192 Prof. Forbes's Researches on the Vibrations which take place 



4th, The time of contact of two points of the metals must 

 be longer than that of the intermediate portions. Art. 37. 



5th, The impulse is received by a distinct and separate 

 proix s at each contact of the bar with the block, and in no 

 case is the metallic connexion of the bearing points in the 

 bar, or those of the block, in any way essential. Art. 38. 



6th, The intensity of the vibration is (under certain excep- 

 tions) proportional to the difference of temperature of the 

 metals. Art. 47. 



60. In order to satisfy these various conditions, we shall 

 find that the range of hypotheses is not great. During my 

 experiments I was for a long time attached to the idea of a 

 thermo-electric action. The hypotheses which I assumed to 

 explain the steps of it I was forced successively to abandon, 

 and the total want of connexion of the order of the metals as 

 vibrators with their thermo-electric properties (and especially 

 the absolute inertness of antimony and bismuth), convinced 

 me, after a long series of experiments, undertaken with this 

 view, that I was wrong. 



61. The strict and simple connexion with the conducting 

 powers of the metals for heat and electricity afforded a firm 

 basis for speculation, and I was soon forced to consider heat 

 as the sole agent in the case, all idea of electricity being ne- 

 cessarily abandoned, as soon as it was established that thermo- 

 electricity had no share in the action. The general laws 

 above quoted seem to be all resolvable into this, " That there 

 is a repulsive action exercised in the transmission of heat from 

 one body into another, which has a less power of conducting 

 it." These repulsions only take place between bodies having 

 a certain amount of conducting power, below which some 

 metals fall ; it must be excitable in a most minute space of 

 time; and is energetic in proportion to the difference of con- 

 ducting power of the substances, and to their difference of 

 temperature. 



62. It seems most probable, therefore, that the repulsive 

 action alluded to, depends on the internal motions of heat it- 

 self. It were easy to frame a hypothesis which would be suf- 

 ficiently plausible, and represent the phenomena. I forbear, 

 however, from doing it at present, because our ignorance of 

 the internal constitution of bodies, and the mechanical process 

 of the conduction of bodies, is such as to render hypothetical 

 reasoning upon such data almost useless. That repulsion 

 does exist in the case of heat can hardly admit of a doubt. 

 The reason that we cannot render it visible in ordinary cases, 

 is no doubt that the repulsion of the heat in two approximate 

 molecules of bodies is to small too be weighed in our balances. 



