No. 1070, Vol. 42] 



NATURE 



laws of action between particles charged with electricity, 

 elements of current, and the like ; then its function 

 ceases, and the rest is a mere matter of mathematical 

 symbols ; by this method the physics and the mathe- 

 matics are sharply divided — the physics occurs in the first 

 few lines of the chapter, the rest of which is mathematics. 

 In the other method the physics and mathematics are 

 kept as closely connected as possible, so that by knowing 

 from physics the kind of results we may expect errors 

 in the mathematical investigations may be detected ; 

 while, on the other hand, our physical conceptions may 

 be extended, and perhaps even the point of view changed, 

 by the results of mathematical transformations. Thus, 

 as Maxwell points out, the two sides of the equation 

 which expresses Green's theorem might have suggested 

 the two ways of regarding electrical phenomena— the one 

 when we confine our attention to the electrified bodies ; 

 the other when we look upon the dielectric as the seat of 

 the phenomena. In the department of physics in which 

 mathematical analysis has won perhaps its greatest tri- 

 umphs, that of gravitational attraction, the first method I 

 is perhaps the most natural ; but in an intricate subject j 

 like electricity, where so much remains to be discovered, 

 and which it is so important to regard from as many points 

 of view as possible, the second method seems infinitely 

 the more likely to lead to an extension of our knowledge. 



M. Bertrand's work is a most favourable example of 

 the first method : it is clearly and gracefully written, and 

 the mathematical part often extremely elegant ; yet, in 

 spite of all this, we must confess to a feeling of dis- 

 appointment on reading the book. We had thought from 

 the publication of Mascart and Joubert's " Lemons sur 

 I'Electricitd et le Magn^tisme," and the excellent transla- 

 tion of Maxwell's "Electricity and Magnetism" by MM. 

 Seligman-Lui, and Cornu, that the ideas introduced by 

 Maxwell, von Helmholtz, and others, were spreading in 

 France ; yet here we have a work written by one of the 

 first scientific men of that country, in which the subject is 

 treated in fundamentally precisely the same way as that 

 in vogue twenty or thirty years ago ; and in fact, with the 

 exception of some results due to M. Marcel Deprez, in the 

 chapter on electro-magnetic machines, there is no reference 

 to any investigation made within the last twenty years. 

 The names of Maxwell and von Helmholtz are not even 

 mentioned in the book itself— though, to be quite accurate, 

 that of Maxwell occurs in the table of contents in con- 

 nection with a particular case of Green's theorem. 



M. Bertrand seems to exact more from the science of 

 electricity, before he deems it worthy to be discussed 

 mathematically, than is exacted from any other science ; 

 thus, for example, he omits all consideration of the effect 

 of the dielectric because there is no satisfactory molecular 

 theory of specific inductive capacity, such as Mossotti 

 attempted by supposing the dielectric to contain conduct- 

 4flg spheres, the specific inductive capacity depending on 

 the ratio of the volume of the spheres to that of the rest 

 of the dielectric. It seems to us that if M. Bertrand were 

 to write a book about optics, he would, if he were con- 

 sistent, leave out everything connected with either refrac- 

 tion or reflection, since no complete molecular theory of 

 these phenomena have been given. The way in which 

 the dielectric affects the lines of force is as definite and 

 sinple as the way in which a refracting medium affects 



the rays of light, and the one is quite as capable of re- 

 ceiving mathematical treatment as the other. 



Again, M. Bertrand, in treating of magnetism, points 

 out that on the theory of magnetic fluids the behaviour 

 of a magnetized body will depend upon the shape of the 

 molecules, and as this is not known he refuses to in- 

 vestigate the magnetic properties of bodies ; he never 

 mentions magnetic permeability, the idea of which, by 

 introducing a new property of bodies, enables us to in- 

 vestigate mathematically their magnetic properties, and 

 express the results of the investigation in terms of quan- 

 titles which can be measured in a physical laboratory. 



In spite of the clearness and elegance of this book, we 

 are afraid that a student who learnt his electricity from it 

 would think, if he read any modern memoirs on the sub- 

 ject, that they dealt with some new and unknown science ; 

 for the mode of regarding the phenomena would probably 

 be entirely different, and many quantities would be intro- 

 duced of whose existence M. Bertrand had given him no 

 hint 



OUR BOOK SHELF. 

 StindevaWs Tentamen \Methodi naiuralis avium dispa- 



nendaruvt te7itamen\. Translated into English, with 



Notes, by Francis Nicholson, F.Z.S., &c. (London : 



R. H. Porter, 1889.) 

 The practice of translating into English memoirs by 

 leading foreign naturalists that may be considered clas- 

 sical is to be highly commended. English ornithologists 

 who are not conversant with German may thus study 

 such important works in their branch of science as 

 Nitzsch's " Pterylographie" and Johannes Miiller's "Voice 

 Organs of Passeres," of both of which excellent English 

 translations exist. 



It is, however, a question whether Sundevall's "Ten- 

 tamen" comes into the category of classical memoirs, or 

 is worth translating if it does. In our opinion it might 

 have been allowed to drop peacefully into oblivion in the 

 obscurity of the original Latin. No particular object is 

 gained by helping to perpetuate a scheme of bird-classi- 

 fication like that of Sundevall, with the details of which 

 no one nowadays can agree. Even the translator has 

 nothing to say for it, except the very general statement 

 that " every serious scheme of classification contains 

 some items of progressive knowledge towards the attain- 

 ment of a complete natural arrangement of the class of 

 birds." It would be very difficult, however, to say what 

 these items are, and the translator gives us no help in 

 the matter. On the other hand, if ornithologists believe 

 that this, the last work of Sundevall, is really important, 

 then it can be certainly said that Mr. Nicholson's trans- 

 lation is good and accurate. 



The introduction, which occupies the first twenty-five 

 pages, is interesting, and so are the notes interspersed 

 through the volume ; but it is clear that the book must 

 be entirely judged by the merits or demerits of the scheme 

 of classification. Prof. Newton (article " Ornithology," in 

 " Encyc. Brit.," ninth edition) has subjected Sundevall to 

 a searching criticism, which seems to us to be perfectly 

 justified. Some of the worst features of the classification — 

 in addition to those mentioned by Prof, Newton — are to as- 

 sociate Serpentarius with any other birds of prey, to place 

 the American vultures near the American kites (an error 

 which is constantly cropping up in spite of the obvious 

 anatomical differences), Glareola among the goatsuckers, 

 &c. Prof. Sundevall's classification is, in fact, most re- 

 actionary in every particular ; it is difficult to believe 

 that it was published in the year 1872— after the appear- 

 ance of so many important papers upon bird classification 

 and structure, such as those of Profs, Huxley and Parker, 



