aving blocks often require that longleaf
pine be 90 per cent heart. This is on the belief that sapwood is
not only more subject to decay, but is also weaker than
heartwood. In reality there is no sound basis for discrimination
against sapwood on account of strength, provided other
conditions are equal. It is true that sapwood will not resist
decay as long as heartwood, if both are untreated with
preservatives. It is especially so of woods with deep-colored
heartwood, and is due to infiltrations of tannins, oils, and
resins, which make the wood more or less obnoxious to
decay-producing fungi. If, however, the timbers are to be
treated, sapwood is not a defect; in fact, because of the
relative ease with which it can be impregnated with
preservatives it may be made more desirable than heartwood.[25]

[Footnote 25: Although the factor of heart or sapwood does not
influence the mechanical properties of the wood and there is
usually no difference in structure observable under the
microscope, nevertheless sapwood is generally decidedly
different from heartwood in its physical properties. It dries
better and more easily than heartwood, usually with less
shrinkage and little checking or honeycombing. This is
especially the case with the more refractory woods, such as
white oaks and _Eucalyptus globulus_ and _viminalis_. It is
usually much more permeable to air, even in green wood, notably
so in loblolly pine and even in white oak. As already stated, it
is much more subject to decay. The sapwood of white oak may be
impregnated with creosote with comparative ease, while the
heartwood is practically impenetrable. These facts indicate a
difference in its chemical nature.--H.D. Tiemann.]

In specifications for structural timbers reference is sometimes
made to "boxheart," meaning the inclusion of the pith or centre
of the tree within a cross section of the timber. From numerous
experiments it appears that the position of the pith does not
bear any relation to the strength of the material. Since most
season checks, however, are radial, the position of the pith may
influence the resistance of a seasoned beam to horizontal shear,
being greatest when the pith is located in the middle half of
the section.[26]

[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]

WEIGHT, DENSITY, AND SPECIFIC GRAVITY

From data obtained from a large number of tests on the strength
of different woods it appears that, other things being equal,