between Metallic Masses having different Temperatures. 435 



cularly as to their distinctness and duration ; a little practice is 

 required to distinguish between the mere mechanical oscillations 

 of a bar once made to vibrate, and a true vibration depending 

 on an impulse received at each successive contact. There is, in 

 general, this difference, even where the sustaining power is very 

 feeble, that whereas the oscillations arising merely from gravity 

 rapidly diminish from the very first, if there be a true sustaining 

 power, they will rather increase in energy and distinctness for 

 some time, from the accumulated effect of successive though 

 small impulses. 



12. We proceed to examine the influence of contingent cir- 

 cumstances upon the intensity of these vibrations. 



1 . Relation to the Specific Characters of the Substances employed. 



13. The first general law of these vibrations may be consi- 

 dered to be, that they never lake place between substances of the 

 same nature. This is probably quite general. An exception is 

 noticed by Mr TREVELYAN, who thinks that he observed a vibra- 

 tion of a copper bar upon a copper block ; I am inclined to think 

 that he had mistaken the oscillations connected with the simple 

 law of gravity for a true vibration, as I have in vain endeavour- 

 ed to repeat the experiment. 



14. The second general law is, that both substances must be 

 metallic. I have never seen a single exception to this law * ; Mr 

 TREVELYAN, however, thinks that he observed a vibration upon 

 glass. It is a remarkable fact, substantiated by experiments which 

 will presently be mentioned, that all metals do not possess the 

 property alluded to. It was natural to divide the metals into 

 two classes, one of which might form the heated bar, the other, 



* I need hardly state it as an exception that I have used galena (the sulphuret 

 of lead) as a block, with success, instead of metallic lead. 



VOL. XII. PART II. 3 K 



