148 HOW 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 off andl may be received in a pan placed beneath. 
Increasing tl:e rapidity of the current enables it to remove 
larger particles, and thus it is easy to separate the soil in- 
to anumber of portions, cach of which contains soil of 2 
different fineness. 
Various attempts have been made to devise precise 
means of separating 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 
throuzhout. The method adopted by the Society of 
Agricultural Chemists of Germany is essentia'ly 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 jfine-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 egg, a walnut, a hazelnut, a pea, etc. Of the fine- 
earth a portion (80 grams) is now 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 I of 
the apparatus, fig. 8., after having poured off the turbid 
water with which it was boiled, into 2, This washing ap- 
paratus (invented by Ndbel) consists of a reservoir, 4, 
