REPLIES TO THE QUARTERLY REVIEWERS. 547 



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hypothesis which enabled Laplace to correct Newton's estimate of 



the velocity of sound — the hypothesis that heat is evolved by the 

 compression each sound-wave produces in the air ? Does he deny that 

 the heat so generated is at the expense of so much wave-motion lost ? 

 Does he question the inference that some of the motion embodied in 

 each wave is from instant to instant dissipated, partly in this way 

 and partly in the heat evolved by fluid friction ? Can he show any 

 reason for doubting that, when the sound-waves have become too fee- 

 ble to affect our senses, their motion still continues to undergo this 

 transformation and diminution until it is all lost ? If not, why does 

 he implicitly deny that the molar motion constituting sound eventu- 

 ally disappears in producing the molecular motion constituting heat ? ' 

 I will dwell no longer on the exclusively-personal questions raised 

 by the reviewer's statements, but, leaving the reader to judge of the 

 rest of my " stupendous mistakes " by the one I have dealt with, I 

 will turn to a question worthy to occupy some space, as having an 

 impersonal interest — the question, namely, respecting the nature of 

 the warrant we have for asserting ultimate physical truths. The con- 

 tempt which, as a physicist, the reviewer expresses for the metaphysi- 

 cal exploration of physical ideas, I will pass over with the remark that 

 every physical question, probed to the bottom, ends in a metaphysical 

 one, and that I should have thought the controversy now going on 

 among chemists, respecting the legitimacy of the atomic hypothesis, 

 might have shown him as much. On his erroneous statement that I 

 use the phrase " Persistence of Force " as an equivalent for the now- 

 generally-accepted phrase " Conservation of Energy," I will observe 

 only that, had he not been in so great a hurry to find inconsistencies, 

 he would have seen why, for the purposes of my argument, I inten- 

 tionally use the word Force : Force being the generic word, including 

 both that species known as Energy, and that species by which Matter 

 occupies space and maintains its integrity — a species which, whatever 

 may be its relation to Energy, and however clearly recognized as a 



1 Only after the foregoing paragraphs were written, did the remark of a distinguished 

 friend show me how certain words were misconstrued by the reviewer in a way that had 

 never occurred to me as possible. In the passage referred to, I have said that sound- 

 waves " finally die away in generating thermal undulations that radiate into space;" 

 meaning, of course, that the force embodied in the sound-waves is finally exhausted in 

 generating thermal undulations. In common speech, the dying away of a prolonged 

 sound, as that of a church-bell, includes its gradual diminution as well as its final cessa- 

 tion. But, rather than suppose I gave to the words this ordinary meaning, the reviewer 

 supposes me to believe, not simply that the longitudinal waves of air can pass, without 

 discontinuity, into the transverse waves of ether, but he also debits me with the belief that 

 the one order of waves, having lengths measurable in feet, and rates expressed in hun- 

 dreds per second, can by mere enfeeblement pass into the other order of waves, having 

 lengths of some fifty thousand to the inch, and rates expressed in many billions per second ! 

 Why he preferred so to interpret my words, and that, too, in the face of contrary impli- 

 cations elsewhere (instance § 100), will, however, be manifest to every one who reads 

 his criticisms. 



