404 



A\4 TURE 



[August 27, 1908 



planation, however, is never ultimate ; so it mav be tliat 

 the process contemplated, and in his last years energetic- 

 ally worked at, by Lord Kelvin is an intermediate step- 

 ping-stone, which must be talien in order to cross to some 

 more stable resting-place beyond ; just as has happened in 

 the case of gravitation. 



The above is an attempt fairly to represent what I con- 

 ceive must have been in the mind of our great leader, and 

 it was a kind of pronouncement which I hoprd to draw 

 from him by the publication above mentioned. If he had 

 been living it would have been presumptuous to try and 

 state more concerning his views than he himself had indi- 

 cated ; and still it is to be hoped that anyone acquainted 

 with his mind on this matter will make the necessary 

 corrections. 



Energy. 



If we now proceed to ask what great generalisation 

 will for ever be associated with Lord Kelvin's name, and 

 in future ages stand out as his greatest achievement, it is 

 not easy amid the wealth of material to focus it clearly. 

 A few days ago I myself should not have been certain, if 

 suddenly catechised, what my answer would be to such a 

 question. But in preparing this address, and reading 

 once more some of his early papers, 1 find nothing greater 

 than what emanated from him in and about the year 1S51, 

 when he was immersed in the doctrine of energv. I do 

 not mean, of course, any single year exactly, but about 

 that period of his life ; for in the records of that time are 

 to be found, I think, his greatest and strongest memoirs. 



The keenness and penetration of his mind at that epoch 

 must have been something astounding. With all his 

 mathematical powers alert, with tremendous natural 

 genius, and extraordinarily vivid interest in phenomena of 

 all kinds, he seized the facts concerning energy as they 

 emanated from Carnot and from Joule, and with them in 

 his mind, more powerfully and persistently than even 

 Helmholtz, he brooded over the whole domain of physics 

 until he elicited therefrom a series of the most beautiful 

 and striking discoveries — discoveries which, as they have 

 gained in familiarity, have perhaps lost something in 

 charm, by constant iteration in text-books and college lec- 

 tures, but which, in their freshness, w-ell repay an atten- 

 tive perusal ; though their form is far inferior to their 

 substance. 



So I expect that the answer of posterity, to the question 

 above mooted, will be that his most immortal work is the 

 development and application of the doctrine of the con- 

 servation of energy, together with the comprehension and 

 elaboration of the laws of thermodynamics. 



Later he became more immersed in the work of the 

 world, managed a great deal of practical business, and 

 made many inventions of surpassing ingenuity; but although 

 all this later work is the best known to the general public 

 — if, indeed, any scientific work can be said to be reallv 

 kno\yn to that body — yet for pure genius, to mv mind, 

 nothing since Newton comes up to his achievement in the 

 fifth and sixth decades of the last century, especiallv from 

 1848 to 1856. 



The comprehensive recognition, and extraordinarv ap- 

 plication to physics, of Carnot's brilliant " Reflections on 

 the Motive Power of Fire," or, as we should now say, 

 On the efficiency of heat engines, must have been largely 

 due to Lord Kelvin's influence, and to the clear and en- 

 thusiastic way in which he took up and developed the 

 subject. It is singular that this discovery of the second 

 law of thermodynamics, which came liistoricallv first, 

 created a real difficulty and obstruction in the recognition 

 of the truth of what is now called the first law; and 

 Joule's work would not only have been rejected bv the 

 Royal Society, as it was, but would have met with a 

 total lack of recognition, or even disdain, had it not been 

 for Lord Kelvin's perception of its value at a meeting of 

 Section A of the British Association in 1847. In fact, 

 the development of the whole subject of thermodynamics, 

 though extensively carried out by Clausius and others, 

 must have received strong initiative from him. 



But it was not the mere recognition of the true nature 

 of heat as a form of energy — so that when work was done 

 by a fall of temperature the heat removed was less than 

 the heat supplied, thereby breaking down the hydraulic 



NO. 202C\ VOL. 78] 



analogy — but it was the way in which, both by Lord 

 Kelvin and Helmholtz, the conservation of energy was 

 applied all over the ground of physics, and especially so 

 as to incorporate electrical phenomena with the rest, in 

 one scheme, that was most remarkable. 



