title in what manner his paper was composed. In
most cases, however, this appellation was applied to the ordinary tar
paper; the impregnating substance was mixed only with coal pitch, or
else a roofing paper saturated with distilled tar. The costly
additions, by the use of which a high grade of roofing paper can
doubtless be produced, largely increased its price, and on account of
the constant fall of prices of the article, its use became rather one
of those things "more honored in the breach than in the observance,"
and was dispensed with whenever practicable. The crude paper is the
foundation of the roofing paper. The qualities of a good,
unadulterated paper have already been stated. At times, the crude
paper contains too many earthy ingredients which impair the cohesion
of the felted fibrous substance, and which especially the carbonate of
lime is very injurious, as it readily effects the decomposition of the
coal tar. The percentage of wool, upon which the durability of the
paper depends very largely, is very small in some of the paper found
in the market. In place of woolen rags, cheap substitutes have been
used, such as waste, which contains vegetable fibers. Since this
cannot resist the decomposition process for any length of time, it is
evident that the roofing paper which contains a noticeable quantity of
vegetable fibers cannot be very durable. To judge from the endeavors
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 ingredien