402 Prof. Stokes on flic Communication of Vibration 



hydrogen gas is mixed with common air, it probably docs not 

 intimately combine, but dissipates the pulsatory impressions 

 before the sound is rigorously formed." 



In referring to Leslie's experiment in which a half-exhausted 



receiver is rilled up with hydrogen, Sir John Ilersehel suggests 

 a possible explanation founded on Dalton's hypothesis that every 



gas acts as a vacuum towards every other*. According to this 

 view there is a constant tendency for sound-waves to be propa- 

 gated with different velocities in the air and hydrogen of which 

 the mixture consists, but this tendency is constantly checked by 

 the resistance which one gas opposes to the passage of another, 

 calling into play something analogous to internal friction, whereby 

 the sound- vibration, though at first produced, is rapidly stilled. 

 Air itself indeed is a mixture ; but the velocities of propagation 

 of sound in nitrogen and oxygen are so nearly equal that the 

 effect is supposed not to be sensible in this case. 



This explanation never satisfied me, believing, as I always 

 have done, for reasons which it would take too long here to ex- 

 plain, that for purely hydrodynamical phenomena (such as those 

 of sound) an intimate mixture of gases was equivalent to a single 

 homogeneous medium. I had some idea of repeating the expe- 

 riment, thinking that possibly Leslie might not have allowed 

 sufficient time for the gases to be perfectly mixed (though that 

 did not appear likely), when another explanation occurred to me, 

 which immediately struck me as being in all probability the 

 true one. 



In reading some years ago an investigation of Mr. Karn- 

 shaw's, in which a certain result relating to the propagation of 

 sound in a straight tube was expressed in terms, among other 

 things, of the velocity of propagation, the idea occurred to me 

 that the high velocity of propagation of sound in hydrogen 

 would account for the result of Leslie's experiment, though in a 

 manner altogether different from anything relating to the propa- 

 gation of sound in one dimension only. 



Suppose a person to move his hand to and fro through a small 

 space. The motion which is occasioned in the air is almost 

 exactly the same as it would have been if the air had been an 

 incompressible fluid. There is a mere local reciprocating motion, 

 in which the air immediately in front is pushed forwards, and 

 that immediately behind impelled after the moving body, while 

 in the anterior space generally the air recedes from the encroach- 

 ment of the moving body, and in the posterior space generally 

 flows in from all sides, to supply the vacuum which tends to be 

 created; so that in lateral directions the motion of the fluid is 

 backwards, a portion of the excess of fluid in the front going to 

 * Encyclopaedia Metropolitans vol. iv. art. Sound. § 108. 



