384 Prof. Tyndall on Laplace's Correction 



himself better acquainted with the path they are of themselves 

 pursuing, and with the progress they have made therein, when 

 he may possibly discover that, though less far advanced than is 

 perhaps desirable, they are nevertheless not quite so ignorant of 

 thermo-molecular physics as he now seems to imagine. 

 I am, Gentlemen, 



Your obedient Servant, 



William Odling. 



P.S. — In your review of Mr. Watts's 'Dictionary of Che- 

 mistry/ after praising my article on atomic weights, for which 

 accept my best thanks, you say that you do not agree with my 

 conclusion that Gerhardt's scale of atomic weights affords a satis- 

 factory representation of the present state of knowledge, but 

 that, on the contrary, a consistent application of the spirit of 

 Gerhardt's reforms would lead us now to go much further than 

 he did. My conclusion certainly was, when writing the article 

 in question, that Gerhardt's system, with a slight extension, 

 afforded a tolerably satisfactory representation of the then state 

 of knowledge. But I quite agree with you that, in consequence 

 of researches made more particularly within the last two or three 

 years, a much further extension in the direction advocated by 

 Messrs. Cannizzaro and Wurtz is now required, which extension I 

 am to have an opportunity of discussing in some future Number 

 of the Dictionary. — W. 0. 



LV. Note on Laplace's Correction for the Velocity of Sound. 

 By John Tyndall, F.R.S.* 



NEWTON calculated the theoretic velocity of sound in air, 

 and found that it differed from the observed velocity by 

 about one-sixth of the whole amount. Laplace accounted for 

 this discrepancy by showing that over and above the changes of 

 elasticity due to changes of density, of which alone Newton took 

 account, the effective elasticity of the medium is augmented by 

 the changes of temperature which occur in the condensations and 

 rarefactions of the sonorous wave. He succeeded in proving 

 that, to obtain the true velocity of sound in air, Newton's result 

 would have to be multiplied by the square root of the quotient 

 obtained by dividing the specific heat of air at constant pressure 

 by its specific heat at constant volume. This ratio was not at 

 the command of Laplace ; but in the absence of experiment, and 

 knowing that the velocity of sound in air had been determined 

 with extreme accuracy, he reversed the process, and deduced from 



* Communicated by the Author. 



