r is
driven off.

On removal from the kiln, the dry wood at once takes up moisture from
the air, even in the driest weather. At first the absorption is quite
rapid; at the end of a week a short piece of pine, 1-1/2 inches thick,
has regained two thirds of, and, in a few months, all the moisture
which it had when air-dry, 8 to 10 per cent, and also its former
dimensions. In thin boards all parts soon attain the same degree of
dryness. In heavy timbers the interior remains more moist for many
months, and even years, than the exterior parts. Finally an
equilibrium is reached, and then only the outer parts change with the
weather.

With kiln-dried woods all parts are equally dry, and when exposed, the
moisture coming from the air must pass through the outer parts, and
thus the order is reversed. Ordinary timber requires months before it
is at its best. Kiln-dried timber, if properly handled, is prime at
once.

Dry wood if soaked in water soon regains its original volume, and in
the heartwood portion it may even surpass it; that is to say, swell to
a larger dimension than it had when green. With the soaking it
continues to increase in weight, the cell cavities filling with water,
and if left many months all pieces sink. Yet after a year's immersion
a piece of oak 2 by 2 inches and only 6 inches long still contains
air; _i.e._, it has not taken up all the water it can. By rafting or
prolonged immersion, wood loses some of its weight, soluble materials
being leached out, but it is not impaired either as fuel or as
building material. Immersion, and still more boiling and steaming,
reduce the hygroscopicity of wood and therefore also the troublesome
"working," or shrinking and swelling.

Exposure in dry air to a temperature of 300 degrees Fahrenheit for a
short time reduces but does not destroy the hygroscopicity, and with
it the tendency to shrink and swell. A piece of red oak which has been
subjected to a temperature of over 300 degrees Fahrenheit still swells
in hot water and shrinks in a dry kiln.

Expansion of Wood

It must not be forgotten that timber, in common with every other
material, expands as well as contracts. If we extract the moisture
from a piece of wood and so cause it to shrink, it may be swelled to
its original volume by soaking it in water, but owing to the
protection given to most timber in dwelling-houses it is not much
affected by wet or damp weather. The shrinkage is