acid (which gives alizarin) and from each other. The
purpurins give somewhat yellower shades than alizarin. Another
trihydroxy-anthraquinone, although not obtained directly from anthracene,
must be claimed as a tar-product. It is prepared by heating gallic acid
with benzoic and sulphuric acids, or with phthalic anhydride and zinc
chloride, and is a brown dye known as "anthragallol" or
"anthracene-brown." The anthracene derivative is in this process built up
synthetically. A sulpho-acid of alizarin has been introduced for wool
dyeing under the name of alizarin carmine, and a nitro-alizarin under the
name of alizarin orange. The latter on heating with glycerin and sulphuric
acid is transformed into a remarkably fast colouring-matter known as
alizarin blue, which is used for dyeing and printing. By heating alizarin
blue with strong sulphuric acid, it is converted into alizarin green.

The great industry arising out of the laboratory work of two German
chemists has influenced other branches of chemical manufacture, and has
reacted upon the coal-tar colour industry itself. A new application for
caustic soda and potassium chlorate necessitated an increased production
of these materials. The first demand for fuming sulphuric acid on a large
scale was created by the alizarin manufacture in 1873, when it was found
that an acid of this strength gave better results in the preparation of
sulpho-acids from anthraquinone. The introduction of this acid into
commerce no doubt exerted a marked influence on the production of other
valuable sulpho-acids, such as acid magenta in 1877, acid yellow in 1878,
and acid naphthol yellow in 1879. The introduction of artificial alizarin
has also simplified the art of colour printing on cotton fabrics to such
an extent that other colouring-matters, also derived from coal-tar, are
largely used in combination with the alizarin to produce parti-coloured
designs. The manufacture of one coal-tar colouring-matter has thus
assisted in the consumption of others.

Artificial alizarin is used in the form of a paste, which consists of the
colouring-matter precipitated from its alkaline solution by acid, and
mixed with water so as to form a mixture containing from 10 to 20 per
cent. of alizarin. The magnitude of the industry will be gathered from the
estimate that the whole quantity of anthracene annually made into
alizarin corresponds to a daily production of about 65 tons of 10 per
cent. paste, of which only