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 deg.  C. (39.2 deg.  F.).  A cubic foot of distilled water at 4 deg.  C. weighs 62.43 pounds.  Hence the specific gravity of a piece of wood with a density of 35 pounds is 35 / 62.43 = 0.561.  To find the weight per cubic foot when the specific gravity is given, simply multiply by 62.43.  Thus, 0.561 X 62.43 = 35.  In the metric system, since the weight of a cubic centimetre of pure water is one gram, the density in grams per cubic centimetre has the same numerical value as the specific gravity.

Since the amount of water in wood is extremely variable it usually is not satisfactory to refer to the density of green wood.  For scientific purposes the density of “oven-dry” wood is used; that is, the wood is dried in an oven at a temperature of 100 deg.C. (212 deg.F.) until a constant weight is attained.  For commercial purposes the weight or density of air-dry or “shipping-dry” wood is used.  This is usually expressed in pounds per thousand board feet, a board foot being considered as one-twelfth of a cubic foot.