Of all the memoirs dealing with the conservation of 

 energy as applied to electricity, perhaps the most striking, 

 though one of the simplest, is Lord Kelvin's early paper 

 on transient currents, or the discharge of an electric 

 capacity ; wherein he gives the whole theory of electric 

 oscillations, in so far as thev can be treated without re- 

 cognising the radiation which accompanies them — a dis- 

 covery reserved for .Maxwell. . . . 



The extraordinary magnitude of the giants in physical 

 science, especially in mathematical physics, during the 

 Victorian era, and, indeed, throughout the nineteenth cen- 

 tury, will probably be recognised more fully by posterity 

 than by us. It will be many generations, probably many 

 centuries, before the general and literary world can re- 

 ceive any adequate impression on the subject, or begin to 

 understand their methods and their more recondite results. 



Specific Heat of Electricity and Volta Force. 



It seems to me an amazing piece of insight which led 

 Lord Kelvin at that date, 1851, to attribute to electricity, 

 even hypothetically and only for convenience, something 

 akin to real specific heat. The fact was really discovered in 

 I.S5I, though he did not verify it experimentally until 

 185(1 (see pp. 246 and 319, &x., of that monument of 

 human power, vol. i. of " Math, and Phys. Papers "). The 

 modern theory of electrons — which are now supposed to 

 be trowing in great crowds through a conducting metal, 

 and which, by their irregular motions, must account for 

 some of the heat energy of a substance, in addition to 

 the much larger portion corresponding to the motion of 

 the atoms — seems to justify this curious expression, 

 "specific heat of electricity," to an unexpected degree: 

 and thermoelectric phenomena may be stated in terms of 

 a definite pressure of these mobile and detached electrons 

 in anv given substance, after the fashion of the pressure 

 of a gas or the osmotic pressure of a salt dissolved in a 

 liquid. 



There is, indeed, no obvious reason for denying that the 

 Volta force might be expressed in this way too, were it 

 not that a perfectly valid vera causa for this effect is to 

 be found at the surface of the metals, where they are in 

 contact with air or other chemically potential material ; 

 and that the magnitude of the effect, so calculated, from 

 electrochemical and thermal data, agrees w^ith observation 

 in absolute as well as in relative value. These and other 

 facts lead me to maintain that Volta force is an incipient 

 display of potential but not actual chemical activity, at 

 the bounding surface of a metal and a dielectric. But I 

 ought to say that Lord Kelvin differed from this view in 

 1.S84, and that he still might not agree with all that is im- 

 plied in this summary statement. 



Thermoelectricity and Gas Theory. 



The splendid way in which the second law of thermo- 

 dynamics was applied to the phenomena of thermoelec- 

 tricity, so as to establish the laws of a thermoelectric 

 circuit, is too well known to demand notice here. The 

 chief features of it are to be found on p. 249 of Lord 

 Kelvin's " Math, and Phys. Papers," vol. i. ; but the enter- 

 prise was, I think, to some extent attended by good for- 

 tune, such as often rewards those who do not hesitate to 

 risk something in the development of a theory, leaving it 

 to be corrected, if necessary, by the future. (J. R. 

 Mayer's theoretical estimate of J is another illustration.) 

 It so happens that the thermoelectric theory has demanded 

 verv little correction, in spite of the intrinsic uncertainty 

 attending application of the second law to an operation 

 which had one irreversible feature about it — which might 

 have been more relevant than it turns out to be — viz., 

 heat conduction. 



As an example of the opposite tendency, however, in 

 Lord Kelvin's mind — for it w-as a mind which at times 

 was extremely cautious — I think I mav instance the diffi- 

 culty he felt about the Boltzmann-Maxwell theory of the 

 distribution of molecular energv. He always seemed to 

 be troubled with a persistent difficulty about the innu- 



