240 SPECULATIVE SCIENCE 



heated end may become cooler, and the cold end warmer. So 

 long as any two parts differ in temperature, change may occur ; 

 but so soon as all parts of the bar are at one temperature, the 

 bar quoad heat can produce no change in itself, and yet if we 

 conceive radiation or conduction from the surface to have been 

 prevented, the bar will contain the same total energy as before. 

 In the first condition, it had the power of doing work, and if it 

 had not been a simple bar, but a more complex arrangement of 

 materials of which the two parts had been at different tempera- 

 tures, this difference might have been used to set wheels going, 

 or to produce a thermo-electric current ; but gradually the 

 wheels would have been stopped by friction producing heat 

 once more, the thermo-electric current would have died out, 

 producing heat in its turn, and the final quantity of heat in 

 the system would have been the same as before. Its distribu- 

 tion only, as in the simple case, would have been different. At 

 first, great differences in the distribution existed ; at last, the 

 distribution was absolutely uniform ; and in that condition 

 the system could suffer no alteration until affected by some other 

 body in a different condition, outside itself. Every change in 

 the distribution of energy depends on a difference between 

 bodies, and every change tends, on the whole, to diminish this 

 difference, and so render the total future possible change less in 

 amount. Heat is the great agent in this gradual decay. No 

 sooner does energy take this form than it is rapidly dissipated, 

 i.e. distributed among a large number of bodies, which assume 

 a nearly equal temperature ; once energy has undergone this 

 transformation, it is practically lost. The equivalent of the 

 energy is there ; but it can produce no change until some fresh 

 body, at a very different temperature, is presented to it. Thus 

 it is that friction is looked upon as the grand enemy of so-called 

 perpetual motion ; it is the commonest mode by which vis viva 

 is converted into heat ; and we all practically know, that once 

 the energy of our coal, boiling water, steam, piston, fly-wheel, 

 rolling mills, gets into this form, it is simply conducted away, 

 and is lost to us for ever ; just so, when the chemical or other 

 energies of nature, contained, say, in our planetary system, once 

 assume the form of heat, they are in a fair way to be lost for all 

 available purposes. They will produce a greater or less amount 



