38 
Proceedings of Boyal Society of Edinburgh. [sess. 
surface, and only then would its aphelion distance begin to shorten, 
when, after a greater or less number of revolutions, grazing his 
surface at every return, its orbit would become circular once more, 
just before being absorbed by the sun. In all the foregoing pro- 
cess the planet, before grazing the sun, would be gaining kinetic 
energy, but at the expense of potential. In other words, it would 
be gaining speed, but at the expense of position, for it would be 
all the time drawing nearer and nearer to the sun ; and although 
its speed would be increased, yet the sun’s hold on it would be 
increased in the same degree, until, as already described, it would 
become ultimately absorbed. Well, now, what would be the 
difference between our hypothetical planet and a revolving par- 
ticle ? The main difference would be in one profound particular,, 
and that is, that in the case of the planet, as the speed increased, 
the centripetal force increased in the same degree, but in the case 
of our particle no such increase of centripetal force would take 
place, for there is no central sun for it ; and supposing our particle 
to be placed near the centre of the bubble, exposed to the full 
pressure of the outside ether, it is quite clear that the pressure 
would be exactly the same in that position as elsewhere. Well, 
then, in the case of the planet, as before said, as the centrifugal 
tendency increased, so would the centripetal, so that it could not 
escape from the sun, but in the case of our particle, no increase of 
centripetal force would take place, and owing to its increased 
centrifugal tendency, induced by increase of speed, it would regain 
its former position, with its former speed, with this difference — 
that its orbit would thenceforth change from a perfectly circular 
to a slightly elliptical one, to go through the same course again,, 
and so on ad infinitum. 
Having now, as I hope, shown by sound analogy that the orbital 
motion of these particles might go on for ever, or so long as the 
pressure existed, we may now inquire into the relative sizes and 
consistencies of the bubbles, or atoms. It is not for me to say 
how bubbles of this kind could originate, but we may suppose' 
that a great number of vortical motions, of various degrees of 
energy, were impressed on the ether; each vortex then would 
expand into spherical bubbles of various sizes, and quantities of 
ethereal particles, according to their various energies and the: 
