the Contact of Bodies having different Temperatures, 11 



ductivity which the mass probably possesses in the direction of 

 the said cleavage might aid in the production of the vibrations. 

 I cut such a piece from a mass of antimony_, and fixed it in a 

 vice, so that the horizontal edge on which the large brass rocker 

 rested was perpendicular to the surfaces of principal cleavage. 

 Loud and sustained vibrations were the consequence. I repeated 

 the experiment in the case of bismuth with equal success ; and, 

 after a little practice, found that the precaution of cutting the 

 substances in the manner just described was wholly unnecessary, 

 and that tones could be obtained with facility, no matter what 

 might be the direction in which the mass was cut. 



We have thus proved antimony and bismuth to be active in 

 one position at least ; but antimony is active both as rocker and 

 bearer. Two irregular masses, the one weighing about a pound 

 and the other five pounds, were so filed down as to present 

 suitable surfaces for rocking. Heated, and placed upon a flat 

 mass of lead, both masses vibrated permanently. These experi- 

 ments add their evidence to that already adduced against the 

 third law; for antimony is a worse conductor than lead, and 

 antimony is here the hottest metal. 



These results appear to leave the theory of Professor Forbes 

 without any foundation. One point only remains to be con- 

 sidered. Professor Faraday attributes the superiority of lead as 

 a bearer to its great expansibility by heat, combined with its 

 deficient conducting power. Against this view Professor Forbes 

 argues in the ingenious manner already described. It cannot be 

 denied that when the supporting metal is a good conductor, a 

 greater quantity of heat will pass into it, during contact, than 

 when it is a bad one. It cannot be denied that the greater the 

 quantity of heat transmitted, the greater will be the expansion ; 

 and hence the conclusion seems unavoidable, that, if the vibra- 

 tion be due to expansion, both rocker and bearer, other things 

 being equal, ought to possess the power of conduction in the 

 highest possible degree. 



Assuming then that the efiects are produced by ordinary ex- 

 pansion, the argument of Professor Forbes, stated in its severe 

 logical form, would be as follows :— 



The greater t'le expansion the greater will be the efi*ect ; but, 



The greater the conducting power the greater is the expan- 

 sion : therefore. 



The greater the conducting power the greater will be the 

 effect. 



This, to all appearance, is conclusive. A slight inadvertence, 

 however, in the use of the term " expansion ^^ appears to deprive 

 the argument of much of its force. In the first proposition the 

 term means expansion in a vertical direction ; for if this be not 



