436 Prof. FORBES on the Vibrations which take place 



the cold block ; and it was also natural to suppose that metals 

 excluded from the one class belonged to the other. I have, 

 however, discovered, that at least two metals are perfectly inert, 

 in either situation, namely antimony and bismuth. 



15. At an early period I found (as Mr TREVELYAN also did 

 independently), that the position of the bar and block were con- 

 vertible ; I mean, that the metal commonly used for the block 

 might be employed for the bar, and vice versa, provided always, 

 that the same metal was always hotter or colder (as experience 

 showed to be necessary) than the other. Thus, Mr TREVELYAN 

 announced that a bar of hot iron or copper placed on a block of 

 cold lead or tin, produced vibrations ; but we shall still have the 

 same phenomena, provided we use a cold bar of lead or tin placed 

 upon a hot block of iron or copper. 



16. Mr TREVELYAN having observed that lead and tin were 

 the metals which required to be cold, and that metals which he 

 designates as " hard," such as iron and copper, must be hot, he 

 naturally draws a division between two classes of metals quite 

 distinct, each of which require certain conditions to produce the 

 vibrations. Mr FARADAY having taken up the subject, found 

 that hot silver vibrated on cold iron, a fact observed by silver- 

 smiths, thus forming a link between the classes, and shewing 

 that a metal which requires to be cold relatively to a second 

 metal, must be hot relatively to a third. Some theoretical views 

 which we shall presently advert to, and to which experience did 

 not seem to be opposed, led Mr FARADAY to the conclusion that 

 the arrangement of the metals with regard to their power of vi- 

 brating with one another, was directly as their conducting power 

 for heat, and inversely as their expansibility. The metal stand- 

 ing highest on the scale of metals thus formed, being necessarily 

 the hot one relatively to the other, which stood lower on the 

 same scale. These observations of Mr FARADAY, were given in 



