NA TURE 



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THURSDAY, APRIL 7, 19 10. 



CRYSlAhJ^OGRAPHY BY A PHYSICIST. 

 Cours de Physique. By Prof. H. Bouasse. Sixieme 



partie, Etude des Symetries. Pp. 424. (Paris : Ch. 



Delagrave, n.d.) Price, 14 francs. 

 ''T^HIS sixth volume of the course of physics, pre- 

 J- pared b}' the author to conform "aux pro- 

 grammes des Certificats et de I'Agregation de 

 Phvsique," deals with the subject of symmetry, both 

 from the general physical standpoint and as it con- 

 cerns crystals. The previous volumes have treated of 

 mechanical physics, thermodynamics, electricity and 

 magnetism, optics, and electro-optics. Besides actual 

 svmmetry, this snth volume also discusses sym- 

 metrical deformation, double refraction in relation to 

 the symmetn.- of crystals, the bearing of optical rota- 

 tion on crystal symmetr}-, and a concluding brief 

 account of liquid crystals. It presents many of the 

 phenomena aeait with by the expert crystallographer 

 from the very different outlook of the pure physicist, 

 and such an outside view, by a competent authority, 

 of a more or less special branch of science is usually 

 of value, although one must not look either for 

 specialised knowledge or intimate acquaintance with 

 the phenomena described. In these days of specialists 

 there are very few men able to deal without error 

 with so many subjects as are included in the com- 

 prehensive course of Prof. Bouasse. 



The reader is very much handicapped at the outset 

 by the lack of any index, either of authors or sub- 

 jects, but it is impossible not to be struck with the 

 fact that the references to original investigations are 

 almost entirely confined to those of French men of 

 science. With the exception of a brief inevitable 

 reference to Miller, whose notation was bound to be 

 mentioned, we look in vain for any recognition of 

 the many important contributions to our knowledge 

 of both general and crystallographic symmetry which 

 have been made during the last twenty years by Bri- 

 tish investigators, such as Maskelyne, Miers, Barlow, 

 and Hilton, or of the large additions to chemical 

 cnstallography effected in this countr3% The work 

 of Sohncke and Schon flies is referred to, and a passing 

 mention of von Fedorow made, but anything approach- 

 ing appreciation of the later most important work 

 of the latter investigator and of Barlow is not forth- 

 coming. Nor could any reference be found to the 

 work on symmetry and cn,-stal structure of Von Lang, 

 Becke, Groth, or Muthmann. Gadolin is referred to, 

 as he published in the French langxiage, but the 

 omission of British and Continental work to such an 

 extent is a serious defect, as is also the fact that no 

 references whatever to the literature of any original 

 memoirs are vouchsafed. 



Having accepted these limitations, however, and 

 the further fact that practical details of crystallo- 

 graphic experimental work is not a feature of the 

 book, an admirable summary of the work of Haiiy, 

 Le\'}-, Fresnel, Mallard, Bravais, Curie, Friedel, 

 Bertrand, Senarmont, and other French men of 

 science, will be found. Moreover, when the author is 

 NO. 21 10, VOL. 83] 



on his own ground of pure physics, and especially in 

 the discussion of such parts of his subject as are 

 not affected by recent foreign work, his matter is ex- 

 cellent, vigorously and interestingly expressed, inci- 

 sive and clear. One of the most valuable parts of the 

 book is the chapter on space-lattices, which includes 

 a capital account of the immensely important, even 

 fundamental, work of Bravais, work which only in- 

 creases in value as the years roll on. It is followed 

 by a good description of the theon.- of "groups of 

 movements," which has resulted in our knowledge of 

 the 230 types of homogeneous structures, based 

 chiefly, however, on the treatise of Schonflies. 



A considerable part of the book is also occupied 

 with the physics of homogeneous deformations, 

 although all mention of work on the thermal dilata- 

 tion of crystals subsequent to that of Fizeau is 

 omitted, and the treatment of elasticity' and elastic 

 deformations is purely theoretical and lacks any refer- 

 ence to recent experimental work. 



When the author enters the domain of the crystallo- 

 grapher it is, unfortunately, to display a complete 

 want of knowledge of the immense progress which 

 has been made, largely by British workers, in experi- 

 mental methods, in the preparation of crystals of a 

 high degree of perfection for the purposes of investi- 

 gation, and of the highly accurate measurements 

 which have been made during the last twenty years 

 on such perfect crystals. The important laws 

 governing the relations between crystalline structure 

 and chemical constitution, especially those relating to 

 the effect of the interchange of the variable elements 

 in isomorphous series, which has been the main out- 

 come of this work, is entirely ignored. It is with 

 great surprise that we read, referring to the use of 

 even an ordinary goniometer provided with telescope 

 and collimator : — 



"Ce procede de haute precision n'est presque 

 jamais employe. C'est que les faces d'un cristal sont 

 loin d'etre des miroirs parfaits. EUes sont souvent de 

 tres petite.c dimensions. EUes presentent souvent des 

 irregularites, des stries. . . . Les faces sont souvent 

 courbes. Enfm, c'est un fait remarquable, les angles 

 des cristaux cristallographiquement les plus beaux ne 

 sont les memes d'un echantillon a I'autre ou d'un 

 angle a son homologue qu'avec une tolerance souvent 

 enorme. II faut entendre par la que, mesurant, avec 

 toute la precision possible, le meme diedre sur une 

 serie d'echantillons, ou les diedres homologues d'un 

 echantillon, on trouve des resultats qui peuvent differer 

 de plus de dix minutes. Pour toutes ces raisons, une 

 precision extreme est illusoire; I'emploi du gonio- 

 metre ordinaire, toujours inutile, est dans bien des cas 

 absolument impossible." 



It is dilTicult to understand how anyone holding such 

 opinions as to the value of exact work in crystallo- 

 graphv should wish to write a text-book on the sub- 

 ject. It is not by disseminating such opinions that the 

 studv of crystallography can be advanced, and young 

 students attracted to a richly repaying and almost 

 unopened branch of scientific investigation. If the 

 worker in crystallography is content to take the first 

 crvstal he alights upon as the best sample procurable, 

 he will, of course, conclude as the author has done. 

 But if he takes the trouble to obtain the most perfect 



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