November 3, 192 1] 



NATURE 



299 



and that this applied not only to liquids, but also 

 to solids. Thus the different crystalline forms 

 were to be viewed as solid crystalline solutions of 

 t-ivo or more molecidar species. Bancroft had 

 already shown that many curious phenomena in 

 the behaviour of "pure" organic substances — 

 such as the dependence of the melting-point on 

 the rate of heating, in cases where no "ordinary " 

 decomposition occurred — could be explained only 

 by the existence of two different molecular species 

 in the liquid, these species existing in inner equi- 

 librium in the " natural " liquid, and the stable or 

 "natural" melting-point corresponding to the 

 point of intersection of the (T, x)-equilibrium 

 curve of the liquid with one of the saturation- 

 curves of the pseudo-binary system. Researches 

 on the tautomeric equilibria of enol-keto systems 

 have greatly extended these results. Prof. Smits's 

 theorv amounts to extending Bancroft's views to 

 the solid crystalline as well as to the liquid state. 



During the last fourteen years Prof. Smits has 

 gradually elaborated his theory on the basis of 

 the thermodynamic treatment of homogeneous 

 and heterogeneous equilibria, and he and his col- 

 laborators have published a very large number of 

 exceedingly interesting researches bearing on the 

 question. One might refer particularly to the 

 very thorough investigation of the phosphorus 

 system. He has also extended his theory in recent 

 years to the electrochemical equilibria existing 

 between atoms, ions, and electrons in both solids 

 and solutions, and has obtained new formulae 

 for the potential differences existing between 

 metals and solutions, etc., and new explanations 

 of such phenomena as polarisation (cathodic and 

 anodic), over-voltage, passivation and activation, 

 etc. 



In the present volume Prof. Smits has given a 

 full account of his theory, of the tests to which 

 he has subjected it, and of its application to many 

 phenomena observed or examined by himself and 

 others. 



In recent years a difficulty has arisen with 

 regard to the newer views of crystal structure 

 obtained by X-ray methods, which may be put 

 briefly thus : If the older view of a crystal as a 

 molecular structure is to be replaced by one where 

 the structural units are atoms, how can we regard 

 a crystalline phase as a solid solution of two 

 different molecular species in inner equilibrium? 

 Prof. Smits deals with this difficulty in a special 

 chapter of his book. He points out that the dif- 

 ferences which characterise his molecular species 

 may be often of a rather fine or subtle type, and 

 that these differences may very well persist in a 

 crystal structure in spite of the close juxtaposi- 

 tion and intimate inter-relationship of the atoms 

 NO. 2714, VOL. 108] 



of different molecules. The present reviewer 

 would suggest that possibly it may be a question 

 of the number and distribution of energy quanta 

 in and amongst the various atoms and atomic 

 groupings. In other words, we may have to do 

 with a kinetic equilibrium of energy-contents 

 rather than with molecular species differentiated 

 by structure in the ordinary sense of this term. 



Prof. Smits is clearly a man who is not afraid 

 of daring generalisations. He has developed his 

 theory fully from the graphical thermodynamic 

 side, and applied it to the explanation of a large 

 number of phenomena not otherwise easily ex- 

 plicable. Some critics may think he has pushed 

 his theoretical views too far, but undoubtedly his 

 theory has cast a flood of light on many obscure 

 phenomena. It has led, and is constantly lead- 

 ing, himself and his collaborators to numerous 

 quantitative experimental researches, and what 

 more can one ask of any theor}? And how many 

 true Baconians are there who can experiment well 

 without a theory to stimulate interest? 



For the present well-written and clear account 

 of his work all chemists owe Prof. Smits a great 

 debt of thanks. F. G. Doxnan. 



The Birds of Australia. 



A Manual of the Birds of Australia. By G. M. 

 Mathews and T. Iredale. Vol. i : Orders 

 Casuarii to Columbae. Pp. xxiv -f- 279-1- plates. 

 (London: H. F. and G. Witherby, 1921.) 

 3 guineas net. 



AUSTRALIA, the birds of which have a fore- 

 most place among those of the world for 

 their variety of form and beauty of plumage, 

 has been fortunate in having two splendid 

 treatises devoted to its avifauna, namely, 

 John Gould's magnificent volumes, published 

 between the years 1840-48, which are pictorially 

 unrivalled among ornithological works ; and 

 the recent valuable and beautiful work by 

 Mr. Mathews. Apart from these great and costly 

 books, there has long been a want of a modern 

 handbook on the subject at a moderate price, not 

 only by those who are specially 'concerned with 

 the Australian ornis, but also by students 

 interested in bird-life generally. Gould realised 

 this, and in 1865 published his excellent and useful 

 " Handbook of the Birds of Australia " in two 

 octavo volumes. From 1865 to the present time 

 our knowledge relating to the birds of Australia, as 

 of air other countries, has advanced prodigiously, 

 and the old handbook is now quite out of date. 

 Very fortunately, however, Mr. Mathews, like his 

 eminent predecessor, has realised that an up-to- 



