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.