mate material agency, to derive mechanical effect from any portion of matter, by cooling it below the temperature of the coldest of the surrounding bodies."
With the present physical constitution of matter this is true, so far as we are yet able to penetrate at least; but since its constitution might have been such as to render such an axiom null, we purpose to inquire into the physical properties of matter which admit of such an enunciation, and the results which in consequence must be predicted for the present universe.
Let us consider a hot spherical body, whose energy we wish to communicate to another equal sphere, absolutely cold, and suppose for a moment that they have a thin outside crust absolutely impermeable to heat, and that the mass of each sphere is concentrated in a single little ball, to whose motion the heat is due, the ball in the cold sphere will of course lie still at the bottom, while the other is flying about at an inconceivable rate; if, now, the two spheres be brought into contact, and an opening be made between them, in a very short time the ball will undoubtedly pass from the hot to the cold sphere, and thus all the energy will be transferred at once, and, if a movable partition were inserted in the passage, all the heat-energy might be transformed into mechanical energy.
If we suppose two balls, instead of one, in each sphere, as soon as one passes from the hot to the cold, it will share its motion with the two already there, and one or more of them may pass back before the second has escaped, and thus at once the relations are rendered more complex, and the chance for availing ourselves of all the energy is diminished. If, now, the number of balls becomes infinite, or if we reduce our imaginary spheres to two real spheres, and substitute molecules for balls, then 1: 
is not an exaggeration of the chance of all the energy being in one of the spheres at any time in the future; on the other hand, the continual tendency is to, and the ultimate result is, absolute equality in temperature.
Clerk Maxwell has made the supposition of a vessel full of air, divided into two portions, A and B, by a division in which there is a small hole; and a being, who can see the molecules, opens and closes this hole so as to allow only the swifter molecules to pass from A to B, and only the slower ones from B to A. He will thus, without the expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermo-dynamics.
By the above mode of reasoning, together with the conclusions drawn from our experience of bodies consisting of an immense number of molecules, the result has been arrived at that the availability of a given amount of energy is determined on a physical basis, and is dependent on the infinite number of particles of which every tangible mass must be composed.
The instance cited by Professor Maxwell is only "applicable to the
