REVIEWS 155 



"if ever we are to solve the difficult problem of the elasticity of matter, it will be 

 by the aid of the phenomena of rotation " ; and later, in a sentence he reveals his 

 attitude not only to the study of gyrostatics but also, one thinks, to the study of 

 mathematics in general. He declares he is not content with the study of gyrostats 

 because they are interesting in themselves but because he realised " that in 

 reconciling the laws of light, the laws of magnetism, the laws of electricity and 

 the laws of matter, they must play an important part." 



The volume concludes by a chronological list of the titles of papers on " Elastic 

 Propagation," with references to the volumes in which they have been published, 

 interspersed by six short papers not previously reprinted. Three of the six are 

 of electrical or of magnetic interest, and a fourth deals with the " Duties of 

 Ether for Electricity and Magnetism." l The Elastic Solid theory of Ether was 

 incapable of accounting for magnetic phenomena and Thomson now puts forward 

 as a tentative theory the hypothesis that two portions of ether occupying infini- 

 tesimal volumes V and V at distance D repel mutually with a force equal to 



(p-i) V.(p'-i)V 

 D 2 



p and p being the respective densities of the two portions of ether considered. 

 If either (p — 1) or (/>' — 1) is negative, the two portions attract each other, and if 

 either becomes zero there is no repulsion. From which it follows that ether of 

 undisturbed natural density experiences neither attraction nor repulsion from the 

 ether, which the author premises is condensed into a positive electron, or from the 

 rarefied ether in the space occupied by a negative electron ; whereas the ether in 

 two positive or two negative electrons would repel each other and there would be 

 apparent attraction between a positive and negative electron. 



This, except for a short paper on " Stress and Strain," brings the volume to an 

 end. To future investigators this and the other volumes will prove invaluable, 

 bringing as they do together so many of the splendid contributions to Physics and 

 Mathematics of perhaps the greatest of English men of Science of the nineteenth 

 century — yea, perhaps the greatest since the author of the Principia. To the 

 serious students of mathematics no training can be finer than that entailed in the 

 disciplinary study of the problems presented in these papers, in which imagination 

 enlarges and makes to live that which in the hands of the pure mathematicians is apt 

 to be the dry bones of severe formalisn. 



F. C. Lea. 



Light, Visible and Invisible. Second Edition. By Silvanus P. Thompson. 

 [Pp. xiii + 369.] (London : Macmillan & Co., 1910. Price 6s. net.) 



The first six chapters of this book are a reprint of the first edition and consist 

 of six Christmas lectures delivered at the Royal Institution in 1896. To these 

 have been added a lecture on Radium given in different places in 1903-4, and 

 a lecture on the Manufacture of Light delivered in 1908 at the York meeting 

 of the British Association. 



There is no attempt at a complete exposition of the subject and higher 

 mathematics is intentionally absent (a few elementary proofs are given in appen- 

 dices) ; but it is surprising how much is included and satisfactorily explained. 



1 See Preface to Baltimore Lectures. 



