622 



NA TURE 



[October 27, 1904 



chosen, clearly and yet artistically drawn, and excel- 

 lently reproduced. The sources from which they were 

 derived are indicated in some cases only, although a 

 number of the unacknowledged figures will be familiar 

 to the readers of zoological literature. 



There is an unduly large number of misprints in the 

 book, especially in the lettering and the inscriptions 

 of the various figures. There is no bibliography. 



G. E. S. 



LIQUID CRYSTALS. 

 Fliissige Kristalle : sowie Plastizitdt von Kristallcn im 

 Allgemeinen, Molekulare Umlagerungen iind Aggre- 

 gatzustanddnderungen. By Dr. O. Lehmann. Pp. 

 vi + 264; atlas of 39 photographic plates. (Leipzig : 

 W. Engelmann, 1904.) Price \l. net. 



WHATEVER may be the ultimate conclusion of 

 physicists concerning the explanation of the 

 interesting phenomena described by the author of this 

 volume under the term " liquid crystals," and however 

 diverse may be the views entertained as to their bear- 

 ing on current molecular theories, there can be no 

 difference of opinion as to the value of the work before 

 us, in which the description of these phenomena is so 

 clearly set forth and so fully illustrated. 



It was in 1889 that Dr. Otto Lehmann, the professor 

 of physics in the Technical High School of Karlsruhe, 

 and the author of many memoirs dealing with the 

 application of microscopical methods to physical re- 

 search, first suggested the use of this term " liquid 

 crystals." The acceptance of the term by physicists 

 and crystallographers has not been by any means 

 universal or unqualified, and in certain quarters it has 

 been received with something like ridicule. The 

 general attitude which, with our present knowledge of 

 the subject, it may be wise to preserve was well 

 expressed by Prof. Miers in an article upon the subject 

 which he contributed to Science Progress of January, 

 1897 :- 



" It will be wise to retain the names crystal and 

 crystalline in their old significations, rather than to 

 extend them so as to include the birefringent liquids 

 whose existence has been established by Lehmann. It 

 may be that these remarkable drops are examples of 

 liquid matter in which particles while free to move 

 are compelled to preserve the same orientation, and 

 differ in this respect from ordinary liquids. But 

 whether this peculiarity of structure, whatever may be 

 its nature, is really analogous to that of solid crystals 

 is a question in which it will be better not to commit 

 ourselves to an answer by applying the same name to 

 .both until more is known about the structure both of 

 liquids and solids." 



It is not probable that the present volume will 

 materially affect the cautious verdict pronounced by 

 Prof. Miers, seven years ago, on behalf of crystallo- 

 graphers and physicists, for although many new and 

 interesting observations are added to those announced 

 in Prof. O. Lehmann 's earlier memoirs, there is 

 nothing in the work before us which can be regarded 

 as supplying absolutely conclusive or crucial evidence 

 on the subject. 



In his original memoir Dr. O. Lehmann was able 

 ■to confirm the observations of Reinitzer and of Gatter- 

 NO. 1826, VOL. 70] 



mann that certain organic compounds possess two 

 melting points, and he showed that, at temperatures 

 between these two melting points, the substances, 

 though possessing the freedom of motion of liquids, 

 affect polarised light like crystals, and, like crystals, 

 exhibit the phenomenon of dichroism. 



In the treatise under review the list of organic 

 compounds exhibiting these remarkable properties is 

 considerably augmented, though all the newly described 

 substances are closely allied in their chemical nature 

 to those previously known. Dr. O. Lehmann pro- 

 poses to divide them into two classes — " Fliessende 

 Krystalle," in which between the two melting points 

 the crystal retains something of its original form, 

 modified by the rounding of the edges and angles, 

 though twO' of them brought into contact have sufficient 

 mobility to enable them to unite; and true " Flussige 

 Krystalle," in which, although the original crystal 

 form is wholly lost and the substance forms rounded 

 and very mobile drops, the double refracting and other 

 optical properties of the crystal are nevertheless re- 

 tained. It is, however, admitted by Lehmann that the 

 distinction between these two classes of substances is 

 neither very definite nor of fundamental character. 



It is impossible in the space at our command, even 

 if such a course were desirable, to enter upon the dis- 

 cussion of the physical relations of solids and liquids 

 which the author bases on his observations. On many 

 points he arrives at conclusions in marked opposition 

 to those maintained by Butschli, Nernst, Ostwald, and 

 other physicists. 



By the aid of the beautiful photographs, so admirably 

 reproduced in the plates, and the numerous diagram- 

 matic figures in the text, the reader will find it possible 

 to follow and understand the very interesting observ- 

 ations of the author. The value of the photographs 

 would certainly have been increased if, in every case, 

 the degree of magnification had been indicated ; and a 

 detailed description of the plates with a good index 

 would have added to the value of this very important 

 treatise. 



THE TESTING OF STEELS. 

 Relations between the Effects of Stresses Slowly 

 Applied and of Stresses Suddenly Applied in the Case 

 of Iron and Steel. Comparative Tests with Notched 

 and Plain Bars. By Pierre Breuil. Pp. vii+i52 + 

 23 plates. Jour. Iron and Steel Inst. Supplement, 

 vol. L\v. (London : Iron and Steel Institute, 1904.) 



CONSIDERATION of the subject of tests and test- 

 ing should be approached with a very open mind, 

 not only because it is the common meeting ground of 

 the engineer and the metallurgist, but because it is 

 a difificult subject of compromise, where the selected 

 method is practically never ideal. Steel is necessarily 

 as often submitted to tests which it is hoped will give 

 a measure of its qualities as to those stresses to which 

 it will be exposed in practice. The latter is the 

 practical ideal test short of behaviour in actual use, 

 which is seldom feasible, as often the finish of the 

 test would require to be left to another generation. 

 Thus the real purpose of tyres or rails is to wear well 

 without breaking; nevertheless, they are often tested 



