

1 66 SCIENCE PROGRESS 



Molecular Association. By W. E.S. Turner, D.Sc. (Monographs on Inorganic 

 and Physical Chemistry. Edited by Alexander Findlay, M.A., D.Sc, 

 F.I.C.) [Pp. viii + 170, with 6 figures.] (London : Longmans, Green & 

 Co., 191 5. Price $s. net.) 

 This book contains the first comprehensive treatment in English of the important 

 subject of molecular association in gases, solutions, and liquids. The mode of pre- 

 sentation exhibits a fine example of critical exposition, only possible indeed when 

 the writer is himself intimately acquainted with his subject. There are eleven 

 chapters, the contents of which may be roughly divided as follows : Molecular 

 Association in Gases ; Molecular Complexity of Dissolved Salts ; Molecular Com- 

 plexity in the Pure Liquid State and the Various Methods, Qualitative and Quanti- 

 tatives Employed to Detect such Complexity ; The Specially Important Case of 

 the Molecular Complexity of Water, particularly in Relation to the Theory of 

 Dynamic Allotropy ; The Connection between Molecular Association and Physical 

 Properties on the One Hand and Chemical Combination on the other. 



As regards the question of association in gases there is little comment to make, 

 the striking feature being the simplicity of the molecular state except in the case of 

 sulphur, selenium, arsenic, and phosphorus. 



Dr. Turner has rightly insisted throughout the book upon the true relation of 

 association to molecular " normality" and to dissociation. One not inconsiderable 

 feature is the impression made on the reader's mind that association is by no 

 means to be regarded as an abnormal and exceptional state of things negligible in 

 generality and importance compared, say, with the phenomenon of dissociation. 

 On reviewing the subject as a whole in the light of this book, the wonder is rather 

 that normal molecular state is realised as frequently as it is in view of the con- 

 clusion brought home again and again that whatever property is taken to be truly 

 characteristic of a given substance, its molecular weight at any rate, in spite of its 

 apparently specific nature, cannot any longer be regarded as such. 



In the chapter upon the molecular complexity of dissolved substances we have 

 a comprehensive and ordered review of a set of phenomena varying much in 

 detail and, one would think, not easy to co-ordinate. Dr. Turner has made it 

 one of the best chapters in the book. Several quite surprising conclusions are 

 arrived at, for example, that electrolytes as a class (organic ammonium salts in 

 particular in various solvents) are strongly associated substances. We have 

 become so accustomed to think of dissociation in connection with electrolytes that 

 association has never received the attention it deserves. Another striking point 

 made by the author is the resemblance between association and the colloidal state. 

 In view of Perrin's work we have been forced to conclude that the distinction 

 between a homogeneous and a heterogeneous solution is one of degree and not of 

 kind, and it would seem that in the phenomenon of molecular association there 

 exists the connecting link making the transition a gradual one. This is borne out 

 by the fact that association of a substance usually occurs in a solvent in which the 

 substance is but little soluble. A further conclusion, which, if substantiated by 

 subsequent work, ought to prove of very great importance, is that the degree of 

 association of a salt is increased by the addition of a second salt, a phenomenon, as 

 the author points out, having something in common with the effect of electrolytes 

 on colloids. 



In dealing with the question of the variation of the apparent molecular weight 

 with concentration it is shown that five different effects may be anticipated (and 

 have actually been realised) supposing molecular combination of solute and solvent 

 to be possible as well as association of the solute. To judge what is really 



