952 Notices respecting Neic Books. 



heat or fluid of light." Young came to the same conclusion. "But 

 Euler many years before had expressed the opinion that light- 

 pressure might be just as reasonably expected on the undulatory 



as on the corpuscular theory." "Euler accounted for the 



tails of comets by supposing that the solar rays, impinging on the 

 atmosphere of a comet, drive off from it the more subtle of its 

 particles." Then follows a full exposition of the contributions of 

 Maxwell, Bartoli, Boltzmann, and a reference to Lebedeu's 

 earlier work. The fuJl experimental verification, of course, belongs 

 to the twentieih century. 



The most interesting and certainly the most important part of 

 this history deals with the development of electrical theory from 

 the time of Maxwell onwards. Maxwell had added a displacement 

 current to the conduction current ; FitzGerald added the 

 convection current, p¥; Hertz and Ht-aviside showed that in the 

 case of moving media the term curl (D.W) must, in accordance 

 with their conceptions, be added in order to obta'n the total 

 current — i. e., that a dielectric which moves in an electric field 

 is the seat of an electric current which produces a magnetic field 

 in the surrounding space. Later on, Lorentz replaced this last 

 term by curl (P. W) which implies that the moving dielectric does 

 Hot carry on the sethereal displacement but only carries along 

 the charges which exist at opposite ends of the molecules of the 

 ponderable dielectric and which are represented by P, the polari- 

 zation. The subject is too complicated and the different issues 

 that arise far too numerous to be summarised here ; but Dr. 

 Whittaker leads the reader on from point to point till (at any rate 

 if he has been long enough acquainted with the subject) he seems 

 to live over again the developments that have taken place but 

 with the fuller understanding which the author's critical acumen 

 imparts. 



The book is excellently edited. We notice 'conduction' instead 

 of ' convection ' on p. 367 (beneath the first formula). The use 

 of an unmistakable small numeral one to stand for current looks 

 very odd in formulas ; it has apparently been adopted because the 

 dot of a small i would be mistakable for time-differentiation 

 since the fluxional notation is commonly adopted. Perhaps the 

 mine of the eldest Becquerel should have been mentioned in 

 connexion with the earliest experimental work in thermo- 

 electricity. Poincare's criticism of Maxwell's stresses probably 

 does not come within the time-rarge of the book but might 

 perhaps be alluded to in a footnote. The present writer has 

 endeavoured to recall the early work on those parts of the subject 

 with which he is most familiar; but in each case on turning up 

 here he has found at least as much as he had recollected. 



It should be added that the book is not intended for an elementary 

 student. A man will require some considerable general knowledge 

 before he will be able satisfactorily to tackle it; but to the 

 properly equipped man it is a mine of erudition and carefully 

 reasoned criticism. 



