1 68 SCIENCE PROGRESS 



colloid, and in order to clear the ground at the outset the author emphasises the 

 fact that Graham's original idea of crystalloids and colloids as being different 

 kinds of matter is erroneous, and that in reality they are only different states of 

 matter. That this latter view is the correct one is evidenced by the fact that so 

 many substances can occur in either of the two forms according to the conditions 

 under which they are produced, and, indeed, von Weimarn has mathematically 

 formulated the factors which cause a substance to assume the one or other state. 

 In dealing with colloids we have to do with a two-phase system, one of which is 

 composed of small separate volumes and is known as the disperse phase, while 

 the other, which is continuous, is known as the dispersion medium ; supposing 

 that both the disperse phase and dispersion medium are liquid, we have what 

 is known as an emulsion, whereas when the disperse phase is solid we have a 

 suspension ; as special cases of these two types we have the so-called emulsion 

 and suspension colloids, or emulsoids and suspensoids, in which the dimensions 

 of the disperse phase are very small indeed, in fact ultramicroscopic. 



Concerning the misconception which still not infrequently exists with regard to 

 the diffusion of colloids, the author says : " It should be particularly observed that 

 Graham's results do not show that, as has been occasionally assumed, there is 

 no diffusion and dialysis of sols. Though his classification into crystalloids and 

 colloids is based upon the differences between them in this respect, he everywhere 

 gives figures for the dialysis and diffusion of colloids. . . ." 



The book is divided into four parts dealing respectively with the general 

 properties of colloids, methods of preparation, adsorption, and applications of 

 colloid chemistry. In the second part will be found a description and discussion 

 of von Weimarn's theory, as well as practical details for preparing a number of 

 typical colloidal solutions. Many interesting applications of colloidal chemistry 

 to sewage and water purification, soil, tanning and dyeing are given in Part IV. ; 

 but with regard to the theory of dyeing we find the somewhat discouraging 

 statement that " the main effect of the advent of colloid chemistry has been to 

 render the question still more complicated by providing yet another set of ex- 

 planations to those already existing, without increasing materially the prospect 

 of a final answer." In conclusion it should be stated that the book is replete with 

 information set forth in a lucid manner, and will be welcomed by students and 

 teachers alike as a valuable contribution to the somewhat sparing list of text-books 

 on colloids in this language. 



P. H. 



GEOLOGY 



The Problem of Volcanism. By Joseph P. Iddings, Ph.B., Sc.D. [Pp. xvi + 

 273, with a folding map and numerous plates.] (Newhaven : Yale Uni- 

 versity Press ; London : H. Milford, Oxford University Press, 1914. Price 

 2\s. net.) 



Under a bequest in memory of Mrs. Hepsa Ely Silliman, lectures are delivered 

 annually in Yale College on some branch of natural science, a wide view having 

 been taken by the testator as to the moral effect of such discourses. Prof. 

 Iddings, in 1914, chose volcanism as his subject, and the present handsome 

 volume embodies his account of modern research on igneous phenomena, and 

 his reasoned argument as to their causes. 



It is not unfair to mention the illustrations at the outset, since we owe them 

 in their present form to the author's judgment and selection. They mostly 



