ill. It is heated in an iron pan to about 90 deg. or 100 deg.C.; the
fabric is drawn through it by means of rollers which at the same time
squeeze out the excess of tar; on coming out of these, the tarred felt
is covered with a layer of sand on both sides by means of a self-acting
apparatus; and is ultimately wound round wooden rolls, in which state it
is sent out into the trade. This roofing-felt is used as a cheap
covering, both by itself and as a grounding for tiles or slates. In the
former case it must be kept in repair by repainting with tar from time
to time, a top covering of sand or small gravel being put on after every
coat of paint.

Coal-tar is also employed for the manufacture of _lamp-black_. This is
done by burning the tar in ovens, connected with brick-chambers in which
the large quantity of soot, formed in this process, deposits before the
gases escape through the chimney. Numerous patents have been taken out
for more efficiently collecting this soot. Most of it is employed
without further manipulation for the manufacture of electric carbons,
printing inks, shoe-blacking, patent leather and so forth. A finer
quality of lamp-black, free from oily and empyreumatic parts, is
obtained by calcining the soot in closed iron pots at a red heat.

_Distillation of Coal-Tar._--Much more important than all applications
of crude coal-tar is the industry of separating its constituents from it
in a more or less pure form by fractional distillation, mostly followed
by purifying processes. Most naturally this industry took its rise in
Great Britain, where coal-gas was invented and made on a large scale
before any other nation took it up, and up to this day both the
manufacture of coal-gas and the distillation of the tar, obtained as a
by-product thereof, are carried out on a much larger scale in that than
in any other country. The first attempts in this line were made in 1815
by F. C. Accum, and in 1822 by Dr G. D. Longstaff and Dr Dalston. At
first the aim was simply to obtain "naphtha," used in the manufacture of
india-rubber goods, for burning in open lamps and for some descriptions
of varnish; the great bulk of the tar remained behind and was used as
fuel or burned for the purpose of obtaining lamp-black.

It is not quite certain who first discovered in the coal-naphtha the
presence of benzene (q.v.), which had been isolated from oil-gas by M.
Faraday as far back as 1825. John Leigh claims to have shown coal-tar
be