December i6, 1920] 



NATURE 



509 



impossibilities of Rayleigh's formula. He next 

 turned his attention to the thermodynamiciil 

 aspect, and this illustrates at its best the groping 

 process, for in fact thermodynamics does not 

 contain the answer at all ; and yet this track 

 guided him to the right solution. He was led to 

 study a certain thermodynamic function (the re- 

 ciprocal of the second differential of the entropy 

 with regard to the energy !), and was struck, by 

 the fact that, with Wien's formula, it was propor- 

 tional to the energy itself — a fact that is really 

 quite accidental. For Rayleigh's formula it was 

 proportional to the square of the energy, and 

 Planck was happily inspired to combine the two 

 forms into one. The result gave him the true 

 formula. It remained to find :i thi-ory to account 

 for it. 



In the course of his effoii-^ lu get this theory, 

 I'lanck turned to the deeper meaning that is 

 attributed to entropy on the kinetic theory, which 

 connects it with probability, and once this was 

 tried it gave the result more simply than might 

 be expected. For in considerations of probability 

 one is bound to work with discrete quantities, ami 

 not with continuous ; and so one must adopt tiie 

 idea of atoms of energy for the calculations, 

 though with the ultimate intention of making 

 thcni infinitely small. But this intention is fru^ 

 trated, because the formula is obtained without 

 going to the limit at all. This was how Planck 

 arrived at his theoretical explanation of the radia- 

 tion formula, and by comparison with experiment 



I he was enabled to deduce two universal constants. 

 I From the first were obtained the earliest really 

 1 good values for the charge of the electron and the 

 I associated constants. Ihe second, he confesses, 

 ! jjerplexed him a good deal, and indeed it would 

 have been surprising if it had not. It was the 

 ; quantum. 



The remainder of the address is occupied with 

 , the later history of the quantum, and it is scarcely 

 too much to say that this is simply the history of 

 i modern physics. Of all its applications, photo- 

 ' electric effect, specific heats of solids at low tem- 

 ' peratures, specific heats of gases, etc., he not un- 

 ' naturally gives pride of place to Bohr's spectrum 

 theory. The address concludes with some specula- 

 tions as to what may be the solution of the almost 

 impossible difficulties with which we are faced. 

 The success of the quantum theory has been, and 

 continues to be, so enormous that it often appears 

 j as if writers had forgotten that the whole present 

 : system of physics is based on a perfectly definite 

 set of mutually contradictory axioms. So it is 

 particularly interesting to hear some views on this 

 question by one of the great authorities. Unfor- 

 tunately, Planck does not seem nearer the solu- 

 tion than are the rest of us, for in one place he 

 throws out the discouraging suggestion that the 

 quantum theory is now in the state in which 

 Komer left the theory of light, so that we may 

 expect to have to wait a long time for the Maxwell 

 of the subject to appear and reconcile the seem- 

 ingly irreconcilable. 



Obituary. 



Spencer Pickering, F.R.S. 



BV the death of Percival Spencer Umfrevillu 

 Pickering at Harpenden on December 5 

 linglish science loses one of its most original 

 and attractive personalities. His death was not 

 unexpected ; for more than a year his friends 

 bad known that Pickering was in a precarious 

 condition, holding on to life by little more than 

 his courage. 



Pickering was born in iJ^.s** "f g(Kxl family, 

 and educated at Kton and Kalliol. His earliest 

 paper was published while still an undergraduate, 

 and it is not uncharacteristic that it was polem- 

 i<al, directed against one of his dons, and con- 

 erned with a basic compoun<l. But his real 

 ictivity began with his appointment as professor 

 of chemistry at Bedford College in 1881. He 

 l)egan to work upon the constitution of double 

 aid basic salts, and passed on to determinatirtns 

 "t thermal phenomena accompanying the forma- 

 tion and solution of salts. Naturally enough, 

 this work led to a general consideration of the 

 process of solution, especially as it was about 

 this time that the \'an't Hoff theory of osmottc 

 pressure and the dissociation into ions of salts 

 in aqueous solutions was Ijcginning to rcvolu- 

 tioni.se the conceptions of chemists. Pickering 

 \ould have none of this theory; his work lay at 

 !ie other end of the scale among strong solii- 

 NO. 2668. VOL. 106] 



tions and powerful electrolytes, and he saw solu- 

 tion as a process of association with formation 

 of hydrates. In a voluminous paper published 

 by the Chemical Society in 1889 he examined with 

 an extraordinary wealth of detail the density, 

 conductivity, heat of dissolution, heat capacity, 

 and expansion of mixtures of sulphuric acid and 

 water, demonstrated breaks of continuity in the 

 graphs representing these properties, and isolated 

 definite hydrates to match the breaks. 



Controversy with the supporters of the ionic 

 hypothesis grew intense, for Pickering was turn- 

 ing out an enormous volume of experimental work, 

 and was standing for his hydrate theory almost 

 Aihixnasius contra mundiim. Between 1889 and 

 1896 he printed no fewer than fifty-six papers on 

 solution with the Chemical .Society alone, many 

 of them of great length, and involving a vast 

 number of exact <lcterminations, all of which 

 were done single-handed and without assistants. 

 The li<le was, however, running against Picker- 

 ing; he and Ostwald were looking at difTcrent sides 

 of the shield; but, though Pickering's work has 

 since fallen into its proper place, at the time it 

 lacked that pragmatic justification of leading to 

 dis<overy which made the dissociation hypothesis 

 so generally aei'cptable to the chemists of the 

 day. In disgust Pickering forsook chemistry, 

 the rapid flow of papers ending abruptly in 1896. 



