REVIEWS 675 



purs. Anisotropic et Orientation des Molecules." The phenomenon and the 

 manner of its variation with temperature, wave-length, etc., are summarised, and 

 the theories proposed by Voigt, Havelock, and Langgvin are discussed. The 

 theory advanced by Mouton is that the molecules possess a double anisotropy : a 

 magnetic anisotropy which causes their rotation in a magnetic field, and an optical 

 anisotropy which produces double refraction. This theory accounts satisfactorily 

 for all the phenomena, but the other theories are not yet decisively disproved. 



A. Cotton discusses in an illuminating manner the evidence which it is possible 

 to obtain as to the properties of symmetry possessed by molecules, and points out 

 the directions in which there are possibilities of obtaining further evidence. His 

 pwn contributions to this subject are important and well known. 



The final memoir is on " Les Mouvements des Particules lumineuses dans les 

 Gaz," by Ch. Fabry. The uncoordinated movements are first dealt with, and the 

 information which can be deduced from the finite breadth of spectral lines is 

 discussed. The well-known application of the theory to the Orion nebula, by 

 MM. Bourget, Fabry, and Buisson, is referred to in illustration. The phenomena 

 of fluorescence, and resonance of gases, and of diffusion of light by gases, are 

 dealt with. The coordinated movements, such as canal and anode rays, are also 

 discussed briefly. 



H. S. J. 



BOTANY 



The Grasses and Grasslands of South Africa. By Prof. J. W. Bews, M.A., 

 D.Sc. [Pp. iv + 161, with 1 map and 24 figures.] (Pietermaritzburg : 

 P. Davis & Sons, Ltd. Price ?s. 6d. net, postage 6d.) 



As the author states in his preface, this work has been written as a preliminary 

 contribution to a big and important subject. From the wide range of aspects 

 dealt with there is necessarily much that will require amplification as the result 

 of further research, but Prof. Bews's presentation forms an excellent rhume of 

 existing knowledge to which the author has himself contributed so much. Both 

 the purely scientific and economic aspects receive due recognition, so .that, 

 whether the reader be practical farmer or botanist, their mutual dependence 

 cannot fail to be appreciated. 



For the assistance of students there is furnished an artificial key to the species 

 and genera of South African grasses, which occupies some forty pages. The 

 value of such a key can, of course, only be gauged by its actual employment in 

 the field, but the almost entire absence of relative characters from this scheme 

 eliminates one of the worst pitfalls for the inexperienced observer. The key is 

 preceded by a glossary, the usefulness of which is, however, somewhat marred by 

 the omission of most of the terms explained in the introduction. The third 

 section is devoted to notes on the principal species of each genus dealing with 

 their ecology, the gross anatomical features of their leaves, economic value, etc. 

 Prof. Bews in the fourth section treats of the developmental phases of grassland 

 and the part played by the Gramineae in the various plant communities dominated 

 by shrubs and trees. The subject-matter of this portion is subdivided according 

 to regions corresponding to broad climatic and edaphic distinctions. 



A particularly interesting feature to agriculturists and ecologists is the de- 

 scription of "substituted" types consequent upon burning or grazing. For 

 example, in the transitional region between the Karroo and the Eastern grass- 

 veld the natural grassland is dominated by the Red Grass (Anthisliria imberbis), 

 but burning or overstocking often results in the preponderance of far less valuable 



