76 RELATIONS OF SOIL TO WATER 



drainage has ceased, the upper portion of the tube is dis- 

 connected, and the amount of water held by the drained soil 

 which it contains is then determined. Wollny's apparatus 

 follows the same principle, and is still more complete 

 (Forschungen der Agrikulturphysik, 1885, 177). It is only 

 by methods such as these that the true amount of water 

 retained by a soil can be ascertained. 



We have previously pointed out that the coarseness or 

 fineness of the particles has no direct influence on the quantity 

 of water that will be held by a mass when fully saturated ; 

 when however we have to deal with the amount of water 

 retained after thorough draining, the size of the particles, or 

 to speak more accurately their extent of surface, becomes the 

 factor having the preponderating influence on the result. The 

 larger are the particles, or the less the internal surface of the 

 mass, the smaller will be the proportion of water retained 

 after draining. 



Mayer separated powdered quartz by sifting into three 

 degrees of fineness : when fully saturated, each of these 

 powders contained more than 40 per cent, of its volume of 

 water ; when thoroughly drained they retained as follows : 



Diameter of Volume of Water retained 



Quartz Particles. per cent, of Total Volume. 



0-9 -2-7 mm 7-0 



0-3-0.9 mm 13-7 



below 0-3 mm 44-6 



The coarsest powder has thus lost by draining about five- 

 sixths of its water, while the finest powder retains after draining 

 about the same quantity of water which it held when fully 

 saturated. Schloesing points out that the very different 

 relation of fine and coarse particles to water may be shown by 

 sifting a sample of moist sand, and then determining the 



