385] H. E. Pulling 187 
which the conditions of the soil-air-water system are repre- 
sented in terms of any two of the three components. 
For convenience the components soil grains and water may 
be selected. The number of points of contact in any gross 
volume are determined by the number, size, shape and ar- 
rangement of the soil grains. In a sufficiently large volume 
(a few cubic centimeters, for arable soils) the soil grains 
may be considered as possessing an average density, size 
and shape, and this average will not change when other 
samples of the same soil are considered. Likewise if two 
samples of the same volume have the same number of soil 
grains, possessing the same average characteristics, it may 
be assumed that the average arrangement of grains is the 
same in each. This will be the more strictly true the longer 
the grains have existed in those volumes, since the forces 
of surface tension and gravity will tend to place them in 
the most stable positions. A relative measure of the proper- 
ties of a mass of soil grains may thus be obtained for any 
one soil in terms of its dry weight per unit of gross volume, 
termed the packing. 
The properties of the water masses in the soil may also be 
considered as being of average character and since these prop- 
erties depend upon the shape and size of the water masses, 
which in turn depend upon the shape and size of the spaces 
about the points of contact of the soil grains, the number 
of these points and the amount of water in the soil, they 
will be sufficiently defined by the amount of water and the 
amount of soil contained in each unit of gross volume. When 
these amounts are determined for samples of any given soil, 
each uniformly packed and permitting only a negligible rate 
of water movement, the data are at hand for plotting the 
experimental approximation to the minimum moisture curve 
between the limits of packing encountered in the samples. 
A method has been devised by the use of which samples 
of any given soil, each of approximately uniform packing, 
may be obtained with water contents so small that the rate 
of water movement is about 0.001 gram in 24 hours through 
