NATURE 715 



If Larmor has failed, as in my opinion he has, does it necessarily 

 follow that a mechanical explanation is impossible? Far from it. I 

 eaid above that as long as a phenomenon obeys the two principles of 

 energy and least action, so long it allows of an unlimited number of 

 mechanical explanations. And so with the phenomena of optics and 

 electricity. 



But this is not enough. For a mechanical explanation to be good 

 it must be simple; to choose it from among all the explanations that 

 are possible there must be other reasons than the necessity of making 

 a choice. Well, we have no theory as yet which will satisfy this con- 

 dition and consequently be of any use. Are we then to complain? 

 That would be to forget the end we seek, which is not the mechanism ; 

 the true and only aim is unity. 



We ought therefore to set some limits to our ambition. Let us not 

 seek to formulate a mechanical explanation; let us be content to show 

 that we can always find one if we wish. In this we have succeeded. 

 The principle of the conservation of energy has always been con- 

 firmed, and now it has a fellow in the principle of least action, stated 

 in the form appropriate to physics. This has also been verified, at 

 least as far as concerns the reversible phenomena which obey La- 

 grange's equations in other words, which obey the most general 

 laws of physics. The irreversible phenomena are much more difficult 

 to bring into line; but they, too, are being co-ordinated and tend to 

 come into the unity. The light which illuminates them comes from 

 Carnot's principle. For a long time thermo-dynamics was confined to 

 the study of the dilatations of bodies and of their change of state. 

 For some time past it has been . growing bolder, and has considerably 

 extended its domain. We owe to it the theories of the voltaic cell 

 and of their thermo-electric phenomena; there is not a corner in phy- 

 sics which it has not explored, and it has even attacked chemistry 

 itself. The same laws hold good; everywhere, disguised in some form 

 or other, we find Carnot's principle ; everywhere also appears that emi- 

 nently abstract concept of entropy which is as universal as the con- 

 cept of energy, and like it, seems to conceal a reality. It seemed that 

 radiant heat must escape, but recently that, too, has been brought 

 under the same laws. 



In this way fresh analogies are revealed which may be often pur- 

 sued in detail; electric resistance resembles the viscosity of fluids; hys- 

 teresis would rather be like the friction of solids.. In all cases friction 

 appears to be the type most imitated by the most diverse irreversible 

 phenomena, and this relationship is real and profound. 



A strictly mechanical explanation of these phenomena has also been 

 sought, but, owing to their nature, it is hardly likely that it will be 

 found. To find it, it has been necessary to suppose that the irreversi- 

 bility is but apparent, that the elementary phenomena are reversible 



