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reactions that are admirably introduced and treated in the present work. Experimental procedure is wholly omitted nor is any attempt made to give a complete citation of all photochemical reactions that have been investigated, though the most important ones, naturally, are discussed in applying theoretical considerations to them. The first chapter contains a list of gaseous and liquid reactions of which the quantum yields have been determined both in direct and in sensitized photochemical reactions. The second chapter, entitled “The Photochemical Primary Process,” is introduced by a splendidly condensed general treatment of atomic and molecular light absorption, continuous and discrete spectra, dissociation and predissociation, followed by the application of these principles to many photochemically important substances for which tables of absorption spectra and diagrams of energy levels are given. The third chapter deals with the photochemical secondary or thermal reactions that follow the primary act. The final chapter treats rather fully the complete kinetics of about a dozen of the most fully investigated photochemical reactions. After examining this excellent treatment of photochemistry from the modern standpoint, one cannot fail t o be impressed by the fact that in spite of its greater experimental difficulties photochemistry has not only repaid thermal kinetics for all that was borrowed, but has enriched it with a knowledge of atomic chemistry that will enable both to advance with enhanced certainty and rapidity, arduous though the way may be. S. C. LIND. Liquid Crystals and Anisotropic Melts. A General Discussion held by the Faraday 204 pp. London: Gurney and Jackson, 1933. Price: Society. 25 x 16 cm.; iv 12/6d. The present volume, reprinted from the Transactions of the Faraday Society, contains twenty-four papers and a general discussion. Two papers, that on the connection between the x-ray patterns of gases and true liquids, and the contribution of Bernal and Fowler on water, are particularly interesting. Water is, we presume, nearly as good a liquid as ever was, and our feeling is that the subject matter of the volume should have been extended to include the most interesting question of structure in so-called isotropic liquids. The book would then have possessed a uniformity which its contents a t present belie. There is no question about the value of the book as a whole. The subject has previously received attention from Continental scientists alone, as reference to a recent volume of the Zeitschrift f a r Kristallographie shows. The names of Vorlander, Kast, Ornstein, Rinne, Zocher, and Ostwald are sufficient guarantee to the authoritative nature of the volume under review. I n addition there are the English contributors, Bernal, W. H. Bragg, Crowfoot, Fowler, Malkin, and Lawrence. Ornstein and Zocher discuss rival theories of liquid crystals; Rinne emphasizes their biological importance. Lawrence makes a valuable contribution to the study of the lyotropic mesomorphs. Until these discussions, little or no study of the true crystalline phases of liquid-crystal forming substances had been made. Bernal and Crowfoot have performed a valuable service in remedying this state of affairs. I t is only by comparison of the densities in the solid and liquid phases that one may obtain clues as t o the modifications of grouping which take place on melting. If the arrangements in the true solid are unknown, speculations about arrangements in true liquids or the mesophases of liquid crystals become mere guess work. The work of Bernal and Fowler is of far-reaching importance. Lack of space prevents anything like an adequate summary of their paper; briefly, however, their
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achievements are these. The accepted crystalline structure of ice has been modified. The unit cell is very much bigger than that found by Barnes and the structure is molecular: four molecules are arranged in rough tetrahedral fashion round a central fifth. This arrangement and various developments of it in space, is, in conjunction with Prim’s theory, the basis of their explanation of the x-ray diffraction patterns of water and their variation with temperature. Water changes from something roughly ice-like at low temperatures to a quartz-like structure a t ordinary temperatures. Further heating tends to produce close packing. The whole volume is of great value to those interested in structural physics and chemistry, particularly from the point of view of the transition from solid to liquid. Need the binding of the Discussions be quite so flimsy? J. T. RANDALL.
