NA TURE 



75 



THURSDAY, MAY 28, 1908. 



THE SCIENCE OF THE ELECTRON. 

 Modern Electrical Theory. By Normnn R. Campbell. 

 Pp. xii + 332; diagrams. (Cambridge: University 

 Press, 1907.) Price 7s. 6d. net. 



IF Franklin had re-visited this globe a dozen years 

 ago he would have found his view of the electric 

 fluid in the desccndent. The work of Hertz and his 

 successors had completed the work which Faraday 

 and Maxwell began in directing the attention of 

 phvsicists to the sether as the region in which real 

 electrical actions proceed. The conductor was made 

 to take a secondary place only as a region fn which 

 electrical energy ceases to be electrical, being dis- 

 -ijjated therein into heat. The supposition of one 

 lectrical fluid (or more) was scouted, and relegated 

 to the daily Press. 



If, however, his visit had been postponed to tliu 

 present day it would be far otherwise. Once more 

 recourse is had to a fluid to explain electrical pheno- 

 i.-na. The story of modern electrical theory is the 

 ~torv of the re-furbishing of an old idea to form the 

 basis of speculative thought. We have Franklin's 

 idea once more in the ascendant ; and the only wonder 

 is that this account of modern electrical theory can 

 be written without a single mention of Franklin's 

 name within its pages. The explanation of the 

 vvonder is, of course, that the elaboration peculiar to 

 the modern theory is relatively so important that the 

 mere assumption of a fluid at all, once taken for 

 ^■ranted, is not worth mentioning. 



The main elaboration consists in assuming that 

 electricity, like matter, is made up of discrete por- 

 tions; and in speaking of "molecules of electricity," 

 r!~ Maxwell did (though he regarded the expression as 

 ■' gross " and " provisional "), or otherwise in speak- 

 ing of " electrons." The idea of " molecules of elec- 

 tricity," based originally on the facts of electrolysis, 

 is needed so greatly to explain the phenomena dis- 

 covered in the last decade in connection with the dis- 

 cliarge through gases and radio-activity that no one 

 questions that it is rightly introduced. 



The present book might be called " The Science of 

 the Electron." It is not written for the expert. 

 "It was begun with the idea of providing a text- 

 book from which students, well-grounded in the ele- 

 mentary branches of physics, might obtain some 

 knowledge of the later developments "; and although 

 this scheme has been departed from in some respects, 

 the above quotation from the preface represents fairly 

 well the general scope of the book. There is no 

 doubt that a volume with this aim was greatly needed, 

 and we do not hesitate to assert that Mr. Campbell 

 has produced one of the most interesting, the most 

 connected, and most comprehensive of all the recent 

 books on the subject. On questions so speculative as 

 those with which our author treats there are bound 

 to be differences of opinion ; and it will be most useful 

 to direct attention to some of these. We will ask Mr. 

 Campbell, therefore, to be content with the above 

 XO. 2013, VOL. 78] 



assurance of our appreciation of his efforts, and will 

 proceed to a few critical remarks. 



The subject and the methods are so speculative that 

 we are throughout on the fringe of knowledge, and 

 often we pass over to the other side. There is, per- 

 haps, scarcely a sustained argument in the volume 

 which will not ultimately be subjected to considerable 

 modification. It is this daring nature of the specula- 

 tion which is one of the charms (though a somewhat 

 dangerous charm) of modern physical writings. The 

 idea of the electron has, indeed, been found so fruit- 

 ful that evervone is employing it for all it is 

 worth. We doubt if there was ever a time in the 

 historv of physical science in which so much un- 

 proved hypothesis was employed. The very success 

 which attends its employment is the tempter which 

 leads one beyond justifiable bounds. Take, for ex- 

 ample, the idea that because the inertia of an elec- 

 tron is explicable in terms of the electromagnetic 

 field which surrounds it, therefore all inertia 

 is explicable in the same way. This idea takes con- 

 crete form in the supposition that all atoms are built 

 up of electrons. The author of this idea no longer 

 worships his creation (the present writer never bowed 

 his knee to it); but Mr. Campbell is still attracted 

 bv it. The result is that while, according to Drude, 

 and in recent years J. J. Thomson, the number of 

 electrons in an atom is quite small (comparable, for 

 example, with the atomic weight or the valency), to 

 Mr. Campbell the number in a radio-active material, 

 and probably in others, is at least as great as 

 400,000. 



In making this estimate Mr. Campbell assumes 

 that it is unlikely that the energy of each electron in 

 an atom is greater than 4'2xio-" ergs, this value 

 being the energy which he calculates as necessary to 

 liberate an electron from the atom. But we must 

 point out that electrons are propelled from radium 

 with nearly the velocity of light, and that therefore, 

 after the work done against the attraction of the 

 positive charge, each possesses energy nearly equal 

 to i x6"t X 10--* xg X io=" ergs, or, roughly, 3xio-' 

 ergs. Preserving his other datum unchanged, this 

 gives a minimum number of (say) 60 electrons to an 

 atom of radium. But the other datum is the energy 

 liberated by a radium atom measured by its heating 

 effect; this energy is supposed to be distributed 

 amongst the electrons in the atom. Yet Rutherford 

 has shown that something like 98 per cent, of the 

 heating effect is due to the alpha particles. Until we 

 know something more definite about the alpha par- 

 ticles no calculation on the author's lines can have 

 any weight. Needless to say, we do not claim any 

 greater validity for the estimate we have made above. 



Our author regards the explosion theory of photo- 

 electric action as unnecessary, and gives the alter- 

 native theory depending upon the accumulation of the 

 effects of forced oscillations. 



" The electron continues to take up energy from 

 the light until a velocity is reached, depending only 

 on the nature of the atom and not on the intensity of 

 the light, when its kinetic energy is sufficient to carry 

 it clear of the attraction of the positive atom." 



