March 19, 1908] 



NATURE 



459 



law of the inverse square after he has established 

 ahnost all the important propositions. But any given 

 proposition can only be established validly by any 

 process of argument from an irreducible number of 

 primary propositions or assumptions. If the assump- 

 tion of which M. Pellat speaks is necessary to the 

 development of the subject, then, if his arguments 

 are sound, that assumption or its equivalent must be 

 introduced at .some stage. The difference between 

 M. Pellal's treatment and that adopted ordinarily is 

 not that the former requires fewer primary proposi- 

 tions than the latter, but that in the former those 

 propositions are introduced as the direct consequence 

 of some experiment, while in the latter they are intro- 

 duced as hypotheses verified subsequently by the 

 agreement of deductions from them with experiment. 

 Now we insist most strongly that it is the latter 

 process which is the more logical, for it is the process 

 bv which experimental sciences are actually de- 

 veloped. It is perniciously misleading to attempt to 

 .applv to such sciences arguments of the kind used in 

 pure matliematics, for it is impossible to deduce 

 any mathematical conclusion whatever from any ex- 

 periment without an hypothesis; there is always an 

 error of experiment. In our opinion, there are only 

 two methods by which a science may be developed 

 logically, neither of which is adopted by M. Pellat. 

 The first is to follow the historical development, 

 pointing out the stages at which hypotheses are in- 

 troduced; the second is to define at the outset the 

 concepts used and the pro|)ositions relating them, and 

 to show that these lead to conclusions in harmony 

 with experiment. 



M. Pellat al.so prefers to develop electrodynamics 

 directly from the mutual action of currents, intro- 

 ducing the concept of magnetism as a subsidiary 

 function, and then applying it to the phenomena of 

 magnetic substances. His reason for this unconven- 

 tional procedure is that magnetism is a fictitious 

 quantity which does not exist, but only behaves as 

 if it existed — a distinction too subtle for our com- 

 prehension. But here surely the author is abandon- 

 ing his logical principles. Magnetism is only intro- 

 duced into the study of current actions because the 

 properties of permanent magnets happened to be 

 examined before those of currents; if the historical 

 order had been reversed, there would have been no 

 need for the conception. If the author is ready to 

 brave all the inconveniences that attend the ignoration 

 of the history of the subject for the sake of logic, 

 surely consistency to his scheme should make him 

 abandon a notion so purely historical as magnetism. 



Remark should also be made on M. Pellat's strange 

 neglect of Maxwell's theory of the electromagnetic 

 field. .\ complete description is given of Hertz's ex- 

 periments on electric waves, but the theory on which 

 alone they are intelligible is relegated to a few pages 

 in an appended note. Rowland's fundamental re- 

 search, proving the identity of the electrostatic and 

 electrom.ignetic conceptions of a current, is referred 

 tj in a brief phrase and attributed to Rontgen and 

 Hertz. In some other places the work shows a lack 

 of proportion in the space that is given to different 

 NO. 2003, VOL. 77] 



subjects ; twenty pages on electrostatic generators 

 seems excessive, but on the other hand the chapter 

 on dynamos and motors is e.xcellent, and contains 

 much that is too often excluded from physical text- 

 books. The last volume is on a somewhat different 

 plane from its predecessors; it is more advanced, and 

 contains discussions of many controversial points. 

 We welcome an excellent last chapter on the elements 

 of gaseous conduction. 



It must not be thought that our remarks imply 

 any disparagement of the work as a treatise for 

 students; criticism has only been directed where it 

 is challenged. English students are not so well pro 

 vided in this subject that they would not welcome a 

 translation. There is only one really serious defect in 

 the book in its present form— the absence of an index. 

 M. Manville's book may be regarded in some 

 respects as an attempt to supplement that of M. 

 Pellat. The author complains that his countrymen 

 have not realised yet the importance of the latest 

 physical research. He thinks that they may have 

 been hindered by the absence of a suitable suminary 

 which renders unnecessary reference to original 

 memoirs, and has set himself to supply the defect. 

 In less than 200 pages he treats of kathode and 

 Rontgen rays, ionisation of gases, radio-activity, and 

 general electron theory. Two subjects are also intro- 

 duced which can hardly be termed modern ; we should 

 have thought that his exposition of the simple facts 

 of electrolysis might have made way for more valuable 

 matter, but apparently he is right in assuming that 

 Maxwell's work has not been assimilated by those 

 for whom he writes. 



M. Manville's project is admirable, but we fear 

 that his powers are not equal to his intentions, for 

 he himself has not mastered these subjects corn- 

 pletely. "There are several actual mistakes, but a still 

 more serious fault is to be found in his failure to 

 show the connection between many of the phenomena 

 which he describes. Though the various methods by 

 which a gas may be rendered a conductor are treated 

 in some detail, the only reference to the modern 

 theory of ionisation, by which these methods may be 

 correlated, is contained in a brief and inadequate 

 paragraph at the end of the chapter. The account of 

 the kathode rays is satisfactory, but there is no refer- 

 ence to the mechanism of the electric discharge in 

 which they have their origin, nor is any distinction 

 made between the electron and the ion which it forms. 

 The chapter on radio-activity is a list of unconnected 

 facts, while the theory of Rutherford and Soddy is 

 dismissed as insutificient for the strange reason that 

 it gives no account of induced activity ! If not more 

 than six pages could be spared for the application of 

 the electron theory to optics, conduction and 

 chemistry, it would have been better to leave such 

 matters unmentioned. In view of recent speculations 

 on the density and rigidity of the aether, it is hardly 

 judicious to describe that medium as " ultra-gaseux." 

 As an example of actual inaccuracy we may quote 

 the statement that solid dielectrics are ionised by 

 Rontgen ravs in the same way as gases.. Our con- 

 fidence in the author's analysis is shaken severely by 



