358 NOTICES OF THE MEETINGS [Jan. 27, 
copper, and brass, placed upon the natural edge of a prism of rock- 
erystal, gave distinct tones; on the clean edge of a cube of fluor 
spar, the tones were still more musical; on a mass of rock-salt the 
vibrations were very forcible. There is scarcely a substance, metallic 
or non-metallic, on which vibrations can be obtained with greater 
ease and certainty than on rock-salt. In most cases a high tempera- 
ture is necessary to the production of the tones, but in the case of rock- 
salt the temperature need not 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 is, it was imagined, sufficient to shew 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 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 ob- 
tained with all of them. Vibrations were also obtained with a brass 
rocker which rested on the edge of a haif-sovereign. These and 
other experiments shew that it is not necessary that the worst con- 
ductor 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 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 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 
