Notices respecting New Boohs. 559 



Batelli and Stephanini [Atti R. Accad. Lincei, 1905, (v ) 14. 

 ii. pp. 3-14), who find that solutions of equal surface tension 

 have equal osmotic pressures, even in cases in which the 

 solutions are not nominally equimolecular. 



In conclusion it may be pointed out that whilst the views 

 advocated above were comparatively novel ten years ago, the 

 idea tbat osmotic pressure depends on the activity of the 

 solvent rather than on that of the solute has now become 

 widely accepted, and has been advocated not only by Poynting, 

 but also by Armstrong (Enc. Brit. xxvi. p. 739), Beilby 

 (B. A. Report, South Africa, 1905, p. 301}, and others. At 

 the present time, therefore, the only points for which any 

 degree of novelty can be claimed are in reference to the 

 mechanism by which the activity of the solvent at the surface 

 of the liquid is reduced by the "blocking action'"' of the 

 solute operating in one direction only, and to the possibility 

 of deducing from the osmotic phenomena information as to 

 the surface structure of liquids. 



130 Horseferry Boad, 

 Westminster, S.AV. 



XLYI. Xotices respecting New Boohs. 



Les Ultramicroscopes et les Objets Ultramicroscojriques. Par A. Cotton 

 et H. Motttox. Paris : Masson & Cie. 1906. Pp. 232. 



THE important part played by the microscope in almost every 

 department of science, pure and applied, readily accounts for 

 the large amount of attention which this instrument has received 

 and for the rapid improvements in its construction — improvements 

 which have rendered it one of the most perfect instruments at our 

 disposal. The limits beyond which a microscope of the ordinary 

 tvpe ceases to be serviceable — limits imposed on it by diffraction 

 phenomena — having been reached, investigators began to turn 

 their attention to means whereby objects too tiny to be visible 

 under ordinary conditions could he so rendered, by the use of 

 special devices, although such devices do not enable us to study 

 the exact form or minute structure of the object viewed. 



An account of the most recent researches in this direction will 

 be found in Messrs. Cotton and Mouton's excellent little treatise, 

 excellent alike from the point of view of the selection and arrange- 

 ment of subject-matter, and that of simplicity of treatment* 

 The subject is introduced by a very clear sketch of the physical 

 theory of the microscope and the limits of visibility. The use of 

 radiations of short wave-length is then dealt with, and in the next 

 chapter is given an account of the conditions under which ultra- 

 microscopic objects may be rendered visible, and a description of 

 the special instruments devised by Siedentopf and Zsigmondy, and 



