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



9. The Physical Condition determining the Velocity of Sound. 

 — It must evidently follow, as a necessary consequence from 

 the above considerations, that the velocity of sound in a gas 

 must be proportional to the velocity of the molecules of the 

 gas ; so that, therefore, the numbers expressing the velocity 

 of sound in different gases and the numbers expressing the 

 velocities of the molecules of different gases must be propor- 

 tional to each other. This is found to be true. Thus, for 

 example, the velocity of the molecules of hydrogen is known 

 to be four times as great as that of the molecules of oxygen; 

 so the measured velocity of sound in hydrogen is four times 

 its measured velocity in oxygen. 



10. The velocity of sound in a gas is, as was to be expected 

 from the reasons before referred to, a certain fixed proportion 

 slower than the normal velocity of the molecules of the gas. 

 Thus the velocity of the molecules of hydrogen at 0° C. is 

 6050 feet per second, whereas the velocity of sound in hydro- 

 gen at 0° C. is 4164 feet per second. The constant ratio ex- 

 pressing the relation between the velocity of sound in a gas 

 and the velocity of the molecules of the gas is given by the 

 number 0'688 very nearly; or the velocity of sound in a gas is 

 0*688 time the velocity of the molecules of the gas ; so that 

 the velocity of sound in a gas maybe simply got by multiply- 

 ing the known velocity of its molecules by this constant. It 

 would seem probable that by taking into account the oblique 

 motions of the molecules in their collisions along the line of 

 propagation of the wave at all conceivable angles, by a system 

 of averages, the absolute value of the velocity of sound in a 

 gas could be determined independently as an a priori problem, 

 direct from the molecular velocity, by mathematics *. 



11. The fact that the velocity of sound in a gas is simply pro- 

 portional to the velocity of its molecules cannot surely but be 

 regarded as a far more simple and satisfactory physical con- 

 dition governing and determining the velocity of sound than 

 the vague idea of " elasticity," or (as assumed) that the velo- 

 city of sound in a gas is proportional to its " elasticity." The 

 definite physical conception of the velocity of the molecules 

 themselves which by their interchange of motion propagate 

 the wave, is surely far preferable to the vague idea of " elasti- 

 city " governing the velocity of the wave. 



12. Velocity unaffected by Density or Pressure. — According 

 to the principles involved in the kinetic theory, therefore, the 

 velocity of sound in a gas is dependent on nothing else but 

 the velocity of its molecules ; or, whatever conceivable condi- 

 tions the gas may be subjected to (such as change of density 



* See Postscript (2). 



