78 RELATIONS OF SOIL TO WATER 



We at once see that the finer the sand, the larger is the 

 proportion of water retained after draining. In the case 

 of the two coarser sands it is evident that we nearly approach 

 the condition of a column composed of uniform particles, the 

 proportion of water retained throughout each column vary- 

 ing very little till the bottom is approached. The finest 

 sand is, on the contrary, clearly a mixture of particles of 

 different size, as it is only in the first two feet that we 

 find a uniform minimum contents; from this point down- 

 wards the proportion of water rapidly increases. 



We have already stated that the reason why a mass of 

 fine particles retains more water after draining than a mass 

 of coarse particles is simply due to the far greater surface for 

 holding water existing in the first instance, and to the much 

 narrower interspaces between the particles. It follows of 

 course that porous particles will have a much greater power 

 of retaining water than solid particles of the same dimensions. 

 Mayer prepared a fine powder from quartz, calcspar, clay- 

 stone, and wood ; the particles were in each case as far as 

 possible of the same size (0-3-0-9 mm. diam.). He then 

 determined the volume of water retained by 100 volumes 

 of each powder after draining. The results were as follows : 



Calcspar. Quartz. Claystone. Wood. 



11.7 ... 13-7 ... 24-5 ... 45-6 



The porous particles thus retained far more water than 

 the solid particles, though all were of approximately the same 

 diameter. 



Schloesing has given some determinations of the weight 

 of water held by fully drained soils. His results were as 

 follows : 



