November i8, 1909] 



NA TURE 



6= 



the law of equipartition has never been proved for 

 purely dynamical systems, and is probably not in 

 general true, it is not surprising that it fails when 

 applied to the eether. 



The Zeeman effect is discussed in chapter iii., and 

 the author is chiefly concerned in showing how little 

 progress has been made in elucidating the phenomena, 

 and how much knowledge of atomic structure we may 

 reasonably hope to gain from study of the facts. 



The electron theory of dispersion is next dealt with. 

 Probably most readers will regret that the author has 

 not found it possible to enter more fully on the prob- 

 lem of the optical properties of metals. 



Optical phenomena in moving bodies forms the sub- 

 ject of the last chapter. The explanation of Fresnel's 

 hypothesis on an electromagnetic basis is one of the 

 most important results contributed by Lorentz to 

 optical theory* The explanation of the result of 

 the Michelson-JSIorley experiment and of the double 

 refraction experiment by Lord Rayleigh and Brace, 

 forms most interesting reading. The hypothesis of a 

 contracted electron is introduced, and with it the 

 question of electric inertia is again raised. The 

 chapter closes with an exposition of Einstein's 

 principle of relativity. 



The book is a most fascinating one, and to those 

 acquainted with Lorentz' former memoirs, it is un- 

 necessary to say that it is written with a lucidity that 

 characterises a master hand. 



W'e venture to offer some observations on the view 

 that negative electrons possess an inertia which is 

 entirely of electric origin. It appears to us that the 

 proof requires considerably more support, both on the 

 experimental and on the theoretical side, than it has 

 yet received. 



For many purposes it is unnecessary to define an 

 electron further than to say that it possesses a charge. 

 But when we come to the question of inertia we have 

 to define the size and shape of the electron. Surface 

 conditions are, of course, determined by the funda- 

 mental equations. We confine attention to the two 

 cases considered by Lorentz, viz., the "rigidly" 

 electrified sphere of .\braham and the contracted elec- 

 tron of Lorentz. It has been claimed that Kaufmann's 

 experiments agree with the spherical electron formula 

 and the view that the ordinary inertia is nil. As a 

 matter of fact, his experiments agree very much better 

 with the contracted electron formula, but make the 

 ordinary inertia quite comparable with the electric 

 inertia for slow speeds. Bucherer's experiments also 

 agree with the contracted electron formula, and make 

 the ordinary inertia nil, but the speed was not so 

 high as in Kaufmann's experiments. On the theoreti- 

 cal side it must be remembered that both formulae are 

 derived from the energy of the steady state, using the 

 "" quasi-stationary state " principle. This principle has 

 been acknow-ledged as quite unsound, and it must in 

 general lead to false results, when, as in the present 

 case, any change of velocity is attended with radiation 

 into the medium. We have reason to believe that 

 any change of velocity is accompanied by a redistri- 

 bution of the charge on the electron, and this in 

 general leads to an expression for the inertia which 

 XO. 2090, VOL. 82] 



differs from that obtained by differentiating the 

 energy of the steady state. 



We may well hesitate to sw-eep away the last scrap 

 of ordinary matter from an electron until the proof 

 rests on some principle more convincing than that of 

 the quasi-stationary state. 



OVR BOOK SHELF. 

 Handbook of Marks on Pottery and Porcelain. By 



\\'. Burton and R. L. Hobson. Pp. xii + 210. 



(London : Macmillan and Co., Ltd., 1909.) Price 



ys. bd. net. 

 This volume supplies a distinct want. Mr. Burton 

 is a practical potter, and the author of numerous works 

 on the history of pottery and porcelain. Mr. Hobson 

 is on the British Museum staff, and there are few 

 men with greater chances of seeing and studying 

 examples of all periods of pottery. The authors, there- 

 fore, speak with authority on subjects connected with 

 ceramics, and although they style it a " reliable pocket 

 volume," it is really a valuable and interesting addi- 

 tion to the bookshelves of collectors and students of 

 pottery. Thousands of authentic marks will, of 

 course, not be found in the volume, but those chosen 

 are, on the whole, thoroughly representative, and the 

 elaborate indices make the work of reference easy. 



It is interesting to note the influence that one 

 factory had on another, as shown by the marks. The 

 Meissen factory was the father of European porce- 

 lain. The Cross Swords from the arms of Saxony, 

 which was used there as their mark, can be seen 

 in Caughley Tournay, Worcester, Derby, and Bristol. 

 Even Meissen itself had in its early days, like so many 

 other factories, marks in imitation of the oriental. 



The short descriptions and the introduction are 

 models of Precis writing, giving in a page or two 

 the history of potting in each country. There is a 

 little confusion in the use of the terms "hard and 

 soft." Hard paste is fired at a high temperature, and 

 the glaze is fired at the heat at which the porcelain 

 matures. Soft paste, on the other hand, is fired at a 

 lower temperature, and the glaze at a still lower one. 

 The terms hard and soft have nothing to do with the 

 hardness and softness, as usually understood. So far 

 as the body of the piece of ware is concerned, some 

 hard paste may scratch with a file more easily than soft. 



The Oriental section is particularly good. It dis- 

 plays very great care, and is a decided advance on 

 anything of a similar kind that has been within the 

 reach of an ordinary collector. 



The scheme of the work takes the reader through 

 the various countries, and the period covered extends 

 from the Middle Ages to about 1850, with a selection 

 of modern marks. It is a very great advantage to 

 have the authors' assurance that none but undoubted 

 marks are illustrated in the volume. Great care seem 

 to have been taken with the dates also, but on page 33 

 the A. R., the cipher of .\ugustus of Poland, the 

 patron of Bottger, is given by the author as 1725-40. 

 Most authorities, and with reason, place this mark 

 considerably earlier. 

 The Races of Man and tkeir Distribution. Bv Dr. 



A. C. Haddon, F.R.S. Pp. x-l-126. (London: 



Milner and Company, Ltd., n.d.) Price is. net. 

 This book gives a description of the various races 

 of mankind as complete as appears to be possible 

 within the compass of a small volume. The physical 

 characters and culture of each race are described as 

 far as it is at present known, and the author has in 

 many cases to confess that. the knowledge is, as yet. 

 very incomplete. In the general classification of 

 mankind the character of the hair is taken as a 



