NATURAL PHILOSOPHY. 173 



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 Pro- 

 fessor 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 siibstances 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 en the edge of a sixpence 

 continuous vibrations were obtained. An iron rocker on the edge of a 

 dinner knife gave continuous vibrations. A fiat brass rocker placed upon 

 the points of two common brass pins, and having its handle suitably 

 supported, gave distinct vibrations. In these experiments, 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, zinc on zinc, silver on silver, tin on tin. The list might be 

 extended, but the cases cited are sufficient to show 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 laAV which first attracted the lecturer's 

 attention. During the progress of a kindred inquiry, 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, copper and brass, placed upon the natural 

 edge of a prism of rock-crystal, gave distinct tones; on the clean edge of 

 a ctibe of fiuor 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 



