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



tendency certainly is to diminish that portion of the elastic force 

 on which the correction of Laplace depends ; and hence the velo- 

 city of sound in such a medium ought to approximate more to 

 that deduced from the formula of Newton than its velocity in 

 air. It is hard to conceive that this effect could be prevented 

 by the rapidity of the vibrations. From the moment any mole- 

 cule receives, by the act of condensation, an accession to its calo- 

 rific motion, that motion is in part wasted upon the sether in 

 which the molecule swings. No conceivable rapidity of sono- 

 rous vibration can, I think, prevent this loss from taking place ; 

 and if it take place in any sensible degree, the correction of 

 Laplace would be no longer applicable, nor could a correct ratio 

 be deduced from the velocity of sound in the medium. 



The necessary data exist for the determination of the theoretic 

 velocity of sound in olefiant and other gases, and from its pitch 

 in organ-pipes its actual velocity in any gas may be deduced. 

 We are thus in a position to compare the actual and theoretical 

 velocities, and to deduce from them the ratio of the two specific 

 heats. If then the radiative power of the gas comes sensibly 

 into play,- it will diminish the actual velocity, and therefore 

 make the ratio of the two specific heats appear less than in the 

 case of air. Conversely, if the ratio for the gas be less than that 

 found for air, it would show that a partial equalization of tem- 

 perature between the condensed and rarefied portions of the 

 sonorous wave had been effected by the radiative power of the gas. 



In the state indicated by this reasoning, the question rested 

 in my mind for a considerable time ; and it is not entirely without 

 diffidence that I now bring it forward. I had thought of an ap- 

 paratus with a view of testing the question, forgetting that the 

 ratio of the two specific heats had been determined in this very 

 way by a celebrated experimenter. Dulong, it is well known, pur- 

 sued this very method in the case of seven gases, three of which 

 were elementary, and four compound*. For the elementary gases 

 he found a ratio which was sensibly the same as that found for air ; 

 this, according to my reasoning, would prove the absence of 

 radiative power on the part of the elementary gases, — a result in 

 exact accordance with my experiments. For the compound gases 

 he found ratios sensibly less than that found for air, and least of 

 all for olefiant gas, which, according to my experiments, is the 

 greatest radiator. In the case of this gas the excess of the 

 ratio above unity is little more than half what it is in the case of 

 air. Dulong of course drew from his experiments conclusions 

 very different from those to which my reasoning points, and which, 

 I submit, need reconsideration. In this state for the present 

 I leave the question to the appreciation of philosophers. Apart 

 * Ann. de Chim. et de Phys. vol. xli. ; and Pogg. Ann. vol. xvi. 



