174 ANNUAL OF SCIENTIFIC DISCOVERY. 



greater ease and certainty than on rock salt. In most cases a high 

 temperature is necessary to the production of the tones ; but in the case of 

 rock salt, the temperature need riot exceed that of the blood. A new and 

 singular property is thus found to belong to this already remarkable 

 substance. It is needless to enter into a full statement regarding the 

 various minerals submitted to experiment. Upwards of twenty non- 

 metallic substances had been examined by the lecturer, and distinct 

 vibrations obtained with every one of them. The number of exceptions 

 here exhibited far exceeds that of the substances which are mentioned in 

 the paper of Professor Forbes, and are, it was imagined, sufficient to show 

 that the second general law is untenable. The third general law states, 

 that "The vibrations take place with an intensity 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 differ- 

 ence, 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 Professor Forbes ; the lecturer, however, 

 had obtained musical tones from both of these substances. The superiority 

 of lead as a cold block, Professor Faraday, as already stated, referred to 

 its high expansibility, combined with its deficient conducting power. 

 Against this notion, which he considers 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 between 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 rocker 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 



