Tlie Surface Toision of Water. — Ladd. 273 
\'. The percentage of secondary shrinkage varied in dif- 
ferent directions, being greatest for the suialler dimensions^ 
The reason for this being, probably, as follows: 
The tendency to establish equilibrium in the mass, due to 
the loss of water on an evaporating surface, is not followed 
by uniform results, owing to the greater amount of 
mass to be moved in the longer direction than in 
the shorter. Consequently the movement and shrinkage are 
more successful along the shorter dimension. More marked 
than this, however, as a cause, is the difference in the resistance 
per unit area of cross section due to the friction on 
the contact surface, at the bottom of the brick, along 
the lateral and longitudinal directions. A slender pen- 
cil of clay can only be dried without breaking, by resting it 
on a series of rollers, or some similar device. 
\'L The tenacity of the clays varied exceedingly at differ- 
ent stages of wetness. 
\"IL In general, clays diminishing the most in bulk 
through secondary shrinkage were, in the dried state, the most 
tenaceous. For example, one clay having a maximum shrink- 
age in a single direction of about 24 per cent, withstood a 
strain of nearly 400 pounds per square inch, while another, 
with a maximum shrinkage of less than 8 per cent, withstood 
a strain but slightly over two pounds per square inch. 
These laboratory experiments explain the nature of many 
€very-day phenomena in the geological world, where materials 
and conditions similar to those of the experiments are so con- 
stantly encountered. From their consideration we see that 
the most direct effects of surface tension acting as capillarity 
in rocks are the wettiiig and transference of water through 
these; shrinkage and consolidation. 
What is the relation of these effects to vegetation? 
Surface tension operates to retain rain-water at the sur- 
face, where it is available to furnish the moisture needed bv 
plant life, and also to draw up from the "water-table" below, 
supplies to renew that removed by evaporation and the re- 
c[uircments of vegetation. It carries, held in solution, with the 
water, the mineral foods which build the tissue and support the 
growing plants. It also forms a crust produced through 
shrinkage, which retards evajiorption. 
