of Edinburgh, Session 1879-80. 705 
stretched thread seems to he capable of rendering all kinds of vibra- 
tions as well as those that come normal to its surface. 
I also tried if heat or cold on the sounding wires would alter their 
powers. I endeavoured to apply ice, but unsuccessfully. It was 
difficult to devise an experiment in which it could be applied and 
removed so quickly as to produce a contrast. It is also probable 
that the difference of temperature between that of ordinary air and 
the freezing point may hardly be great enough to become sensible to 
the ear. With the Bunsen lamp it was, however, different. It can 
be easily applied and easily withdrawn. I clamped between two 
binding screws about 2J inches of No. 25 platinum wire. I 
attached a thinner platinum wire to it, which acted as the thread 
of a parchment telephone pulled transversely. On applying the 
lamp the sound became sensibly louder and remained so at a white 
heat. On cooling it again fell off. In the same manner I listened 
to the effect of a No. 18 soft iron wire. There was a singular in- 
crease of loudness just up to the point below which iron became 
visibly hot, then there was a decided falling off as the wire reached 
a full red, but it still continued sounding so far as could be judged 
under the loudness that it had before the lamp was applied. When 
the lamp was withdrawn the sounds waxed and grew less in reverse 
order. When the telephone threads were wires of copper and iron 
the same was observed. When the sound begins to diminish the 
iron is quite hard, so it is not due to the softening of the iron, 
which may to some extent account for the falling off at a white heat. 
In results like these it is possible to discuss the question of mole- 
cular impact versus expansion, on a new footing. In the case of the 
platinum the result is much as one would expect, for the rate of in- 
crease of its electric resistance and of its expansion are generally 
allowed to increase at high temperatures. The increase of sound 
may thus be associated with the one as well as with the other. Iron, 
however, is exceptional. De la Bive thought that the sounds of 
wires were in proportion to electric resistance except in the case of 
iron which stood quite by itself. Here, again, in reference to high 
temperatures it is quite peculiar. This Society has more than once 
learned from Professor Tait, and those who have worked with him, 
of the critical temperature of iron about a dull red heat, in reference 
to the specific heat of electricity, thermal and electric conductivity, 
