between Metallic Masses having different Temperatures. 193 



Consequently, two bodies equally heated and placed together, 

 manifest no sensible repulsion. In such a case every portion 

 of heat is kept in equilibrium by the equal and opposite re- 

 pulsions of the molecules on each side of it, which is the case 

 when heat is uniformly diffused through a body, and which is 

 manifested by that universal tendency to diffusion. Hence 

 the element of heat is in a state of equilibrium, and the only 

 force which could be excited successfully to produce a sepa- 

 ration, would be between the heat residing in the last mole- 

 cule of one body, and the first of a separate one in contact 

 with it, but not bound to it by cohesive attraction. Suppose, 

 however, this second or free body cooler than the other, a 

 current of heat will be immediately created, which, as it is 

 more or less easily received by the cold body than parted 

 •with by the hot, will create a stagnation, or a rarefaction of 

 the elements of heat, respectively; in the former case pro- 

 ducing a repulsive action, or recoil through the whole string 

 of elements set in motion; in the latter we are led to antici- 

 pate that the action would be attractive. If this view be cor- 

 rect, (and being theoretical, I do not attach great importance 

 to it,) it is easy to see why repulsion takes place only when 

 the cool body has less conducting power than the hot, and 

 why the repulsive energy depends on the difference of these 

 conducting powers. In the case of very bad conductors, such 

 as antimony and bismuth, I conceive that the current has not 

 had time to establish itself. 



63. In the case of electricity, a remarkable similarity of 

 effect is observable, depending on the conducting power of 

 the material through which it passes. All those remarkable 

 repulsive actions which produce destructive effects in the case 

 of lightning, take place during the accumulation of impulses 

 in bad conductors*. 



64. I have been led to entertain the idea of a new species 

 of mechanical agency in heat, not from a love of introducing 

 novel principles, but after having been driven by experiment 

 from the hypotheses to which I was at first entirely attached. 

 Although the mechanical effect of the repulsive power of heat 

 cannot be said ever to have been demonstrated, experiments 

 are not wanting which seem to be quite inexplicable without 

 its aid, or some other principle not yet recognised in science. 



65. Several ingenious French experimentalists have fur- 



* I might point out another analogy in the sudden and forcible action 

 of the hydraulic ram, where the accumulated effect of small impulses pro- 

 duces sudden and intense results, but 1 am afraid of extending unwarrant- 

 ably such speculative analogies. The two preceding paragraphs of this 

 paper have been somewhat modified since it was first read. 

 Third Series. Vol. 4. No. 21. March 1834. 2 C 



