May 1 6, 1901] 



NA TURE 



51 



said, "will be found to contain a connected account of 

 everything essential to a first course of modern electrical 

 theory." 



But in chapter vi. the definition of potential is a mathe- 

 matical one. The distinction between scalar and vector 

 quantities is drawn, and it is pointed out that in many 

 cases the line integral of a vector between two points is 

 independent of the path, and that in this case the vector 

 is said to have a potential, the value of the line integral 

 being the difference of the potentials at the two points 

 which are taken as the e.\tremities of the path. 



A number of mathematical propositions connected with 

 the theory of potential are then proved or illustrated in 

 a very interesting way ; but the application of the theory 

 to the fundamental facts discussed in the earlier chapters 

 is hardly attempted. 



The beginner might be given some idea of the nature 

 of potential without being asked to grasp the meaning of 

 a line integral. Faraday's and Maxwell's notions as to 

 the tension along the lines of force and the pressure 

 perpendicular to them which occurs in a dielectric 

 medium may be used, without the introduction of sym- 

 bols, to explain the simple attractions and repulsions 

 described in the earlier chapters ; the link between the 

 ancient observations and the modern theory is wanting, 

 and the loss to the reader is very marked. 



The same want is illustrated in the two following 

 chapters. The quantity K, the specific inductive 

 capacity of a medium, is defined in the usual way in §70, 

 and a footnote tells us " it is identical with the per- 

 mittivity or dielectric co-efficient K." This statement is 

 repeated in the next chapter, on electric action in 

 dielectrics, but the author does not explicitly establish 

 the connection ; a few words at the end of §86 would do it, 

 the words, however, are wanting. 



Or again, K is defined as the ratio of the polarisation, 

 or the intensity of the electric displacement, to the force. 

 Now the force has a perfectly definite meaning, and the 

 inductance K can be defined in unambiguous terms ; why 

 then make it depend on " a peculiar distortion called 

 electrical displacement " which is " roughly represented 

 by supposing every tube of force to be divided into cells 

 by elastic membranes firmly attached to the tube, these 

 cells being completely filled with incompressible liquid. 

 The distortion does not displace the sides of the tube, 

 but it displaces the liquid a little way along the tube, in 

 the direction of the force F, further displacement being 

 prevented by the elastic resistance of the membranes." 



The inductance of a dielectric is too important a 

 physical quantity to be defined in terms of something 

 which can only be explained by an incomplete analogy ; 

 it is surely better to say that the force between two given 

 charges is found to depend on the medium in which 

 they are placed, so that the complete law of force is 

 F = cv"/Kr-, where K is a constant for a given medium, and 

 is known as the permittivity or inductance of the medium. 

 Then the statements in §80 as to the modification of 

 fundamental formula; follow naturally ; as it is, they seem 

 to the reader to depend on the analogy between the flow 

 of a liquid and electric displacement, and not to rest on 

 an experimental basis. 



The earlier chapters on magnetism are clear and good, 

 §148, giving the reason why [a bar of soft iron sets 

 NO. 1646, VOL. 64J 



parallel to the lines of force, may be specially commended. 

 Chapter xiii. gives a useful development of magnetic 

 theory ; the proof of the relation, however, between B and 

 H, §158, might be given in fuller detail, and a reference 

 to §83 as well as to §90 would not be misplaced. 



The rest of the book is taken up with the theory of 

 electric currents and electro-magnetism, and can, on the 

 whole, be warmly commended. The description of in- 

 struments, ammeters, voltmeters and the like is brought 

 up to date. At times, possibly, almost too much is 

 attempted for the space available, e.g., in the very con- 

 densed account of the ballistic galvanometer in §200. 

 Again, some preliminary account of a voltaic cell is 

 needed before §213, which begins " In a circuit consist- 

 ing of a battery of four similar cells." 



In places the book would be improved .by a more 

 distinct reference to the fundamental experiments on 

 which the various laws are based. Thus in chapter xix., 

 after a reference to a statement as to the force exerted 

 on a wire carrying a current in a magnetic field, we pass 

 on to "two fundamental formuki:.'' These formula; give 

 the electrical and mechanical forces on a conductor 

 carrying a current when in a magnetic field, and various 

 important deductions are drawn from them in an admir- 

 able manner in the following paragraphs. But we miss 

 any clear indication of the method by which these two 

 fundamental formukt are deduced from experimental 

 results. 



The chapter on dynamos is specially good ; there is 

 sufficient detail to enable the student to grasp the 

 prmciples which underlie the action of the various forms, 

 while at the same time the book is not overburdened with 

 accounts of small differences of construction which, 

 though they are of great importance to the student of 

 dynamo design, have no place in a general te.xt-book. 



Enough, perhaps, has been said to show the value of 

 the book. Prof Everett has rendered a real service to 

 his readers by his new edition ; the book is one which is 

 sure to become popular and to be valued alike by teacher 

 and by student. 



AN ESS A Y IN CRITICAL BIBLIOGRAPHY. 

 The Periodic Classification and the Problem of Chemical 

 Evohitioti. By G. Rudorf. Pp. xvi -I- 228. (London : 

 VVhittaker and Co., 1900.) Price ^s. bd. 



THE object of this work, as stated by the author in 

 his preface, is one which should command hearty 

 approval. The author aims at presenting a summary of 

 the work done and the speculations advanced in the 

 particular field indicated in the title. The publication 

 of such summaries has long been customary in Germany, 

 and it is to be hoped that the custom may become 

 more common in England. Most text-books pub- 

 lished in this country suffer from one or other of two 

 defects. Either they are very elementary in scope and 

 wholly didactic in treatment, or they are diffuse in 

 treatment and of unmanageable size. This work cer- 

 tainly does not fall under either condemnation. It deals 

 with a difficult subject, and is rather suggestive and 

 argumentative than didactic. On the other hand, it is 

 neither unreasonably long nor over-elaborate in treatment. 

 Indeed, it sometimes errs in the other direction. 



