paper. These pores are highly injurious to
it, as they facilitate a process of decomposition which will ruin it
in a short time.

Roofing paper can be called good only when it is essentially made from
woolen rags, and contains either very few or no earthy additions. It
is beyond doubt that the durability of a roofing paper increases with
the quality of wool fiber it contains--vegetable fibers and earthy
additions cause a direct injury. Reprehensible altogether is any
combination with lime, either in form of a carbonate or sulphate,
because the lime enters into chemical combination with the
decomposition products of the tar.

The general nature of gravel is too well known to require description.
The grains of quartz sand are either sharp cornered or else rounded
pieces of stone of quartz, occasionally mixed with grains of other
amorphous pieces of silica--such as horn stone, silicious slate,
carnelian, etc.; again, with lustrous pieces of mica, or red and white
pieces of feldspar. The gravel used for a tar paper roof must be of a
special nature and be prepared for the purpose. The size of its grains
must not exceed a certain standard--say, the size of a pea. When found
in the gravel bank, it is frequently mixed with clay, etc., and it
cannot be used in this condition for a roof, but must be washed. The
utensils necessary for this purpose are of so simple and suggestive a
nature that they need not be described. Slag is being successfully
used in place of the gravel. It is easily reduced to suitable size, by
letting the red hot mass, as it runs from the furnace, run into a
vessel with water. The sudden chilling of the slag causes it to burst
into fragments of a sharp cornered structure. It is next passed
through a sieve, and the suitably sized gravel makes an excellent
material, as it gives a clean appearance to the roof.

The thinking mind can easily go one step further and imagine that,
since the tar contains a number of volatile hydrocarbons, it might be
made more adaptable for impregnation by paper by distilling it, as by
this process the fluid would lose its tendency to evaporate and the
percentage of resinous substances increase. Singular to say, there was
a prejudice against the employment of distilled tar, entertained by
builders and people who had no knowledge of chemistry. Increasing
intelligence and altered business circumstances, however, brought
about the almost universal employment of distilled tar, and ev