made to improve the coal tar, it may be concluded that this material
does not fully comply with its function of making the roofing paper
perfectly and durably waterproof. The coal tar, be it either crude or
distilled, is not a perfect impregnating material, and the roofing
paper, saturated with it, possesses several defects. Let us in the
following try to ascertain their shortcomings, and then express our
idea in what manner the roofing paper may be improved. It was
previously mentioned that every tar roofing paper will, after a
greater or smaller lapse of time, assume a dry, porous, friable
condition, caused by the volatilization of a part of the constituents
of the tar. This alteration is materially assisted by the oxygen of
the air, which causes the latter to become resinous and exerts a
chemical influence upon them. By the volatilization of the lighter tar
oils, pores are generated between the fibers of the roofing paper,
into which the air and humidity penetrate. In consequence of the
greatly enlarged surface, not only the solid ingredients of the tar,
which still remain unaltered, are exposed to the action of the oxygen,
but also the fibers of the roofing paper are exposed to decomposition.
How destructive the alternating influence of the oxygen and the
atmospheric precipitations are for the roofing paper will be shown by
the following results of tests. It will have been observed that the
rain water running from an old paper roof, especially after dry
weather, has a yellowish, sometimes a brown yellow color. The
supposition that this colored rain water might contain decomposition
products of the roofing paper readily prompted itself, and it has been
collected and analyzed at different seasons of the year. After a
period of several weeks of fair weather during the summer, rain fell,
and the sample of water running from a roof was caught and evaporated;
the residue when dried weighed 1.68 grammes. It was of a brownish
black color, fusible in heat and readily soluble, with a yellow brown