236 Royal Institution. 



with an intensify proportional {within certain limits) to the difference 

 of the conducting powers of the metals for heat, the metal having 

 the least conducting power being necessarily the coldest." The 

 evidence adduced against the first law appears to destroy this one 

 also ; for if the intensity of the vibrations be proportional to the 

 difference of the conducting powers, then where there is no such 

 difference there ought to be no vibrations. But it has been proved 

 in half a dozen cases, that vibrations occur between different pieces 

 of the same metal. The condition stated by Professor Forbes was, 

 however, reversed. Silver stands at the head of conductors ; a strip 

 of the metal was fixed in a vice, and hot rockers of brass, copper, 

 and iron were successively laid upon its edge : distinct vibrations 

 were obtained with all of them. Vibrations were also obtained with 

 a brass rocker which rested on the edge of a half-sovereign. These 

 and other experiments show that it is not necessary that the worst 

 conductor should be the cold metal, as affirmed in the third general 

 law above quoted. Among the metals, antimony and bismuth were 

 found perfectly inert by Pi-ofessor Forbes ; the lecturer, however, 

 had obtained musical tones from both of these substances. 



The superiorit)' of lead as a cold block. Professor Faraday, as 

 already stated, referred to its high expansibilitj'', combined with its 

 deficient conducting power. Against this notion, which he con- 

 siders to be " an obvious oversight," Professor Forbes contends in 

 an ingenious and apparently unanswerable manner. The vibrations, 

 he urges, depend upon the difference of temperature existing be- 

 tween the rocker and the block ; if the latter be a bad conductor 

 and retain the heat at its surface, the tendency is to bring both the 

 surfaces in contact to the same temperature, and thus to stop the 

 vibration instead of exalting it. Further, the greater the quantity 

 of heat transmitted from the rock to the block during contact, the 

 greater must be the expansion ; and hence, if the vibrations be due 

 to this cause, the effect must be a maximum when the block is the 

 best conductor possible. But Professor Forbes, in this argument, 

 seems to have used the term expansion in two different senses. The 

 expansion which produces the vibration is the sudden upheaval of 

 the point where the hot rocker comes in contact with the cold mass 

 underneath ; but the expansion due to good conduction would be an 

 expansion of the general mass. Imagine the conductive power of 

 the block to be infinite, that is to say, that the heat imparted by the 

 rocker is instantly diffused equally throughout the block ; then, 

 though the general expansion might be very great, the local expan- 

 sion at the point of contact would be wanting, and no vibrations 

 would be possible. The inevitable consequences of good conduction 

 is to cause a sudden abstraction of the heat from the point of con- 

 tact of the rocker with the substance underneath, and this the lec- 

 turer conceived to be the jirecise reason why Professor Forbes had 

 failed to obtain vibrations when the cold metal was a good conductor. 

 He made use of blocks, and the abstraction of heat from the place of 

 contact by the circumjacent mass of metal was so sudden as to ex- 

 tinguish the local elevation on which the vibration^ depend. In the 



