148 ^ao^y crops feed. 



of extreme division. This fine matter is in many cases 

 clay (kaolinite), or at least consists of substances resulting 

 from the weathering of the rocks, and is not, or not chiefly, 

 rock-dust. Between this impalpably fine matter and the 

 grains of sand retained by a sieve, there exist numberless 

 gradations of fineness in the particles. 



By conducting a slow stream of water through a tube 

 to the bottom of a vessel, the fine particles of soil are 

 carried otf and may be received in a pan placed beneath. 

 Increasing the rapidity of the current enables it to remove 

 larger particles, and thus it is easy to separate the soil in- 

 to a number of portions, each of which contains soil of a 

 diflferent fineness. 



Various attempts have been made to devise precise 

 means of sepaiating the materials of soils mechanically 

 into a definite number of grades of fineness. 



This may be accomplished in good measure by washing, 

 but constant and accurate results are of course only at- 

 tained when the circumstances of the washing are uniform 

 throughout. The method adopted by the Society of 

 Agricultural Chemists of Germany is essentially the fol- 

 lowing ( Versuchs Stationen, VI, 144) : 



The air-dry soil is gently rubbed on a tin-plate sieve 

 with round holes three millimeters in diameter; what passes 

 is weighed as fine-earth. What remains on the sieve is 

 washed with water, dried, weighed, and designated as 

 gravel, pebbles, stones, as the case may be, the size of the 

 stones, etc., being indicated by comparison with the fist, 

 with an Q^g., a walnut, a hazelnut, a pea, etc. Of the^/?;/e- 

 earth a portion (30 grams) is riow boiled for an hour or more 

 in water, so as to completely break down any lumps and 

 separate adhering particles, and is then left at rest for 

 some minutes, when it is transferred into the vessel 1 of 

 the apparatus, fig. 8., after ha\ ing j)oured off" the turbid 

 water with which it was boiled, into 2. This washing ap- 

 paratus (invented by Nobel) consists of a reservoir, A^ 



