490 SCIENCE PROGRESS 



the clay fraction in soils has been studied by many workers, but the points 

 of similarity and dissimilarity between these materials (to say nothing of 

 the different meanings attached to the same word in the two industries) 

 are at present almost unexplored. 



In this connection it is instructive to compare Dr. Mellor's paper on the 

 plasticity of clays with those of Mr. Comber on the mechanism of fiocculations, 

 and Mr. Morrison on the formation of the hard layer, known as a " pan," 

 in soil. 



Returning to the sections in which the papers are grouped, the section 

 dealing with soil moisture contains an introductory paper by Mr. B. A. Keen, 

 who discussed the significance of the various divisions and equilibrium 

 points recognised in the soil moisture by American investigators with special 

 reference to the method of the freezing-point depression. Prof. Od6n con- 

 tributed a note on the hygroscopicity of clay in which he showed that this 

 quantity depended not only on the surface area of the clay particles, but 

 also on its chemical nature and constitution. Two papers are also con- 

 tributed in the section by American investigators — on the soil solution in 

 relation to the plant, by Prof. Hoagland, and on osmotic phenomena, by 

 Prof. ShuU. The former deals with the relation between the concentration 

 of salts in the soil solution, determined by analysis of water extracts and 

 other means, to the growth of plants ; the latter paper is devoted to various 

 theories of the osmotic action of the root hairs in the passage of water from 

 the soil into the plant. 



In the concluding paper of this section Dr. Hackett gives an account of 

 experimental work on the ascent of liquid through granular media, in which 

 it is shown that the laws of capillary flow are obeyed for the initial rise which 

 takes place in the tube-like spaces between sand grains. This comparatively 

 rapid initial rise is followed by a slow ascent, which is apparently not explained 

 by the theory of capillary flow. 



A number of the essential properties of soil are conferred upon it by 

 the organic matter present. These properties are discussed by Mr. Page in 

 the introductory paper to section 3. Prof. Oden gives a summary of his 

 important investigations on humus, in which he has demonstrated the 

 existence of humic acid, and thrown considerable light on the question of 

 the origin and cause of soil acidity. Dr. SaUsbury discussed from the 

 ecological standpoint the vertical distribution of soil acidity in soils, and 

 showed that under conditions of undisturbed natural vegetation, the acidity 

 decreased from above downwards. 



The opening paper in the section dealing with adsorption phenomena is 

 by Mr. Fisher, who critically discusses the application of the empirical adsorp- 

 tion equation to the reactions between soil and some inorganic substances. 

 Mr. Crowther deals with some aspects of soil acidity, such as the methods for 

 measuring the hydrogen-ion concentration of soils, and the action of neutral 

 salts on acid soils ; Mr. Morison discusses the causes of pan formation, 

 which appears to be due to the translocation of colloidal material from the 

 surface layer of soil and its subsequent coagulation at a certain depth, where 

 it has a cementing efiect on the soil particles and produces a hard impervious 

 layer. 



In the final section. Prof. Oden describes his method of mechanical 

 analysis, by which he obtains a curve showing the distribution of the weight 

 (or number) of particles corresponding to given radii. Mr. Comber deals 

 with mechanism of flocculation, and brings forward evidence to show that 

 the clay fraction can be regarded as having a siUceous protection, Avhich 

 impresses emulsoid properties on the clay. 



The verbatim report of the discussions following the papers has already 

 been mentioned. It adds much to the value of the papers themselves. 



B. A. K. 



