THE AVAILABILITY OF ENERGY. 247 



mate material agency \ to derive mechanical effect from any portion 

 of matter, by cooling it below the temperature of the coldest of the sur- 

 rounding 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 enuncia- 

 tion, and tbe results which in consequence must be predicted for the 

 present universe. 



Let us consider a hot spherical body, whose energy we wish to com- 

 municate 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 mole- 

 cules for balls, then 1 : co 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 molecides, 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 p>hysical 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 



