30 Britain's Heritage of Science 



of organized matter, either endowed with vegetable life, or 

 subject to the will of an animated creature. 



3. Within a finite period of time past, the Earth must 

 have been, and within a finite period to come the Earth must 

 again be. unfit for the habitation of man as at present 

 constituted, unless operations have been, or are to be, 

 performed, which are impossible under the laws to which 

 the known operations going on at present in the material 

 world are subject. 



The third of these statements must necessarily apply 

 not only to this earth but to the whole universe, and there 

 is therefore no escape from the conclusion that the material 

 universe, as we know it, is like a clockwork which is slowly 

 but steadily running down. 



It was reserved to Clerk Maxwell to perceive the reason 

 of our inability to check the gradual degradation of energy. 

 Heat is essentially a disorderly motion, the particles of 

 matter in a body which is apparently at rest moving 

 irregularly in all directions. We are unable to convert this 

 irregular into a regular motion, and it is this limitation of 

 our powers which prevents our making full use of molecular 

 energy as a source of mechanical work. Speaking of the 

 second law of thermodynamics, Maxwell says : . . . . 

 " it is undoubtedly true, as long as we can deal with bodies 

 only in mass, and have no power of perceiving or handling 

 the separate molecules of which they are made up. But 

 if we conceive a being whose faculties are so sharpened that 

 he can follow every molecule in its course, such a being, 

 whose attributes are still as essentially finite as our own, 

 would be able to do what is at present impossible to us. 

 For we have seen that the molecules in a vessel full of air 

 at uniform temperature are moving with velocities by no 

 means uniform, though the mean velocity of any great 

 number of them, arbitrarily selected, is almost exactly 

 uniform. Now let us suppose that such a vessel is divided 

 into two portions, A and B, by a division in which there is 

 a small hole, and that a being, who can see the individual 

 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 to pass from B to A. He will thus, without 



