The Surface Teiision of Water. — Ladd. 275 
Indirectly, it makes possible (?) a growth of vegetation 
which further and so largely protects the soil from erosion. 
Moreover, this vegetation is a source of chemical decomposing 
agents which accelerate soil accumulation — a process further 
facillitated by capillary penetration of the solid rock. 
The following generalizations, though containing familiar 
facts, are grouped together here as an outline of the features 
of the circulation of water in soils.* 
First — The soils are composed of particles of mineral 
matter of different sorts, chiefly quartz, feldspar and clay 
(kaolinite), and these occur in sizes rano^ing from that of 
coarse gravel down to minute rock fragments .0001 of a milli- 
meter in dimater.f They are also of different shapes, — the 
quartz grains varying in form from angular to well rounded, 
while the clay or kaolinite constituents are commonly present 
as thin flat scales. 
Second — The state of aggregation of these, and the rela- 
tive amounts of each present in different soils, varies enor- 
mously. 
Third — The attraction of the soil particles for water varies 
with their chemical nature; and particles of the same material 
have a varying "affinity," under different conditions (not now 
well understood). 
Fourth — Salts and organic matter, in solution, modify the 
value of the surface tension of the liquid, the former generally 
increasing, the latter decreasing it. 
Fifth — The circulation of water is of two sorts, viz., (a) flow 
due to hydrostatic pressure, or gravity, and (b) flow^ due to 
capillarity. 
Sixth — The permeability of the soil to water moving under 
the influence of gravity depends largely on the size of the 
tubes. The smaller the tubes, the slower the rate of flow. 
Seventh — The penetration of the soil by water acting un- 
der the influence of capillarity depends largely, also, but in a 
*For general discussion see Whitney, op. cit. 
tin this connection Whitney states the interesting facts; that on 
the 'average about 50 per cent of the volume of soils is space occupied 
only by air and water; that in a cubic foot of soil the grains have, 
on an average, at least 50,000 square feet of surface; and that soils 
containing from 10 to 30 per cent of clay consist, respectively, of from 
four to twelve billion grains. 
