Prof. Challis on the Theory of the Velocity of Sound. 4:07 



sudden condensation of the air, I suggested that as experiment 

 only showed that the effect is to raise the temperature when the 

 developed heat acts on a ver)^ limited portion of air, we are not 

 justified in supposing the same effect to take place when the air 

 is unlimited; and that the developed heat, being in the first 

 moment of its generation in the state of radiant heat, and being 

 allowed to radiate indefinitely, does not sensibly change the tem- 

 perature of the air at the position where it is generated. This 

 is the supposition which Professor Stokes alludes to in the April 

 Number of the Philosophical Magazine, p. 306. 



As some advocates of Laplace's theory are of opinion that that 

 theoiy assigns a vera causa for the excess of the velocity of sound 

 above Newton's value, in refutation of that opinion I appeal to 

 Laplace's exposition of his own views. It is clear that he thought 

 it necessary to establish the theory upon certain laws of the 

 action of caloric on the atoms of matter, of the atoms of caloric 

 on each other, and of the relations of free and latent heat. But 

 in the existing state of our knowledge of the theoiy of heat, these 

 laws can only be regarded as hypothetical. The supporters, 

 however, of Laplace' s'theory, instead of referring to these views, 

 have substituted hypotheses of a difi"erent kind, leading to the 

 same results. In the article by Professor Stokes already referred 

 to (p. 306), these hypotheses are introduced in the following 

 terms : " That in the case of small sudden condensations (positive 

 or negative) the increase of temperature is ultimately propor- 

 tional, cteteris paribus, to the condensation, will not, it is presumed, 

 be called in question." In this sentence there are involved 

 three distinct and unsupported hypotheses : first, that there is 

 increase of temperature in fluid of unlimited extent, experiment 

 only proving that this is the case when the fluid is confined 

 within narrow limits; secondly, that this increment of temperature 

 is in exact proportion to the increment of density ; thirdly, that 

 the increments of temperature are simultaneous with the gene- 

 ration of the increments of heat by which they are produced, 

 whereas all analogy would lead us to expect that time must elapse 

 between the effect "and the generation of the cause producing the 

 eflFect. For these reasons I assert that Laplace's theoiy, m what- 

 ever way it be viewed, rests on hypotheses. 



It is unnecessaiy for me to make any remark on the investi- 

 gation by which Professor Stokes determines the effect of the 

 radiation of lieat on the propagation of sound, because that in- 

 vestigation proceeds on the hyi)otliesis of that very increase of 

 temperature, the reality of wliich it has l)een the purport of the 

 foreiroing observations to call in question, and the object of it is 

 to calculate the effect of radiation due to such increase of tem- 

 perature. 



