56 THE MECHANICAL ANALYSIS OF SOILS [chap. 



in one, the hydraulic method (Hilgard, Schoene, Nobel), 

 soil is washed by successive currents of water of veloci- 

 ties calculated to carry particles of the required size 

 according to the table on p. 37 ; in the other, the sedi- 

 mentation method of Osborne, Knop, and Schloesing, 

 the soil is suspended in water and allowed to stand, the 

 separation being effected either by the times required 

 for the particles to settle down through a fixed distance, 

 or by the distances fallen in a given time. The method 

 to be described is based upon the latter principle. The 

 hydraulic method requires special apparatus, and is only 

 suited to laboratories entirely devoted to soil analysis. 



Method of Analysis. 



I. Ten grams of the air-dry fine earth are weighed 

 out into a beaker or basin and treated with 100 c.c. of 

 A T JS hydrochloric acid ; the soil is well worked up with a 

 rubber pestle (made by fixing a glass rod into a small 

 solid rubber bung) until all the lumps of clay, etc., are 

 broken up. If the soil contains much calcium carbonate, 

 a further addition of acid may be required. 



The object of the acid is to dissolve the carbonates and 

 humates, and thus loosen the particles in any aggre- 

 gates where chalk or humus form the cement. With- 

 out this preliminary treatment the amount of clay 

 found will be largely determined by the proportion 

 of humus present ; the soil of an arable field, for 

 example, will show more clay than the soil of an 

 adjoining pasture, when the sedimentation is made with 

 water alone. But after the preliminary treatment with 

 acid to remove the humus, both fields will show the 

 same proportion of clay (as they should do, since they 

 are of the same origin), and only differ in the amount 

 of humus they have accumulated a temporary factor. 



After standing with the acid for an hour, the whole 

 is thrown on a tared filter and well washed until all acid 



