1854.] OF THE ROYAL INSTITUTION. 357 
and not feeling satisfied with the explanation, undertook the further 
examination of the subject; his results are described in a highly 
ingenious paper communicated to the Royal Society of Edinburgh in 
1833. He rejects the explanation supported by Professor Faraday, 
and refers the vibrations to ‘‘ a new species of mechanical agency in 
heat ”—a repulsion exercised by the heat itself on passing from a good 
conductor to a bad one. This conclusion is based upon a number of 
general laws established by Professor Forbes. If these laws be 
correct, then indeed a great step has been taken towards a knowledge 
of the intimate nature of heat itself, and this consideration was the 
Lecturer’s principal stimulus in resuming the examination of the 
subject. 
He had already made some experiments, ignorant that the subject 
had been further treated by Seebeck, until informed of the fact by 
Professor Magnus of Berlin. On reading Seebeck’s interesting 
paper, he found that many of the results which it was his intention 
to seek had been already obtained. The portion of the subject 
which remained untouched was, however, of sufficient interest to 
induce him to prosecute his original intention. 
The general laws of Professor Forbes were submitted in succession 
to an experimental examination. The first of these laws affirms that 
“< the vibrations never take place between substances of the same nature.” 
This the Lecturer found to be generally the case when the hot 
rocker rested upon a block, or on the edge of a thick plate of the same 
metal; but the case was quite altered when a thin plate of metal 
was used. ‘Thus, a copper rocker laid upon the edge of a penny- 
piece did not vibrate permanently ; ,but when the coin was beaten out 
by a hammer, so as to present a thin sharp edge, constant vibrations 
were obtained. A silver rocker resting on the edge of a half-crown 
refused to vibrate permanently; but on the edge of a sixpence con- 
tinuous vibrations were obtained. An iron rocker on the edge of 
a dinner knife gave continuous vibrations. A flat brass rocker 
placed upon the points of two common brass pins, and having its 
handle suitably supported, gave distinct vibrations. In these ex- 
periments the plates and pins were fixed in a vice, and it was found 
that the thinner the plate, within its limits of rigidity, the more 
certain and striking was the effect. Vibrations were thus obtained 
with iron on iron, copper on copper, brass on brass, zine on zinc, 
silver on silver, tin on tin. The list might be extended, but the 
cases cited are sufficient to shew that the proposition above cited 
cannot be regarded as expressing a ‘ general law.” 
The second general law enunciated by Professor Forbes is, that 
“both substances must be metallic.” This is the law which first attracted 
the Lecturer’s attention. During the progress of a kindred enquiry, 
he had discovered that certain non-metallic bodies are endowed with 
powers of conduction far higher than has been hitherto supposed, 
and the thought occurred to him that such bodies might, by suitable 
treatment, be made to supply the place of metals in the production 
of vibrations. This anticipation was realized. Rockers of silver, 
DD 
