

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



This is an inevitable certainty by the acceptance of the kinetic 

 theory ; and therefore it is clear that a change of density of 

 the air, by adding to the number of air-molecules (as when air 

 has been forced into a vessel), cannot possibly influence the 

 velocity of propagation of a wave by the air so long as the 

 molecular velocity remains constant. 



14. Obviously when air has been forced into a vessel and 

 thereby its density increased, this has simply the effect, by 

 adding to the number of molecules, of increasing the number 

 of collisions among the molecules ; but this cannot affect in the 

 least the velocity of propagation of the sound-wave, for the 

 simple reason that it cannot affect the velocity of the mole- 

 cules. So the increased pressure of the air against the sides 

 of the vessel (considered to represent increased " elasticity ") 

 cannot possibly influence the velocity of the sound-wave, this 

 increased pressure being merely due to an increased number 

 of molecules colliding against the sides of the vessel. 



The above considerations may perhaps be made more ob- 

 vious by imagining the case of a number of couriers pro- 

 pagating a message with no intervening mechanism through 

 which they can communicate with each other (as in analogy 

 with the molecules of a gas). Then the velocity of propaga- 

 tion of the message will solely depend on the velocity of the 

 couriers themselves. Increased number of couriers (corre- 

 sponding to increased number of molecules in a gas or in- 

 creased density) will have no effect on the velocity of propa- 

 gation of the message, provided the velocity of each courier 

 remains the same. So with the molecules of a gas, which 

 according to the kinetic theory, are interchanging motion 

 among themselves with no means of acting upon each other 

 excepting by direct impact. Thus the velocity of a wave in 

 a gas can be determined solely by the velocity of its mole- 

 cules, and by nothing else ; or the sole physical condition de- 

 termining the velocity of sound in a gas is the velocity of its 

 molecules. "yL? 



15. Variation of Specific Gravity. — So, therefore, also vari- 

 ation of specific gravity in gases can have no influence on the 

 velocity of the sound-wave, unless the molecular velocity be 

 changed. One known consequence of the kinetic theory is, 

 that equal volumes of different gases all contain the same 

 number of molecules, so that therefore the specific gravity of 

 a gas is proportional to its molecular weight. A cubic foot of 

 oxygen contains the same number of molecules as a cubic foot 

 of hydrogen ; but the specific gravity of oxygen is sixteen 

 times that of hydrogen, and the oxygen molecule is sixteen 

 times as heavy as the hydrogen molecule, and (as is known) 



