between Metallic Masses having different Temperatures. 191 



57. Hence it appears to be quite obvious, that as far as 

 conducting power is concerned, both bar and block should have 

 it in the highest possible degree. It would be quite essential, 

 too, upon this explanation, that the cold metal shoidd expand 

 more than the hot one, otherwise the loss of elevation by the 

 contraction of the warm metal will equal or exceed the van- 

 tage-ground for the new vibration gained by the expansion of 

 the cold one. By both these criteria Mr. Faraday's theory 

 seems to be deficient: I need only point out the position of 

 zinc, which, with greater expansibility than lead or tin, occupies 

 so high a place in the list of vibrators, and cannot be used as 

 the cold metal with any other except silver: according to the 

 theory, zinc ought to vibrate far better upon zinc than upon 

 lead or tin. Silver, again, vibrates upon cold iron, although 

 its expansibility is a half greater. Such facts as these seem 

 absolutely unaccountable upon the hypothesis of expansion. 



58. The objections which I took in limine to the explana- 

 tion of Sir John Leslie and Mr. Faraday, (which was adopted 

 by Mr. Trevelyan in the paper printed in the Edinburgh 

 Transactions,) were strengthened, and I may say rendered 

 decisive, by my subsequent experiments, the results of which 

 have been detailed in a previous part of this paper. For 

 nearly two years I have been constantly expecting to see some 

 systematic examination of these curious facts; but the public 

 seems to have rested satisfied with the ascription of them to a 

 simple and acknowledged effect of heat. They have hardly 

 been noticed in the Journals, and foreigners complain of the 

 few data afforded by English works on the subject. An article 

 by Professor M'uncke of Heidelberg, in Poggendorff 's Amialen*, 

 consists chiefly of the translation of a very brief notice, which 

 I had published in the Edinburgh Journal of Science f, but 

 contains no new observations. Having shown the reasons 

 which led me to dissent from the opinions at first proposed, 

 I shall now explain the views which I have been led to enter- 

 tain from the study of the phenomena. 



.59. I shall first recapitulate the general laws at which we 

 have arrived. 



1st, The vibrations never take place between substances of 

 the same nature. Art. 13. 



2nd, Both substances must be metallic. Art. 14. 



'5rd, The vibrations take place with an intensity propor- 

 tional (within certain limits) to the difference of the con- 

 ducting powers of the metals for heat (or electricity), the 

 metal having the least conducting power being necessarily 

 the coldest. Art. 80. 



• ]ft3,>, No. III., |>. 466. f New Scries, No. XI. 



