August i, 1895] 



NATURE 



315 



large enough to admit the diameter of the disc one way, 

 and a little more than its thickness the other. The coil 

 carries two pole-pieces, one at each end, which project 

 from opposite sides a little way round the disc. Thus 

 •opposite poles are stretched out as it were from the coil 

 round the disc in the same direction. An alternating 

 current passed round the coil magnetises both these pole- 

 pieces and the disc, and the repulsion between the 

 .adjacent similar polarities of the disc and pole-pieces 

 produces the rotation, the polarities of both being of 

 •course reversed with the current. The disc is wound 

 with closed coils, so that the induced currents augment 

 the turning couple developed. This arrangement is 

 further developed into a "multipolar motor"; but in 

 •neither case is there any clear statement of how the 

 •action depends on hysteresis. 



In connection with these and similar devices it would 

 have been interesting to have had some estimate of 

 •efficiency, but generally speaking, in no part of the book 

 is there any discussion of this most important question. 

 Indeed, when the word energy is used it seems to bear 

 .a somewhat peculiar sense. For e.\ample, at p. 8i we 

 have a statement as to the "energies" of the field and 

 the armature, and the importance of these being equal if 

 for a given sum the motor is to have the greatest 

 efficiency. This passage is a little difficult of interpret- 

 ation, if the word energy is to be taken as it ought to be 

 in its technical sense throughout, though it is not very 

 hard to make out the idea intended. 



By far the most interesting portion of the book to a 

 student of electricity generally is Part ii. The alter- 

 nator of high frequency which Mr. Tesla used is fully 

 described, and the arrangements for using it explained 

 in the first of the lectures already referred to. The 

 phenomena produced are set forth in the remaining 

 chapters with numerous illustrations which render the 

 •descriptions very easy to follow. The whole subject of 

 high frequency phenomena is very intimately connected 

 •with the researches of Hertz on the one hand, and the 

 work of Mr. Crookes on the other, and forms a most 

 inviting field of research for e.xpcrimentalists who possess 

 the necessary equipment. Whether always the theo- 

 retical view taken by Mr. Tesla is correct, is matter for 

 legitimate difference of opinion. For one thing, we do 

 not think that there is any difference at all between 

 •electric force produced by what is properly called electro 

 static action and that produced by electro-magnetic 

 action. The distinction is only mathematical — the 

 former force can be derived from a potential function, 

 the latter cannot — and in a sense only expresses our 

 ignorance of the mode of production of the force. But 

 perhaps we are mistaken in supposing that Mr. Tesla 

 regards the electric forces in these two cases as different 

 in nature. 



To every |)hysical inquirer the perusal of these lectures 

 cannot but be of the greatest benefit. It will again re- 

 mind him that the field of research is unlimited, and 

 quicken his scientific enthusiasm, if not to taking part in 

 the work of this particular part of it, to at least prosecut- 

 ing with renewed vigour the intiuiry, whatexcr it is, which 

 lies ready to his hand. 



It was reported a few weeks ago that all the apparatus 

 and machinery belonging to Mr. Tesla had been de- 

 NO. 1344, VOL. 52] 



stroyed by fire. Every reader of his researches must 

 sincerely sympathise with Mr. Tesla in his loss of 

 valuable appliances and still more valuable time. That 

 he at once set himself to repair the loss is only what 

 was to be expected from his character ; let us hope that 

 it may result in such improvements of his means of 

 experimenting as may, in some measure at least, make 

 up for his disappointment, if it is not, what is perhaps 

 too much to suppose, turned into a blessing. 



A. Gr.'vv. 



OUR BOOK SHELF. 

 An Introduction to Chemical Crystallograpliy. By Andreas 



Fock, Ph.D., translated and'edited by William J. Pope, 



with a preface by N. Story-Maskelyne, M.A., F.R.S. 



Pp. iSgand xvi. 8vo. (Oxford: Clarendon Press, 1895.) 

 This little book is issued by the Clarendon Press as a 

 companion volume to Maskelyne's " Morphology of 

 Crystals," which was recently reviewed in these columns. 

 It is far from being a mere translation of Fock's " Einlei- 

 tung in die chemische Kr\-stallographie," which was 

 published in 1888. That book contained a useful sum- 

 maiy of the leading facts known about the origin and 

 growth of crystals, and the general relations between 

 their chemical composition and other properties, especially 

 as regards isomorphism and the properties of mixed 

 crj'stals. .All this is contained in the present volume, 

 which is, moreover, less sketchy than the earlier book, 

 and the somewhat numerous inaccuracies which dis- 

 figured the German edition have been corrected. But it 

 is in the additional matter that the chief alteration is to 

 be found, .\bout fifty pages have been introduced, con- 

 taining a survey of those important contributions to our 

 knowledge of crystals which have recently been made 

 from the side of physical chemistry ; the remarkable 

 theoretical researches of \'an t'Hoft'and Willard Gibbs, 

 and the quite recent experimental investigations of 

 Bakhuis Roozeboom, to which they gave rise, are here 

 ver)- happily summarised and brought within the reach of 

 the English elementary student. 



In order to give a comprehensive survey of the origin 

 and growth of crystals, it is necessary to take into account 

 the properties of the solutions from which they separate, 

 and several chapters are accordingly devoted to such 

 subjects as the relations between osmotic pressure and 

 concentration, the separation of double salts and those 

 containing water of crystallisation, the conditions of 

 equilibrium in a solution containing various solutes (to 

 employ a convenient word suggested by Prof. Maskelyne 

 in his preface as a term for the substances dissohed), and 

 the resulting variations in the isomorphous mixtures which 

 cr>-stallisc from such solutions ; all these are subjects 

 of great importance, which have up to the present time 

 met with no adequate treatment in English text-books. 



A treatise which merely summarises without criticism 

 loses much piquancy and interest, and also some value as 

 a guide to students. This objection may fairly be urged 

 against Fock's book, which appears to accept without 

 question all the observations reported by the author. It 

 would have been better, for example, to indicate the 

 insecure nature of some of the evidence which rests only 

 upon microscopical observation, such as that of Lehmann 

 and \'ogelsang. 



This book remains, nevertheless, an excellent survey of 

 chemical crystallography, brought fully up to date, and 

 one which will, we hope, open the eyes of English chemists 

 to a new field of work. 



Mr. Pope's translation is both fluent and accurate : he 

 is further responsible for some of the new matter intro- 

 duced into this edition. The book is lucid, readable, 

 and interesting, and is one which does credit to the 

 Clarendon Press. 



