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NOTICES OF NEW BOOKS. 
The Physical Properties of Colloidal Solutions. By E. F. Burton, 
B.A., Ph.D. 1921. viii + 221 pp., 18 illustrations. 2nd edition. 
Published by Longmans, Green and Co., 39 Paternoster Row, 
London, E.C. Price 12s. §d. net. 
That Professor Burton’s book should have reached a second edition 
already shows not only how rapidly the science of colloidal and molecular 
physics has advanced, but also that many research workers and students 
have found in it just the information which they needed. The ground 
covered is large, including the Brownian Movement, coagulation of 
colloids, together with a mass of detail dealing with optical properties, 
cataphoresis, endosmose, and kindred “ borderland ” subjects. 
To microscopists the chapter dealing with the ultra-microscope will 
naturally appeal. Here we find the Helmholtz-Rayleigh and Abbe 
theories of resolution given due prominence (such fundamental concepts 
being far too often overlooked), together with the chief forms of ultra- 
microscopic illumination. 
Stress is laid on the limitations of the ultra- microscope as an instru- 
ment of research, both from the point of view of ordinary observational 
work and in the domain of ultra-micrometry. 
The mathematical treatment throughout the book follows along 
conventional lines ; it suffers in part from being somewhat too restricted, 
especially in regard to Einstein’s classical work on Diffusion, and the 
deductions of Helmholtz and Lamb with respect to cataphoresis. Again, 
though the subject is suggested, we are not given overmuch information 
about the range of validity of Stokes’s Law (which has recently been 
called in question), a matter which concerns ultra-microscopists very 
deeply in their observations on the distribution of particles. 
The volume teems with fascinating problems for workers in colloidal 
physics, but is one well worth the attention ef microscopists ; there is 
perhaps no greater need than the bridging of the gulf which tends to 
divide microscopists and physicists. How narrow the gulf really is 
Professor Burton shows with a conviction born of experience. 
The book is well printed, and the diagrams are excellent, with the 
exception of Fig. 4, which is too small and obscure to be helpful. 
The very compendious references bo original memoirs are sure to be 
of great assistance both to microscopists and to those who study the 
subject of colloids for its intrinsic or industrial interest. F. I. G. R. 
Aggregation and Flow of Solids. By Sir George Beilby, F.R.S. 
1921. xvi + 256 pp., 34 plates. Published by Macmillan and 
Co., Ltd., London, W.C.2. Price 20s. net. 
The reader will not have proceeded far with the study of this book 
without coming to the conclusion that it constitutes a labour of love on 
the part of the author, and to some extent of those who have been 
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