WATER CAPACITY OF SOILS 65 



sible quantity of water when all the interspaces between the 

 particles are filled by that liquid, the soil is then perfectly 

 saturated. The quantity of water which a soil may contain 

 when in a saturated condition is thus determined by the 

 volume of its interspaces. The circumstances which determine 

 the proportion of interspaces in a soil have been already men- 

 tioned (p. i), and should be referred to once more before 

 pursuing the subject any further ; the experimental results 

 we are about to describe will serve to illustrate the principles 

 there laid down. 



The proportion of interspaces in any volume of soil may 

 be ascertained by determining the quantity of water which 

 it holds when perfectly saturated. It may also be ascertained 

 by calculation, if both the apparent specific gravity of the 

 dry soil and the real specific gravity of its mixed constituents 

 are known. Thus, if we deduct the weight of i litre of the dry 

 soil from the weight of i litre of the solid soil constituents, 

 and divide the difference by the specific gravity of the soil 

 constituents, we obtain the volume of the interstices in i litre 

 expressed in terms of water. 



Numerous determinations in natural gravel and sand of 

 various degrees of fineness, and in artificial mineral powders, 

 show that so long as the material itself is non-porous, the 

 interstices are generally about 40 per cent, of the whole 

 volume. The volume of the interstices tends to rise when 

 the particles are very small, owing to the looser packing 

 under these conditions ; it tends also to rise when the particles 

 are of uniform dimensions. It falls when the variation in 

 size is considerable, owing to the closer packing which then 

 takes place from the insertion of small particles in the 

 spaces between larger ones. 



F 



