4 o PHYSICAL CONSTITUTION OF SOIL 



compound particles when it is rather less than half saturated 

 with water. In this happy condition of moisture there is 

 sufficient freedom of internal movement, without the cohesion 

 of the particles being unduly diminished. When this favour- 

 able condition exists, the formation of compound particles is 

 brought about by the alternate expansion and contraction 

 of the mass. 



The expansion and contraction of the soil may be simply 

 due to the difference between the day and night temperature, 

 the small alteration in volume being made effectual by its 

 frequent repetition. Much larger changes in volume may, 

 however, be brought about by alternate frost and thaw, and 

 by alternate drying and wetting; and these larger changes 

 produce naturally more speedy results. In freezing, water 

 expands to Jfths of its previous volume, and the expansion 

 of a dry loam after rain will easily exceed this proportion. 



In each case in which a moist soil is exposed to the 

 conditions assumed above, we have an unequal expansion and 

 contraction taking place in different parts of the mass. The 

 soil consists of particles of very various size and nature, 

 packed in various ways, coated by films of water of different 

 thickness, and with colloid matter irregularly distributed 

 throughout it. In such a mass, the cohesive force being 

 different in different parts, and the internal strains and 

 pressures also unequal, separations take place along the lines 

 of least resistance, and the mass becomes divided into groups 

 of particles, which as the operation progresses become more 

 and more isolated from each other 1 . During both frost and 



1 It is to be recollected that compound particles are, as already stated, only 

 ormed in soils of mixed constitution. Frost will, of course, disintegrate 

 a uniform moist sand, but in such a case no compound particles, but a powder 

 composed of individual grains, will be produced. 



