216 Professor Joseph Larmor [May 1, 



which it started; it now dominates the whole range of physical science. 

 It is only on its validity that our confidence is based, that we can treat 

 the interactions of the finite bodies of our experience by strict mathe- 

 matical and dynamical reasoning, entirely leaving aside, as self- 

 balanced and inoperative, those erratic though statistically regular 

 motions of the molecules, forming a very considerable part of the total 

 energy, which constitute heat in equilibrium. 



This fundamental basis of our knowledge of inanimate nature, 

 thus acquired from clues suggested by human industrial improvements, 

 still retains an aspect essentially anthropomorphic ; it is conditioned 

 by the limitations of our outlook as determined by the coarseness of 

 our senses, as Maxwell seems to have been the first definitely to per- 

 ceive. For the case of an ultra-material sentient creature of bodily 

 size so small as to be comparable with a single chemical atom, his own 

 sensible physical universe would be controlled by some fundamental 

 law possibly of quite difi:'erent type, while the phenomena which are 

 prominent to us would take on for him a cosmical character as regards 

 both time and space. We can ourselves catch partial glimpses of such 

 a transformed physical universe, not subject to ordinary laws of matter 

 in bulk, in the phenomena of high vacua, where the gaseous molecules 

 come nearly individually before our attention and can almost be 

 counted, and in the recent cognate phenomena of radio-activity either 

 spontaneous or electrically excited. The boundary of demarcation of 

 this new world from the universe which is dominated by the principle 

 of available energy is naturally ill-defined: its exploration sheds light 

 on both, and is perhaps the most interesting of the present activities 

 of theoretical and practical physics. 



Here also Lord Kelvin has played a part. Already, in 1852, he 

 had prefixed to one of his papers the title ' On the Sources available 

 to Man for the Production of Mechanical Effect,' as if in anticipation 

 of this anthropomorphic side of the subject, first broached apparently 

 by Maxwell in 1871 at the end of his "Theory of Heat," where he 

 points out that it is only man's inability to obstruct passively the 

 individual molecules at will that prevents the whole of their energy 

 from being available, and shows how sentient agents capable of doing 

 this could reverse the otherAvise irrevocable course of diffusion of the 

 energy in a gaseous medium. 



Perhaps Thomson's own most systematic pronouncement on the 

 inner significance of these relations is a short paper in ' Proc. R. S. 

 Edin.,'* of date February 1874. He points out that the changes con- 

 templated in abstract dynamics are strictly reversible ; while in actual 

 physical phenomena the absence of reversibility is conspicuous, a fact 

 which was already embedded in the principle of dissipation of energy 

 in 1852. Now "the essence of Joule's discovery that heat is diffused 

 energy is the subjection of physical phenomena to dynamical law." 



* Also Nature, ix. 1874, pp. 421-424 ; Phil. Mag., March 1892, pp. 291-299. 



