PHYSICAL COMPOSITION OF SOILS. 



89 



The fine particles flock together, forming little pellets, which refuse to 

 be separated by the sieve. This difficulty can, of course, be partly 

 overcome by previously separating the clay from the sand by means of 

 water, as detailed above ; but even then it will be found that so 

 soon as the grain-sizes fall much below -gL of an inch (i millimeter) 

 the same difficulty is experienced, so long as the sand is dry. By 

 playing a small stream of water upon the sieve, however, all the parti- 

 cles beyond the -g-^ of an inch may be successfully separated from the 

 coarser portion ; and for many practical purposes the separation need 

 be carried no farther. 



Use of Water for Separating Finest Grain-Sizes. The scientific 

 investigator, however, must of necessity proceed to separate the finer 

 grain-sizes from each other, since, as will presently be shown, they 

 influence the tilling qualities of the soil to a much greater degree than 

 do the coarser particles. Such farther separation can be accomplished 

 only by the aid of water. 



Subsidence Method. \Yhen a small amount of soil is 

 stirred up in water, and is afterward allowed to stand for some 

 time, the different grain-sizes will settle consecutively in ac- 

 cordance with their sizes (or weights) ; the smallest ones 

 settling latest, and the clay only remaining suspended, as 

 stated above. So long, however, as any considerable amount 

 remains suspended in the water, the latter is not only denser 

 but especially more viscid than if the clay were absent. In 

 order therefore to obtain correct results by any method in- 

 volving the use of water, it is necessary to remove the clay be- 

 fore proceeding to the separation of the granular sediments. 

 This, as has been already stated, is approximately accom- 

 plished by allowing the soil, when diffused in water after 

 proper disintegration, to settle for 24 hours from a column of 

 water 200 mm. high, whereby all grain-sizes, of and above .01 

 mm. diameter are removed from the turbid liquid. This 

 sedimentation is then repeated until after 24 hours the water 

 becomes clear. The clay is then determined in the ; ' clay 

 water" by evaporation or precipitation; the granular sedi- 

 ments may then be successfully separated by sedimentation. 



The U. S. Bureau of Soils uses for the separation of clay, 

 instead of subsidence for 24 hours, the more expeditious pro- 

 cess of contrifuging the turbid soil water in appropriate glass 

 cylinders, by the aid of an electric motor; and thus in a rel- 



