[ ( ^ 51 ] 



XCIII. Notices respecting JSew Boohs. 

 A History of the Theories of jEther and 'Electricity from the 

 Aye of Descartes to the close of the Nineteenth Century. By E. T. 

 AVhittaker, tic.D., F.lt.S. Pp. xv + 475. London : Longmans, 

 Green & Co. Dublin : Hodges, Figgis & Co., 1910. Price 

 12 5 . 6d. nett. 



"I T is not often that an historian exhibits an equal insight info his 

 subject to that which Dr. Whittaker shows in this remarkable 

 work. In reading it one is over and over again struck with 

 wonder at how he has found time in the midst of his other duties 

 even to collate the vast amount of material which the book 

 contains. Bat it is not a mere collection. Each stage in discovery 

 is subjected to a clear criticism which places it accurately in 

 relation to preceding and subsequent discoveries. The whole must 

 have involved a most painstaking examination of even out-of-the- 

 way and little known theories over which " the iniquity of oblivion 

 lias scattered everywhere her poppy." The result is a volume 

 which will be Gf immense value to the physicist in giving him a 

 clear conception of the nature of the gradual advance which has 

 taken place from the time of Descartes onwards. It may be 

 divided roughly into two portions, viz., that dealing with light, and 

 that dealing with electricity : these, of course, run side by side 

 and ultimately coalesce into one subject. One important point in 

 the treatment is that in the presentation of the mathematical 

 development, the notation and method of proof are in every case 

 modernised so that they become immediately iutelligible. This 

 is done without sacrificing in any way the original character of 

 the argument, so that nothing essential is lost and much is 

 gained in the way of lucidity. As an illustration of the exhaustive 

 way in which Dr. Whittaker deals with a subject, take the case 

 of the pressure of light. On this subject " an experiment was 

 performed in 1708 by Homberg, who imagined that he actually 

 obtained the effect in question ; but Mairan aud Du Fay in the 

 middle of the century, having repeated his operations, failed to 

 confirm his conclusion." " The subject was afterwards taken up 

 by Michell, who ' some years ago,' wrote Priestley in 1772, 

 ' endeavoured to ascertain the momentum of light in a much 



more accurate manner He exposed a very thin and 



delicately-suspeuded copper plate to the rays of the sun concen- 

 trated by a mirror and observed a deflexion ' A similar 



experiment was made by A. Bennet [the inventor of the gold-leaf 

 electroscope] who directed the light from the focus of a large lens 

 on writing paper delicately suspended in an exhausted receiver ' but 

 could not perceive any motion distinguishable from the effects of 

 heat.' " These experiments were made of course in connexion 

 with the corpuscular theory, and Bennet concluded from his 

 negative result that •' heat and light may not be caused by the 

 influx or rectilineal projections of fine particles but by the 

 vibrations made in the uuiversallv diffused caloric or matter of 



