304 Royal Society of Edinburgh. 



the inquiry as soon as the remarkable fact *.vas announced by Mr. Tre- 

 velyan, and was induced to prosecute it to a considerable extent ex- 

 perimentally, in consequence of being dissatisfied with the only plau- 

 sible explanation yet offered, — that of the successive expansions of 

 the cold metal by the hot one at the point of contact, at each succes- 

 sive vibration, which was conceived to afford the necessary impulse 

 or maintaining power. 



In this paper the phenomena of sound are first discussed, which, 

 with Mr. Faraday, the author imputes solely to the number of vibra- 

 tions taking place in a given space of time. This seems completely 

 proved by observation j and the note depends upon the frequency of 

 the oscillations, which have been observed as high as between 700 

 and 800 in a second, and must often be greatly more frequent. The 

 phenomena of vibration are next considered, as affected by the nature 

 of the metals, by the form of the masses, and by temperature. The 

 order of the metals as vibrators is the following,— meaning that the 

 cold metal must always stand lower in the list than the hot one, and 

 that the force or intensity of vibration is, generally speaking, propor- 

 tional to the space intervening between two metals on the list : — 

 silver, copper, gold, zinc, brass, platinum, iron, tin, lead, antimony, 

 bismuth. Antimony and bismuth are placed at the bottom of the list, 

 because no other metal is capable, under any circumstances which 

 have been examined, of producing vibrations in conjunction with those 

 two metals : they are the only metals yet observed which, when heat- 

 ed, do not vibrate on cold lead. 



From experiments detailed at considerable length in the paper, the 

 author is led to the following practical conclusions, which, whatever 

 may be the fate of the hypothesis which he is disposed to found upon 

 them, will, he conceives, be viewed as valuable in themselves :— 

 1 . As far as has been observed, the vibrations never take place be- 

 tween substances of the same nature. 2. Both substances must be 

 metallic. 3. The vibrations take place with an intensity proportional, 

 within certain limits, to the difference of the conducting powers of the 

 metals for heat (or electricity), the metal having the least conduct- 

 ing power being necessarily the coldest. 4. The time of contact of 

 two points of the metals (between which the oscillations take place) 

 must be longer than that of the intermediate portions. 5. The im- 

 pulse is received by a distinct and separate process at each contact 

 of the bar with the block, and in no case is the connexion of these 

 points in any way essential. 6. The intensity of vibration is (with 

 certain exceptions) proportional to the difference of temperature of 

 the two metals. 



From these data the author first endeavours to show that the hy- 

 pothesis of expansion is untenable, by tracing closely the process of 

 communication of heat, and proving that it must lead to several con- 

 clusions totally at variance with experiment, and particularly that, as 

 far as conducting power for heat is concerned, both the hot and the 

 cold metal should possess it in the highest degree. The author is led 

 by the striking analogy of the powerful repulsive action of electricity 

 in passing from a good to a bad conductor, to infer a similar pro- 



