i6 4 SCIENCE PROGRESS 



connected with optically active compounds, both organic and inorganic, while 

 his crystallographic work has included investigations, not only of the external 

 morphology of crystals, but also of the internal structure as revealed by X-rays. 



The earlier chapters of the book are concerned with the general problem 

 of symmetry and the derivation of the theoretically possible types, the latter being 

 based on the methods of Schonflies. Numerous examples of the various types 

 taken from biology as well as crystallography are given, and the symmetry 

 demonstrated by means of excellent illustrations. Although the subject-matter 

 is such that a simple presentation is difficult, the discussion is admirably lucid and 

 sufficiently non-mathematical to be easily followed. Unfortunately a fresh notation 

 for the symmetry groups is introduced, and a tabular comparison with those 

 already in use would have been useful for purposes of reference. The succeeding 

 two chapters deal with the application to crystallography, and cover not only the 

 geometrical theory, based on the limitations imposed by Haiiy's law, but also the 

 hypotheses which have been advanced regarding crystal structure, as well as 

 the results of the X-ray investigations of the latter. 



The geometrical theory of crystal structure is now fairly complete, and, as 

 several elaborate treatises on it have already been published, in the present book 

 only a general account of the derivation of the space lattices and point systems 

 is given. On the mechanical side, however, the solution of the problem is still 

 very incomplete, and this is instanced by the unsatisfactory nature of the structural 

 theories which have been proposed, while X-ray analysis still suffers from the 

 disadvantage that the results have sometimes been interpreted in more than one way. 

 The author's discussion of the valency-volume theory is scarcely impartial, as 

 comparatively slight mention is made of the weighty objections which have been 

 raised against it. With regard to the interpretation of X-ray analysis, the view 

 that is taken is that, while the crystal structure is built up by the interpenetration 

 of space lattices, each point of which is occupied by an atom, the chemical 

 molecule maintains its entity in the solid state. Although the latter cannot be 

 held to be proved or disproved, yet it seems scarcely correct to say that from the 

 crystallographic point of view the atoms can be arranged into molecules in any 

 arbitrary fashion. Geometrically this may be so ; mechanically it cannot be, and 

 crystallography is as much concerned with the mechanical side as with the 

 geometrical. Most of the arguments which have been advanced in favour of 

 the view that the " solid molecule " of sodium chloride contains one sodium and 

 one chlorine atom are equally favourable to the view that the " solid molecule " 

 contains a sufficiently large number of atoms of each type to give a first approxi- 

 mation to an unlimited point system. The description of the author's experimental 

 verification of the limitation of the Laue radiogram — owing to the centro- 

 symmetrical nature of the radiation the thirty-two crystal classes are reduced 

 to eleven — ought to have been completed by an explicit statement of Friedel's 

 earlier theoretical deduction. 



The latter portion of the book is occupied by a very able and complete account 

 of Pasteur's law and the associated phenomena. Emphasis is rightly laid on 

 several points which are often lost sight of ; for example, that compounds may 

 contain "asymmetric" carbon atoms and yet be inactive and non-resolvable, and 

 that optical activity depends on the compound possessing only axial symmetry. 

 The recent work in this field is critically surveyed and an indication given of the 

 immediate problems awaiting solution. One is glad to find that the author cannot 

 accept Werner's supposed resolution, by spontaneous crystallisation, of a compound, 

 the racemic form of which is less soluble than the active antipodes ; according 



