ants.

16. _The decay of roots_, etc., is allowed to proceed, because the
preservative influence of too much water is removed. Wood, leaves, or
other vegetable matter kept continually under water, will last for ages;
while, if exposed to the action of the weather, as in under-drained
soils, they soon decay.

The presence of too much water, by excluding the oxygen of the air,
prevents the _comminution of matters_ necessary to fertility.

[How much heat does water take up in becoming vapor?

Why does water sprinkled on a floor render it cooler?

Why is not a cubic inch of vapor warmer than a cubic inch of water?

Why does a wet cloth on the head make it cooler when fanned?

How does this principle apply to the soil?]

17. _The evaporation of water, and the consequent abstraction of heat
from the soil, is in a great measure prevented_ by draining the water
out at the _bottom_ of the soil, instead of leaving it to be dried off
from the surface.

When water assumes the gaseous (or vapory) form, it takes up 1723 times
as much _heat_ as it contained while a liquid. A large part of this heat
is derived from surrounding substances. When water is sprinkled on the
floor, it cools the room; because, as it becomes a vapor, it takes heat
from the room. The reason why vapor does not feel hotter than liquid
water is, that, while it contains 1723 times as much heat, it is 1723 as
large. Hence, a cubic inch of vapor, into which we place the bulb of a
thermometer, contains no more heat than a cubic inch of water. The
principle is the same in some other cases. A sponge containing a
table-spoonful of water is just as _wet_ as one twice as large and
containing two spoonsful.

If a wet cloth be placed on the head, and the evaporation of its water
assisted by fanning, the head becomes cooler--a portion of its heat
being taken to sustain the vapory condition of the water.

The same principle holds true with the soil. When the evaporation of
water is rapidly going on, by the assistance of the sun, wind, etc., a
large quantity of heat is abstracted, and the soil becomes cold.

When there is no evaporation taking place, except of water which has
been deposited on the lower portions of soil, and carried to the surface
by capillary attraction (as is nearly true on under-drained soils), the
loss of heat is compensated by that taken from the moisture in the
atmosphere by the soil, in the above-named manner.

This cooling of the soil by