672 



NATURE 



[May 27, 1922 



present state of a subject which he has done so much 

 to develop, Sir J. J. Thomson has performed a real 

 service to science. 



Although the intervention of the war delayed for 

 some time the fulfilment of the prophecy made by the 

 author in his preface to the first edition, that the 

 method of positive rays would be of service in con- 

 nection with important chemical problems, it has since 

 been so signally verified that the scientific world has 

 been shaken, and the distant reverberations have re- 

 echoed even through the columns of the daily press. 

 The discovery, made by means of the positive rays, 

 that the chemical elements are in many cases mixtures 

 of substances of almost identical properties but of 

 different atomic weights is, indeed, one which may 

 well justify the distinguished author of the method of 

 positive ray analysis in his conviction " that as yet 

 we are only at the beginning of a harvest of results 

 which will elucidate the process of chemical combina- 

 tion, and thus bridge over the most serious gap which 

 at present exists between physics and chemistry." 

 In addition, however, to advances of which no one 

 could be completely ignorant, many of us were aware 

 that a considerable volume of research of a less startling, 

 but not necessarily less important, character had 

 accumulated in the eight years which have elapsed 

 since the publication of the first edition of " Rays of 

 Positive Electricity," and will welcome this well- 

 balanced account of the present state of the subject 

 as a whole. 



The growth of the subject is indicated by the size 

 of the present volume, which is almost twice that of 

 its predecessor. So much has been added to the text 

 and so many sections have been rewritten, that although 

 here and there a critical reader may detect, by some 

 imperfectly concealed join, that a portion of the old 

 text still survives, the book is to all intents and pur- 

 poses a new production. It cannot be said that the 

 present volume is quite so easy to read as its pre- 

 decessor. The subject has grown not merely in size, 

 but also in complexity, and all the author's well-known 

 powers of exposition are required, at times, to guide 

 the reader through the very complicated phenomena 

 of the discharge tube. The journey is, however, 

 lightened by the skill with which the author succeeds 

 in visualising the most abstruse physical phenomena, 

 and is illuminated by the frequent flashes in which, 

 rising from the particular to the general, he links the 

 happenings in his discharge tube with some of the 

 outstanding problems of physics and chemistry. 

 Thus new light is thrown, not only on such problems 

 as the disintegration of metals and the production and 

 absorption of gases in the discharge tube itself, but also 

 on the origin of spectra, the mechanism of ionisation, 

 NO. 2743, VOL. 109] 



the varieties of chemical combination, and the structure 

 of atoms and molecules. 



An interesting account is given of the various 

 methods which have been evolved for producing and 

 measuring the positive rays, including, of course, the 

 mass-spectrograph employed by Dr. Aston for the 

 investigation . of isotopes. It is thus possible to com- 

 pare the relative advantages and limitations ' of the 

 different methods. The excellent plates at the end of 

 the volume leave little doubt that, when accurate 

 measurements of the masses of the particles are re- 

 quired, the much greater dispersion which the mass- 

 spectrograph makes possible gives it undoubted advan- 

 tages over other methods. It is, however, not adapted 

 for the investigation of the intricate phenomena of the 

 discharge itself, and it is mainly to the latter problem 

 that Sir J. J. Thomson has devoted his own re- 

 searches. The reproduction of some of the author's 

 more recent photographs shows the considerable 

 advance which has been made in the technique of the 

 experiments, while new methods of measuring the 

 plates, indicated in the text, add greatly to the accuracy 

 of the measurements. 



In addition to the now well-known parabolas, the 

 author directs particular attention to the existence of 

 numerous other secondary lines of very varied appear- 

 ance and to the remarkable 'variations in brightness 

 or " beading " seen on many of the parabolas in the 

 photographs. Considerable space is given to the 

 elucidation of these appearances, and from them the 

 author derives, not only many practical hints for 

 eliminating some of the ambiguities which previously 

 existed as to the nature of the particles to which a 

 given line on the plate must be ascribed, but also much 

 interesting and valuable information as to the pro- 

 cesses going on in the discharge tube itself and the 

 mechanism by which the positive rays are produced. 

 It is impossible to pursue the argument in a review, 

 but it forms by no means the least interesting part of 

 the work. 



In one respect we fear that the author may un- 

 intentionally be misleading his readers, and that is 

 in the statement that the technique of the subject is not 

 difficult. Here we feel that Sir J. J. Thomson is, 

 unconsciously, undervaluing the very exceptional 

 experimental skill which he and his able assistants 

 have shown in this prolonged and difficult series of 

 researches. In particular the electrical method of 

 measuring the rays, which alone seems capable of 

 giving those metrical results which are so desirable 

 for the solution of many of the outstanding problems, 

 has not until the last few months found any successful 

 exponent since the original experiments of the author 

 himself. We can, however, cordially agree that any 



