708 Proceedings of Royal Society of Edinburgh. [sess. 
quickening, and the other kind through a slowing down of the 
motion of these same particles, one would think that as the 
molecules are all in contact the motion should spread with equal 
facility through bodies of all kinds, generally speaking ; at least 
that there should not he the enormous differences that there 
actually are without something special to account for them. And 
there is something very special indeed, and complicated, hut, 
broadly speaking, very simple though complicated in its effects. 
The whole secret lies in the difference of speed of the particles of 
different bodies. When bodies in motion are placed in contact 
the exchange of motion between them will depend upon the 
difference between the motion of the one and that of the other, 
if their motion is both alike, each will receive from the other as 
much as it gives, and there is nothing lost or gained by either, 
but if their motion is greatly different the one will lose a great 
deal and the other gain as much. As a partial confirmation of the 
above, viz., that the secret lies in the difference of particle-speed, 
I appeal to the broad fact that the greater the atomic volume 
compared with weight of bodies the less will those bodies conduct 
both of heat and electricity. Now the atomic volume of my 
hypothetical atoms depends entirely upon the speed of the 
particles, and upon the number of particles (the atomic weight) 
upon which that speed is impressed. For, granting the same 
speed to be impressed upon different quantities of ethereal 
particles, the radii of the resulting bubbles would be proportional 
to the cube roots of the atomic weights, and, as I have already 
pointed out, the volume would be proportional to the square of 
the speed multiplied by the number of particles. But there is 
another and more striking fact to which I appeal for confirmation 
of my idea that difference of particle-speed is the secret cause 
of the difference in the conducting powers of bodies. And that 
fact is that the greater their radiating powers for heat the less is 
their conducting powers for electricity and also for heat. The 
latter fact may seem to follow from the former, for what is 
radiated cannot be conducted ; but wait a little. It is true that 
conducting bodies the more they are heated conduct electricity 
worse, but this is not true of insulators. Heat helps their 
conducting powers. Now, it is evident that in differently 
