24'. 



NA TURE 



[January i i, 1906 



ference of polarised light follows a normal path, but 

 the- part relating to biaxial crystals is unusually full. 

 The later chapters deal with absorbing media, dis- 

 persion, structurally active media, and magnetically 

 active media ; each of these phenomena is shown to 

 follow from suitable modifications in the expression 

 for the light vector; the interesting question of the 

 constitution of the ether which could give rise to 

 such modifications is, of course, in the author's 

 scheme passed by. But while this is necessarily the 

 case, the analysis given is very full and complete, and 

 Mr. Walker has added to the literature of the subject 

 a book of real value. The book has been printed at 

 the Cambridge Press and published by the Syndics, 

 and their share of the work is admirable. 



LIQUID CRYSTALS. 

 Kristallinische Flussigkeiten und flussige Kristalle. 



By Rudolf Schenck. Pp. viii+158. (Leipzig: 



Wilhelm Engelmann, 1905.) 

 ^T^HK announcement of the discovery of liquids that 

 A were doubly refracting and dichroic by Prof. 

 Lehmann some fifteen years ago was received with 

 considerable mistrust, for the possession by a liquid 

 of these properties which had hitherto been associated 

 solely with the solid crystalline state seemed at first 

 sight almost inconceivable, and quite inconsistent 

 with the generally accepted ideas as to the molecular 

 tactics of liquids and crystals. The very name of 

 liquid crystal seemed to be self-contradictory. Leh- 

 mann \ results, however, were soon confirmed by 

 other physicists, one of the most active amongst 

 whom was Dr. Schenck, the writer of the present 

 work on the subject. 



Several explanations of Lehmann's observations 

 were offered, based on the assumption that he had 

 worked with liquids containing impurities. Quincke 

 supposed them to consist of solid crystalline particles 

 surrounded bv a film of liquid, and Tammann en- 

 deavoured to explain their properties by assuming 

 them to be an emulsion of two liquid phases. On the 

 other hand, Lehmann pointed out that it was not 

 justifiable to consider these cases as if the) were 

 isolated instances of irregular properties, since the 

 behaviour of these liquids apparently so anomalous 

 may be reconciled with that of other crystalline 

 media if we consider the part played by the rigidity 

 of crystals in maintaining their crystalline form. His 

 investigations have shown that the rigidity of dif- 

 ferent crystals varies within wide limits. The 

 majority of those we know best offer considerable re- 

 sistance to deformation, while some, like yellow phos- 

 phorus, are quite soft, and others, such as the oleates, 

 have so little rigidity that the force of surface tension 

 is sufficient to deform the crystal from its true geo- 

 metrical shape; in the limiting case, that of />-a/oxy- 

 anisol and the other liquids investigated by Lehmann, 

 the rigidity has become so small compared with the 

 force due to surface tension that the crystal when 

 placed in a liquid of equal density assumes a spherical 

 form. 



Lehmann's work was entirely microscopic, but 

 NO. 1889, VOL. 73] 



macroscopic investigations were undertaken by other 

 investigators. A study of the physical properties of 

 the birefrint^ent liquids, particularly of their viscositv 

 and dielectric constants, and an unsuccessful attempt 

 to resolve them by cataphoresis, showed that each of 

 them was without doubt a single substance, and thus 

 the hypotheses put forward by Quincke, Tammann, 

 and other authors were disproved. 



Prof. Lehmann's monograph on these bodies, which 

 was reviewed in Nature recently (vol. lxx., p. 622, 

 I 1904), consists mainly of an account of the results 

 1' of his microscopic investigations and of the theory 

 he has formulated to explain these. A very important 

 part of the work was thus left undescribed, and Dr. 

 Schenck 's book covers the ground omitted by Leh- 

 mann, and, in addition, gives a short summary of 

 the latter's experiments. 



The preparation of the various substances that have 

 been found to yield anisotropic liquids is described in 

 detail, also the determination of their physical con- 

 stants. The investigation of the surface energy o) 

 the liquids indicates that there is no sudden change 

 in their molecular weight at the temperature at which 

 the anisotropic liquid passes into the isotropic con- 

 dition. The viscosity curves, however, show a large 

 break at this temperature, the isotropic liquids being 

 in some eases the more viscous. The density curves 

 show a similar discontinuity. The two liquids have 

 different specific heats, and there is a small but 

 definite heat of transformation of one form to the 

 other. 



Dr. Schenck has given a very complete account of 

 our knowledge of these anomalous liquids, which 

 have great interest both for the chemist and physicist, 

 and his book will be of great service to those who 

 wish for information about them. It is clearly 

 written and arranged, and contains a number of 

 diagrams and plates. Of theories as to their nature 

 hi' wisely fights very shy, and it seems that consider- 

 ably more work is needed before we shall be able to 

 form any clear idea as to their molecular structure. 



H. B. H. 



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