), drying and then
dropping it into water, it floats. The air-filled cell cavity or
interior reduces its weight, and, like an empty corked bottle, it
weighs less than the water. Soon, however, water soaks into the cell,
when it fills up and sinks. Many such cells grown together, as in a
block of wood, when all or most of them are filled with water, will
float as long as the majority of them are empty or only partially
filled. This is why a green, sappy pine pole soon sinks in "driving"
(floating down stream). Its cells are largely filled before it is
thrown in, and but little additional water suffices to make its weight
greater than that of the water. In a good-sized white pine log,
composed chiefly of empty cells (heartwood), the water requires a very
long time to fill up the cells (five years would not suffice to fill
them all), and therefore the log may float for many months. When the
wall of the wood fibre is very thick (five eighths or more of the
volume, as in Fig. 20, _b_), the fibre sinks whether empty or filled.
This applies to most of the fibres of the dark summer-wood bands in
pines, and to the compact fibres of oak or hickory, and many,
especially tropical woods, have such thick-walled cells and so little
empty or air space that they never float.

[Illustration: Fig. 20. Isolated Fibres of Wood.]

Here, then, are the two main factors of weight in wood; the amount of
cell wall or wood substance constant for any given piece, and the
amount of water contained in the wood, variable even in the standing
tree, and only in part eliminated in drying.

The weight of the green wood of any species varies chiefly as a second
factor, and is entirely misleading, if the relative weight of
different kinds is sought. Thus some green sticks of the otherwise
lighter cypress and gum sink more readily than fresh oak.

The weight of sapwood or the sappy, peripheral part of our common
lumber woods is always great, whether cut in winter or summer. It
rarely falls much below forty-five pounds, and commonly exceeds
fifty-five pounds to the cubic foot, even in our lighter wooded
species. It follows that the green wood of a sapling is heavier than
that of an old tree, the fresh wood from a disk of the upper part of a
tree is often heavier than that of the lower part, and the wood near
the bark heavier than that nearer the pith; and also that the
advantage of drying the wood before shipping is most important in
sappy and light