

 not as if it were a case of a few molecules — say a dozen, when 

 the mean velocity of the six moving in one direction might 



