ARTICLES 579 



on the formation of compound particles. He suggests that the 

 clay particles are protected by a coating of an emulsoid character, 

 and thus partake of the properties of an emulsoid. The larger 

 soil particles, such as silt, by themselves are suspensoid in 

 character. Soil aggregates are supposed to be built up around 

 a large nucleus, the outer layers of the compound particles 

 being composed of clay particles, which impose their emulsoid 

 nature on the whole aggregate, and therefore, normally, on 

 the whole soil. One practical application of this view lies in 

 the fact that it may be possible to devise an ameliorative 

 treatment for a class of soils known as " silt soils." Mechanical 

 analysis shows the presence of a large proportion of the " fine 

 silt " fraction, and very little clay. The difficulty of working 

 these soils in practice is ascribed by Comber to the inability 

 of the small amount of clay to impose its emulsoid character 

 on the large suspensoid surface exposed by the fine silt, and one 

 possible remedy advanced is to apply emulsoid materials 

 before liming. 



It is now necessary to consider briefly some other conse- 

 quences arising from the loose packing together of the soil-grains. 

 There is a certain amount of space not taken up by soil par- 

 ticles, and this will be occupied by the soil water and soil 

 atmosphere, whose proportions will vary with the soil conditions. 

 The volume of this pore space varies roughly from 30 per cent, 

 to 50 per cent., according to the type of soil, and is usually least 

 in sandy soils. The physical conditions within it are of vital 

 importance, since they will control the available water supply 

 to the plant roots, the environment of the micro-organisms 

 engaged in producing plant food, and so on. In order to form 

 a mental picture of the pore space, it may be regarded as a 

 bundle of capillary tubes, irregular in length, width, and 

 direction, along which the soil moisture and soil atmosphere 

 circulate. For mathematical purposes it is necessary to in- 

 troduce certain assumptions : we may take the soil-grains as 

 spheres all of the same radius and packed together in a sym- 

 metrical manner. We must, in fact, consider an " ideal," rather 

 than a natural, soil. Under these conditions the dimensions 

 of the capillary tubes can be expressed in terms of the 

 common radius of the spheres, and calculations may be made of, 

 for instance, the rate at which water will percolate through a 

 given depth of such a soil. King, in America, has used this 

 conception of the ideal soil for the purpose of defining various 

 actual soils. The percolation of a fluid through tubes con- 

 taining soil was measured, and compared with the percolation 

 through tubes of sand, all the sand-grains in any one tube being 

 approximately of the same diameter. The diameter of the 

 sand particles in a tube which gave the same percolation as a 



