70 NATURE AND PROPERTIES OF SOILS 



be the determination of the percentage of each grade, or group, 

 of particles as based on the original sample. This is precisely 

 what every method of mechanical analysis in which water is 

 utilized aims to do, although the irregularity in the shape of 

 the particles prevents to a certain extent a perfect separa- 

 tion. The apparatus and technique of the various methods 

 employed are generally rather complicated. 



One of the earliest and most useful methods to be perfected 

 was the separation of the various grades of soil by simple 

 subsidence in a column of still water. This is commonly 

 spoken of as the Osborne beaker method. 1 The determination 

 is very simple. The soil sample is first fully deflocculated and 

 thrown into suspension, each particle functioning separately. 

 Beakers are commonly used as containers, but any vessel that 

 is relatively deep will do for the determination. The larger 

 particles, gravel and sand, will of course settle first, and the 

 finer silts and clays may be decanted off. As the sands carry 

 finer particles down with them, the suspension and subsidence 

 must be repeated a number of times. The sands are later 

 dried and sieved into their respective groups. The silt and 

 clay particles, thus decanted, may be separated from each 

 other by subsidence as above described. The time necessary 

 for such decantation as will leave in suspension only particles 

 below a given size is determined by the examination of a drop 

 of the suspension under a microscope fitted with an eyepiece 

 micrometer. In this way the size of the particles decanted 

 may be measured accurately. (See Fig. 14.) 



The four steps in this method of separation are : defloccula- 

 tion of the sample; separation by successive subsidence and 

 decantation ; evaporation to dryness of the separates and the 

 sieving of the sands; and the weighing of the separates and 

 the calculation of percentages based on the original dry sam- 



1 Osborne, T. B., Methods of Mechanical Soil Analysis; Ann. Rep. 

 Conn. Agr. Exp. Sta., 1886, pp. 141-158; 1887, pp. 144-162; 1888, pp. 

 154-157. 



