March 17, 19 10] 



NATURE 



63 



out the place of the planet Neptune from the perturba- 

 tions of Uranus gave a result which, when found to 

 agree with obser\ation. was generally regarded as 

 affording a much more forcible proof of the truth 

 of the gravitational theory than all the induction 

 which preceded it, and the same thing mav be said 

 of results in other departments of phvsics, which 

 illustrate the predictive value of a true theon,-. A 

 reference to Adams and Leverrier, and the discovery 

 of Neptune, was made in the address, but after 1862 

 it was omitted by the advice of Prof. Tait, who 

 thought that the subject of Neptune had been "ridden 

 to death." 



The provinces of heat, electricity and magnetism, 

 which are referred to in the introductory address as 

 lying in great measure outside the scope of dynamics, 

 it was Thomson's destiny to bring under the sway 

 of the science of forbes. He is already, when in 

 Pans, meditating on the results set forth' in Gauss's 

 great memoir on attracting and repelling forces var>-. 

 ing inversely as the square of the distance, and 

 noticmg how the general theorems there given lead to 

 conclusions which were afterwards expressed in the 

 language of the theory of energ>'. The letter (quoted 

 on p. 130) is interesting in this connection and in some 

 others. 



"April S, 1845. To-day, in the laboratory (of 

 Physique at the Coll. de France, M. Regnault Prof ) 

 I got the idea which gives the mechanical effect 

 necessary to produce any given amount of free elec- 

 tricity, on a conducting or non-conducting body If 

 m IS any electrical element, v the potential 6f the 

 whole system upon it. the mechanical effect necessary 

 to produce the distribution is 2mi'. . Also the 



theorem of Gauss that Smv is a mimim. when v 

 IS const., shows how the double inti which occurs 

 when we wish to express the action directly, 

 may be transformed into the diff.-co. of a simple 

 intl taken with reference to the distance between 

 the two spheres. . . . This has confirmed my resolu- 

 tion to commence experimental researches, if ever I 

 make any with an investigation of the absolute force 

 of statical electricity As yet each experimenter has 

 --nl> compared intensities by the deyns of their electro- 

 jTieter. 



I Here we have the train of ideas in progress which 

 jed, no doubt, to some of the series of papers on the 

 nathematical theory of electricity which were pub- 

 ished later in the Cambridge and Dublin Mathe- 

 natical Journal. But what is still more particularly 



be remarked is the determination to measure forces 

 n absolute units. In the discussions of Lord Kelvin's 

 rork which have appeared in print, hardly sufficient 

 mportance has been attached to the part which Thom- 

 on played in the working-out of the scheme of abso- 

 Jte measurements the beginnings of which were 

 lade by W. Weber and Gauss. Perhaps, as he con- 

 2ssed later in one of his addresses, he never succeeded 



1 getting the capacities of the levden jars in his 

 iboratory expressed exactly in absolute units, but it 

 ecame possible to obtain a fair estimate of these 

 opacities, and to measure, also in absolute units, by 

 leans of the beautiful electrometers which he after- 

 -ards made, the potentials to which the jars were 

 larged, and therefore to say approximately, in er^s 



XO. 2107, VOL. 83]' 



how much energy was stored up in a particular jar 

 when charged to the measured potential. Each experi- 

 menter, he says, expressed his results in terms of the 

 deflections of his own electrometer : not merely was 

 that the case, but currents were measured bv each 

 experimenter' in divisions on his own gklvano- 

 meter, and the insulation resistance of a cable at 

 one time could only be compared with its value 

 at another time by using the same instruments as 

 before and reproducing exactly the former conditions. 

 All this had to be swept away and an absolute 

 system substituted when Atlantic cables began to 

 be laid; but an enormous amount of exceedingly 

 valuable work, both theoretical and experimental, 

 had to be done ere a proper system could be elabor- 

 ated. No small amount of this was accomplished 

 by Thomson and his volunteer laborator\' corps at 

 Glasgow, in the "coal hole at the old college," as 

 some members of that corps have since described the 

 famous "first laboratory for students." Then the 

 toil which the members of the British Association 

 Committee undertook in working out, perfecting, 

 and realising the system of units! It was work 

 which did not attract public attention or strike the 

 public fancy; and yet hardly anything else has done 

 more to render possible practical applications of elec- 

 tricity in all their modern ramifications. 



In the early 'fifties came the papers on the 

 theory of heat. The account of Carnot's theory of 

 the motive power of heat, with its determination of 

 Carnot's function from Regnault 's experiments on 

 steam, valuable as it was, seems to have led Thom- 

 son's thoughts into a kind of groove, from which, 

 when Joule's proof that heat and work were equiva- 

 lent was published, he had some diflficulty in escaping, 

 and which involved him in considerable perplexity. 

 It is, as Prof. Thompson says, entirely to the credit 

 of Clausius that he saw clearly at once the full force 

 of Joule's discovery, and accepted implicitly the first 

 law of thermodynamics to which that discovery 

 pointed. After that the necessar>^ modification of Car- 

 not's theor>- followed immediately, and Carnot's notion 

 of a cycle of operations enabled the whole of the 

 immediate consequences of the true dynamical theory 

 to be worked out. A little later, but independently, 

 Thomson also arrived at the true theory, and by an 

 "axiom," or rather postulate, very differently ex- 

 pressed from that employed by Clausius, but on the 

 whole equivalent, showed that the efficiency of all 

 ideal thermodynamic engines, no matter what their 

 working substances were composed of, had the same 

 value. This in Thomson's hands led afterwards to 

 his definition of absolute temperature, a conception 

 which Prof. Tait used to insist, in the pages of 

 Nature and elsewhere, was of the most enormous 

 importance, and ought to be set forth at the outset 

 in every treatise on the subject, 



Thomson's great paper, followed up as it was by 

 developments and applications of the theory in his 

 later writings, was destined to e.xert a profound in- 

 fluence on the study of thermodynamics both in this 

 country and abroad. This result was in part due 

 to the peculiarities of his treatment of the subject, 

 which were characteristic of his practical genius! 



