itro-groups, and there results a sulpho-acid of
dinitro-alpha-naphthol. This was discovered in 1879 by Caro, and
introduced as "acid naphthol yellow." It is now one of the standard yellow
dyes.

The history of another important group of colouring-matters dependent on
naphthalene begins with A. v. Baeyer in 1871 and with Caro in 1874. Two
products formerly known only as laboratory preparations were called into
requisition by this discovery. One of these compounds, phthalic acid, is
obtained from naphthalene, and the other, resorcin or resorcinol, is
prepared from benzene. Phthalic acid, which was discovered in 1836 by
Laurent, is a product of the oxidation of many benzenoid compounds.
Chemically considered it is a di-derivative of benzene, _i.e._ two of the
hydrogen atoms of benzene are replaced by certain groups of carbon,
oxygen, and hydrogen atoms. We have seen how the replacement of hydrogen
by an ammonia-residue, amidogen, gives rise to bases such as amidobenzene
(aniline), or diamidobenzene. Similarly, the replacement of hydrogen by a
water-residue, hydroxyl, gives rise to a phenol. The group of carbon,
oxygen, and hydrogen atoms which confers the property of acidity upon an
organic compound is a half-molecule of oxalic acid--it is known as the
carboxyl group. Thus benzoic acid is the carboxyl-derivative of benzene,
and the phthalic acid with which we are now concerned is a
dicarboxyl-derivative of benzene. It is related to benzoic acid in the
same way that diamidobenzene is related to aniline. Three isomeric
phthalic acids are known, but only one of these is of use in the present
branch of manufacture. The acid in question, although a derivative of
benzene, is most economically prepared by the oxidation of certain
derivatives of naphthalene which, when completely broken down by energetic
oxidizing agents, furnish the acid. Thus the dinitronaphthol described as
Manchester yellow, if heated for some time with dilute nitric acid,
furnishes phthalic acid. The latter is made on a large scale by the
oxidation of a compound which naphthalene forms with chlorine, and known
as naphthalene tetrachloride, because it contains four atoms of chlorine.

The other compound, resorcinol, was known to chemistry ten years before it
was utilized as a source of colouring-matters. It was originally prepared
by fusing certain resins, such as galbanum, asafoetida, &c., with
caustic alkali. Soon after its discovery, viz. in 1866, it was sh