The most volatile portion of the tar evaporated soon, whereby the
coating became thicker and finally dried. The bad properties of the
coal tar, pointed out elsewhere, made it very unsuitable even for this
purpose, and experiments were instituted to compound mixtures, by
adding other ingredients to the tar, that should more fully comply
with its function. It may be said in general that the coating masses
for roofs can be divided into two classes: either as lacquers or as
cements. To the former may be classed those of a fairly thinly fluid
consistency, and which contain volatile oils in such quantities that
they will dry quickly. Cements are those of a thickly fluid
consistency, and are rendered thus fluid by heating. It is not
necessary that the coating applied should harden quickly, as it
assumes soon after its application a firmness sufficient to prevent it
from running off the roof. Coal tar is to be classed among lacquers.
If it has been liberated by distillation from the volatile oils, it is
made better suited for the purpose than the ordinary kind. The mass
contains much more asphaltum, and after drying, which takes place
soon, it leaves a far thicker layer upon the roof surface, while the
pores, which had formed in the roofing paper consequent on drying, are
better filled up. Nevertheless, the distilled tar also has retained
the property of drying with time into a hard, vitreous mass, and
ultimately to be destroyed by decomposition.--_The Roofer_.

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A PHYSICAL LABORATORY INDICATOR.