moisture, the work of filling should be resorted to.

[Illustration: FIG. 118.--A Cavity Filled in a Tree that Should Have
Been Cut Down. Note how the entire interior is decayed and how the tree
fell apart soon after treatment.]

Filling should be done in the following manner: First, the interior
should be thoroughly freed from diseased wood and insects. The
chisel, gouge, mall and knife are the tools, and it is better to
cut deep and remove every trace of decayed wood than it is to leave
a smaller hole in an unhealthy state. The inner surface of the
cavity should then be covered with a coat of white lead paint, which
acts as a disinfectant and helps to hold the filling. Corrosive
sublimate or Bordeaux mixture may be used as a substitute for the
white lead paint. A coat of coal tar over the paint is the next
step. The cavity is then solidly packed with bricks, stones and
mortar as in Fig. 119, and finished with a layer of cement at the
mouth of the orifice. This surface layer of cement should not be
brought out to the same plane with the outer bark of the tree, but
should rather recede a little beyond the growing tissue (cambium
layer) which is situated immediately below the bark, Fig. 120. In
this way the growing tissue will be enabled to roll over the cement
and to cover the whole cavity if it be a small one, or else to grow
out sufficiently to overlap the filling and hold it as a frame holds
a picture. The cement is used in mixture with sand in the proportion
of one-third of cement to two-thirds of sand. When dry, the outer
layer of cement should be covered with coal tar to prevent cracking.

[Illustration: FIG. 119.--A Cavity in the Process of being Filled.]