for the Velocity of Sound. 385 



the velocity of sound the ratio of the two specific heats. It 

 was found to be 1*421. 



We now know that the excess of this number above unity, 

 namely 0*421, expresses the amount of heat consumed in external 

 work when the air is allowed to expand under constant pressure ; 

 and from this number we can at once deduce the mechanical 

 equivalent of heat. The almost absolute identity of the equiva- 

 lent thus found, with that established by Mr. Joule by direct 

 experiment, leaves no shadow of doubt upon the mind as to the 

 correctness of Laplace's result. 



Still, notwithstanding this striking verification, and the great- 

 ness of the names connected with this question, I have for some 

 years thought it probable that the legitimacy of Laplace's pro- 

 ceeding in calculating the ratio of the two specific heats depends 

 upon an accident. At the time he wrote, and for a long time 

 subsequently, it was universally believed that elastic fluids pos- 

 sessed no sensible radiative power; and the practical absence 

 of this power is virtually assumed in his calculation. He sup- 

 posed that the heat and the cold produced their full effect in 

 changing the elasticity of the air; that there was no loss of 

 heat either by radiation or conduction ; and he would at the time 

 have extended the same assumption to elastic fluids generally, 

 and deduced from any one of them the ratio of the two specific 

 heats. 



The point to which I now wish to direct attention is whether, 

 in the present state of our knowledge, this extension of the 

 assumption is warranted; and whether the correctness of La- 

 place's result may not depend upon a peculiarity of air unproved 

 in his day, but which distinguishes it from most other elastic 

 fluids. 



My experiments taught me some years ago, and every addi- 

 tional day's experience only confirms the result, that air pos- 

 sesses no sensible power of absorption or radiation ; hence a con- 

 densation in air, in presence of its associated rarefaction, has no 

 sensible power to neutralize, by radiation, the differences of tem- 

 perature in the condensed and dilated portions of the sonorous 

 wave. But while air is thus neutral, other gases, ammonia and 

 olefiant gas for example, show themselves competent to absorb 

 80 or 90 per cent, of the entire radiation from an obscure source ; 

 and they possess a radiative power proportionate to their enormous 

 absorbing power. Let us imagine then a series of sonorous waves 

 propagated through an atmosphere of olefiant gas ; every heated 

 condensation is a powerful radiator, and every chilled dilatation 

 is a powerful absorber. These two portions of a wave cannot, I 

 think, subsist for an instant in the presence of each other with- 

 out loss of heat on the one hand, and gain on the other. The 



