[Footnote 23:  Bul. 80:  The commercial hickories, p. 50.]

[Footnote 24:  Loc. cit.]

Specifications for paving 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, 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