the Contact of Bodies having different Temperatures, 7 



The number of metals capable of this action might, I doubt 

 not, be greatly extended. Thus far we have obtained vibrations 

 with 



Iron upon iron, 



Copper on copper, 



Brass on brass. 



Silver on silver, 



Zinc on zinc. 



Tin on tin ; 



and these, I think, are sufficient to show that the first general 

 law of Professor Forbes does not stand the test of experiment. 



Seebeck indeed had already proved the untenableness of this 

 law. His method of experimenting has been followed in one or 

 two of the cases above described. The placing of the heated 

 rocker upon pointed wires is his idea. Rockings are very readily 

 obtained in this way ; but when tones are required, the sharp 

 edge will, I think, in general be found preferable. 



The second general law of action stated by Professor Forbes 

 is as follows : — 



^' Both substances must be metallic.'* 



This is the case which first excited my attention ; for even 

 granting the final explanation given by Professor Forbes to be 

 the true one, the necessity of the law before us does not at all 

 follow. Previous to entering upon the present subject, I had 

 found that rock-crystal and rock-salt possessed conducting 

 powers not much, if at all, inferior to some of the metals ; and 

 this led me to suppose that either or both of these substances 

 might possibly be made to exhibit the action which the above 

 law restricts to metals. 



My first attempts failed through want of delicacy, as first 

 attempts generally do. But a little practice suggested the means 

 of imparting to the rocker the requisite degree of mobility. 

 Crystals of quartz were cut in such a manner, that wien the 

 rocker was laid upon them, a very slight force was sufficient to 

 cause it to oscillate. By this means I had the satisfaction of 

 obtaining distinct vibrations from a brass rocker placed upon 

 rock-crystal. 



I refrain from entering into a more exact statement of the 

 manner in which the crystals were cut ; for subsequent expe- 

 rience proved that there is no difficulty in obtaining the effect, 

 without any artificial preparation whatever. 



I shall now proceed to describe the results obtained with non- 

 metallic bodies. 



1. Rock-crystal. — The brass rocker already described was 

 heated, and placed upon the natural edge of the prism; the 



