the crushing strength parallel to the grain, fibre stress at
elastic limit in bending, and shearing strength along the grain
of wood vary in direct proportion to the weight of dry wood per
unit of volume when green. Other strength values follow
different laws. The hardness varies in a slightly greater ratio
than the square of the density. The work to the breaking point
increases even more rapidly than the cube of density. The
modulus of rupture in bending lies between the first power and
the square of the density. This, of course, is true only in case
the greater weight is due to increase in the amount of wood
substance. A wood heavy with resin or other infiltrated
substance is not necessarily stronger than a similar specimen
free from such materials. If differences in weight are due to
degree of seasoning, in other words, to the relative amounts of
water contained, the rules given above will of course not hold,
since strength increases with dryness. But of given specimens of
pine or of oak, for example, in the green condition, the
comparative strength may be inferred from the weight. It is not
permissible, however, to compare such widely different woods as
oak and pine on a basis of their weights.[27]

[Footnote 27: The oaks for some unknown reason fall below the
normal strength for weight, whereas the hickories rise above.
Certain other woods also are somewhat exceptional to the normal
relation of strength and density.]

The weight of wood substance, that is, the material which
composes the walls of the fibres and other cells, is practically
the same in all species, whether pine, hickory, or cottonwood,
being a little greater than half again as heavy as water. It
varies slightly from beech sapwood, 1.50, to Douglas fir
heartwood, 1.57, averaging about 1.55 at 30 deg. to 35 deg. C., in terms
of water at its greatest density 4 deg. C. The reason any wood
floats is that the air imprisoned in its cavities buoys it up.
When this is displaced by water the wood becomes water-logged
and sinks. Leaving out of consideration infiltrated substances,
the reason a cubic foot of one kind of dry wood is heavier than
that of another is because it contains a greater amount of wood
substance. ~Density~ is merely the weight of a unit of volume,
as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre.
~Specific gravity~ or relative density is the ratio of the
density of any material to the density of distilled water at 4 de