576 SCIENCE PROGRESS 



this country and in America — to show quite definitely that the 

 colloidal theory, which has extended into so many different 

 branches of science, will be of the greatest use to the soil 

 physicist in his study of the very complicated physical constitu- 

 tion of soil. 



Soil Physics may be divided into four broad sections : 



1. The dimensions of the individual particles and the 



manner of their arrangement. 



2. Soil moisture. 



3. Soil temperature. 



4. Soil atmosphere. 



Discussion of each of these sections, of which the first 

 two are the most important, follows immediately below. 



(i) Dimensions and Arrangement of Soil Particles 



Since soils contain particles of all sizes down to ultra- 

 microscopic dimensions, it is possible to classify soils by sorting 

 the particles into groups or fractions, each of which contains 

 all particles within certain specified limits of diameter. This 

 process is known as mechanical analysis, and, by suitably 

 choosing the various diameters, the characteristics of the soil 

 can be estimated with reasonable accuracy from the proportions 

 of each soil fraction present. The details of the process vary in 

 different countries ; the customary division in British practice 

 is as follows : 



Conventional name Average diameter 



of fraction. of particles, mm. 



Fine gravel .... above i -o 



Coarse sand 

 Fine sand 

 Silt . 

 Fine silt 

 Clay . 



I-O to Q'2 



0*2 to 0*04 

 0-04 to o-oi 



O'OI to 0-002 



below 0'002 



The separation, which is based on the assumption that 

 the individual soil particles follow Stokes's well-known law 

 for the fall of a sphere in a viscous liquid, may be done either 

 by sedimentation or elutriation. In the former case the soil 

 fractions are obtained by repeated settling in beakers through 

 a fixed depth of water for a definite time, and in the latter, 

 by means of upward currents of water of known velocity, 

 which carry off all particles below a certain size. 



A separation of the soil particles into six fractions must be 

 regarded as of only quahtative value, although for most practical 



