September 8, 1904] 



NA TURE 



451 



The author, here and elsewhere, keeps in close touch 

 with the actual experimental conditions, and makes 

 frequent reference to the historic controversies of the 

 earlv days of long-distance signalling. This chapter 

 is followed by an account of the author's investiga- 

 tion of the electrostatic field associated with a given 

 current system. An interesting special case is worked 

 out in detail, viz., the two-dimensional field produced 

 by a current sheet flowing round an infinitely long 

 cylinder, an impressed E.M.F. being localised in a 

 generator. 



In the treatment of induced currents which follows, 

 Prof. Brillouin departs widely from the order of ideas 

 now usually adopted. He confines the discussion to 

 fixed circuits in a uniform non-magnetic medium, and 

 takes as starting point Felici's experiments on the 

 induction of currents in a secondary circuit, by 

 making or breaking a given current in a primary. 

 Proceeding in the old action-at-a-distance manner, he 

 gets first a formula for the inductive action of an 

 element of the primary circuit on an element of the 

 secondary, and from this obtains the coefficient of 

 mutual induction and the vector potential. Some 

 cases of induction coefficients are worked out, and 

 then follows an exhaustive and critical analysis of 

 Kirchhoff's great memoir of 1857, in which the finite 

 rate of propagation of electric effects along a wire 

 was established. 



Perhaps the most novel feature of the book, at 

 least to an English reader, is the way in which the 

 question of open circuits is approached. The author 

 begins by adding to his vector potential a term which 

 goes out on integrating round a closed path. This 

 term is affected by an arbitrary constant which 

 appears also in the complete electric force derived 

 from the new vector potential. The value of this 

 constant is then chosen so as to make the divergence 

 of the electric force still equal to 4ir times the charge. 

 This preserves what the author calls the " unity of 

 the electric force," i.e., it makes the ponderomotive 

 force on unit charge identical with the current-pro- 

 ducing force which enters into Ohm's law. When 

 we have reached this point we find that the new 

 term in the vector potential has given us Maxwell's 

 displacement current. It is then shown that its 

 identification as a true current makes all currents 

 closed, and is justified by its electromagnetic effects. 

 The magnetic force is then introduced " pour la com- 

 modity de langage," as the vector the time-rate of 

 which is the curl of electric force; and such things as 

 magnet-poles need not exist at all. 



To readers brought up on Maxwell and Heaviside 

 this electrostatic method of arriving at things will 

 come as a sharp disturbance to the " normal piling " 

 of their electrical ideas. A similar disturbance would 

 be produced in the theory itself by the introduction of 

 a little iron into its system. We shall probably 

 understand the reason for the adoption of this pro- 

 cedure if we remember that Prof. Brillouin wrote 

 when "raffaire Cremieu " was at its height, and 

 before Pender crossed the Atlantic to see what the 

 matter was. An exposition which linked Maxwell's 

 NO. 18 19, VOL. 70] 



views to the earlier theories was specially natural at 

 that time, in view of the doubts suggested touching 

 relations which had come to be regarded as the 

 " solid ground of Nature." If a revision of belief had 

 been shown to be necessary, some such harking-back 

 to earlier positions as is displayed in the present 

 book would have become essential. 



The concluding section of the lectures is occupied 

 with a discussion of the problems of the Hertz 

 oscillator and of the oscillations proper to spherical 

 and spheroidal conductors. A full account is given 

 of the recent work of Prof. Pearson and Miss Lee 

 on the field of the Hertzian doublet as modified by 

 the damping of the oscillations. In the discussion 

 of the spheroid the author supplements the work of 

 Abraham and Maclaurin, specially in the direction of 

 numerical evaluation of the functions involved. 



W. B. M. 



MILK IN RELATION TO DISEASE. 



Bacteriology of Milk. By Harold Swithinbank, of 

 the Bacteriological Research Laboratory, Denham, 

 and George Newman, M.D., D.P.H., Medical 

 Officer of Health of the Metropolitan Borough of 

 Finsbury, and formerly Demonstrator of Bacteri- 

 ology in King's College, London. With special 

 chapters also by Dr. Newman on the Spread of 

 Disease by Milk and the Control of the Milk Supply. 

 Pp. XX + 605; illustrated. (London: John Murray, 

 1903.) Price 25s. net. 



THE public is beginning to recognise the import- 

 ance of milk and its products from the dietetic 

 and hygienic point of view, and public authorities are 

 becoming alive to the necessity for safeguarding the 

 miUi supply from adulteration, from the addition of pre- 

 servatives, and from contamination with filth and the 

 germs of disease. The appearance of this work, a 

 large volume of 600 pages, is therefore opportune. It 

 is a treatise on milk in its relation to disease rather 

 than, as its title implies, an account of the general 

 bacteriology of milk, for while such subjects as the 

 souring of milk and the various fermentations it under- 

 goes are dealt with in 55 pages, tuberculosis in relation 

 to milk, epidemics of disease due to infected milk, the 

 legal enactments regulating milk supply, &c., occupy 

 sorne 350 pages. 



As a general criticism, in the reviewer's opinion 

 some of the matter introduced might without detriment 

 have been omitted, thereby giving more space to 

 certain subjects that at present receive somewhat scant 

 treatment. Thus an attempt has been made to deal 

 generally with bacteriological technique, the prepar- 

 ation of culture media, and examination of water and 

 air, instead of limiting the matter in these directions to 

 that special to the subject. The pages on the agglutin- 

 ation reaction, on preventive inoculation in enteric 

 fever, and on the bacteriological diagnosis of 

 diphtheria seem to be quite unnecessary. The 

 chapter on the description of species of milk bacteria,' 

 occupying some 60 pages, also gives for the majority 



