28 Dr. Charles J. B. Williams oti the Production 



equally necessary for the production and propagation of so- 

 norous vibrations ; for if the elasticity of some molecules be 

 less than that of others, the reaction, being less prompt (4.), 

 will produce vibrations not consentaneous with those of the 

 others, and may impair or even destroy them, and this the 

 more effectually the more irregular and varied these mo- 

 tions are. Hence in bodies of mixed density the vibrations 

 do not continue, and the sound heard is only a stroke or 

 knock. 



Now, to understand more clearly the relative power of dif- 

 ferent conductors with regard to sound, we will take in con- 

 trast the relations of two, which differ greatly, steel and air. 



6. When a piece of the former, freely suspended, is struck, 

 the impulse is propagated through the particles in the manner 

 just described (4.), until it is expended in forcing them into an 

 excursion at the opposite surface : then, their elasticity coming 

 into play will determine their recoil with a similar excursion 

 on the other surface, and then back again, until the disturb- 

 ing force is lost by friction, &c.* The continuance of the 

 vibrations and the production of a tone are here independent 

 of surrounding bodies. In air, on the other hand, one ele- 

 ment of molecular elasticity (2. 4.), attraction, is wanting; 

 hence, after an impulse has been applied to a body of it, this 

 fails to produce a continued tone without the aid of reflecting 

 walls of some denser matter. 



7. Another remarkable difference between air and a sono- 

 rous solid is, that tones of volumes of the former become 

 deeper in proportion to their size; whilst, up to a certain limit, 

 enlarging the bodies of solids increases the rapidity of their 

 vibrations, and therefore heightens their tones. The cause of 

 this difference has been sufficiently investigated with regard 

 to air ; but although the fact is familiar, I have not met with 

 a close examination of the cause of the lowering the tone of a 

 solid by reducing its thickness. The greater proportional re- 

 sistance of the air is not a sufficient reason, for the tone is 

 nearly as much diminished in vacuo ; and the diminished in- 

 ertia of the thinner body would probably be enough to coun- 

 terbalance this influence. The true cause I believe to be, 

 that in solids of small thickness the impulse is not expended 

 on reaching the opposite surface with the vibration proper to 

 the material ; hence the impulse continues to operate, and forces 

 the particles into an increased and therefore prolonged excur- 

 sion (6.), which, by causing further condensation, augments 

 their elastic force, and enables them to overcome the impulse. 



• See Sir John Ilerschei's paper on Absorption, as just referred to. — Edit.] 



