between Metallic Masses having different Temperatures. 457 



Consequently this amount, or d a, will be proportional to 



f ( 0). K. dt 



E being the measure of expansibility, c the capacity for heat of 

 the substance, taken by volume not by weight. 



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

 ducting 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 should expand more 

 than the hot one, otherwise the loss of elevation by the contrac- 

 tion of the warm metal will equal or exceed the vantage-ground 

 for the new vibration gained by the expansion of the cold one. 

 By both these criteria Mr FARADAY'S theory seems to be defi- 

 cient : 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 hy- 

 pothesis 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 de- 

 cisive, 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 ac- 

 knowledged 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 



