386 



NATURE 



\SepL 7, 1876 



Society, were at the time supposed to disprove the 

 well-known laws first given by Coulomb. Thomson, 

 however, at the age of twenty-one, undertook the exami- 

 nation of the results of Snow Harris, and showed that, 

 instead of being out of harmony with the laws of Coulomb, 

 they were, so far as they went, confirmatory of those 

 laws. He pointed out clearly the precautions necessary 

 in experiments on the elementary laws, and he showed 

 that it was through a misunderstanding as to the condi- 

 tions of the simple laws enunciated by Coulomb that 

 Snow Harris was led into error. In this memoir we find 

 also the first steps towards making Faraday's new theory 

 of induction the basis of the mathematical theory of elec- 

 tricity. In subsequent papers this method of proceeding 

 to the mathematical theory is completely worked out ; 

 and, reading together the memoirs of Faraday and of 

 Thomson, we cannot help being struck with the way in 

 which the notion of lines of force and lines of flow of 

 heat fascinated the minds and guided the intuition of our 

 two greatest investigators. 



We cannot here follow Sir William Thomson in detail 

 through his series of papers on electrostatics and on mag- 

 netism. They were collected and pubhshed in 1872, 

 with notes and most important additions, in a volume of 

 600 pages. It is greatly to be desired that the same 

 may be done for the very numerous memoirs on other 

 physical subjects with which he has enriched the Trans- 

 actions and Proceedings of a host of learned Societies. 



In 1846 Mr. Thomson became editor of the Cambridge 

 and Dublin Mathematical Journal, a post which he held 

 for about seven years. Among the contributors to the 

 journal during his editorship he could count Stokes, 

 Cayley, De Morgan, Liouville, Salmon, Sir William 

 Rowan Hamilton, and many other distinguished mathe- 

 maticians ; while from his own pen proceeded many me- 

 moirs of great importance. It was about this time, also, 

 that he contributed to Liouville's Journal de Mathe- 

 matiques the memoirs in which he unfolded his principle 

 of " electric images." By means of this principle, which 

 he in his first letter likens to Brewster's kaleidoscope, he 

 shows how, by simple geometrical principles, to solve 

 many problems of an apparently very complicated nature, 

 as to the distribution of electricity on a system of conduc- 

 tors under the influence of a given electrified system. 

 The veteran Liouville, concluding a note suggested by 

 the letters of Mr. Thomson, writes of his own develop- 

 ments of the theory : " Mon but sera rempli, je le repute, 

 s'ils peuvent aider k bien faire comprendre la haute im- 

 portance du travail de ce jeune g^om^tre, et si M. Thom- 

 son lui-meme veut bien y voir une preuve nouvelle de 

 I'amitid que je lui porte et de I'estime que j'ai pour son 

 talent." 



His electrostatic researches led Thomson to the inven- 

 tion of very beautiful instruments for electrostatic measure- 

 ment. The subject of electrostatic measurement occu- 

 pied much of his attention from the very earliest, when 

 he was obliged to call attention to the defects of the elec- 

 trometers of Snow Harris. His labours in this direction 

 have produced the quadrant electrometer, which is em- 

 ployed for all kinds of electric testing in telegraph con- 

 struction, and for the registration of atmospheric electricity 

 at Kew Observatory ; the portable electrometer, for atmo- 

 spheric electricity and for other purposes in which the 



extreme sensitiveness of the quadrant-electrometer is not 

 required ; and the absolute electrometer, which serves 

 for reducing the scale readings of other instruments to 

 absolute measure, and which was used by Thomson in his 

 measurement of the electrostatic force producible by a I 

 Daniell's battery and in many other investigations. Those ^ 

 who have seen the collection of electrometers in the Loan 

 Collection at South Kensington will not think it too much 

 to say that to Sir W. Thomson is due our present syitem 

 of practical electrometry. 



But while thus engaged in investigations in electro- 

 statics and magnetism, there were many other branches 

 of science that were receiving from him advancement in 

 a not less remarkable way. There is no part of his work 

 of higher importance than his investigations on the 

 Dynamical Theory of Heat. These were communicated 

 in a series of papers to the Royal Society of Edinburgh, 

 the first of which was given in 1849. It was a critical 

 account of Carnot's memoir of 1824, " Reflexions sur la 

 Puissance Motrice du Feu." Though Rumford and Davy 

 had, in the beginning of this century, experimentally dis- 

 proved the material theory of heat, their experiments and 

 arguments were unheeded and nearly unknown ; and it 

 was only after 1843, when Joule actually determined the 

 dynamical equivalent of heat, that the great truth that heat 

 is a mode of motion was admitted and appreciated. Thus 

 Carnot, although dissatisfied with it, was obliged to adopt 

 the material theory of heat in 1824 ; and, regarding heat 

 as indestructible, spoke of the letting do.vn of the heat 

 from a higher to a lower temperature, and looked on the 

 production of work by the heat engine as a phenomenon 

 analogous to that in which water, descending from a 

 higher to a lower level, does work by means of a water- 

 wheel. Thomson, among the first to appreciate the im- 

 portance of Joule's results, set himself to alter the theory 

 given by Carnot into agreement with the true theory ; 

 and in the series of papers referred to, placed the whole 

 science of Thermodynamics on a thoroughly scientific 

 basis. In 1846 he first suggested the reckoning of tem- 

 perature on an absolute thermodynamic scale independ- 

 ent of the properties of any particular substance. Sub- 

 sequently, in consequence of experimental investigations 

 of the thermodynamic properties of air, and other gases, 

 made in conjunction with Joule, he showed how to define 

 a thermodynamic scale of temperature having the con- 

 venient property that air thermometers and other gas 

 thermometers agree with it as closely as they agree with 

 one another. This system of reckoning temperature 

 gives great facility for the simple expression of thermo- 

 dynamic principles and results. 



Having here mentioned Joule and Thomson together, 

 we cannot omit to remark that some of the most admirable 

 researches in thermodynamics were those undertaken in 

 conjunction by these two attached friends. 



Among the many important results of Sir W. Thom- 

 son's investigations in thermodynamics, one of the most 

 remarkable was his discovery of the principle of dissipa- 

 tion of energy, announced by him in 1852. During any 

 transformation of energy of one form into energy of 

 another form there is always a certain amount of energy 

 rendered unavailable for further useful application. No 

 known process in nature is exactly reversible, that is to 

 say, there is no known process by which we can convert 



