mical
reasons. In 1877, however, Nietzki worked out a very good process for the
preparation of quinone from aniline by oxidation with sulphuric acid and
bichromate of soda in the cold. This placed the production of quinone on a
manufacturing basis, so that when a demand for hydroquinone sprung up, the
wants of the photographer were met by the technologist. Eikonogen is
another organic reducing agent, discovered by the writer in 1880, and
introduced as a developer by Dr. Andresen in 1889. It is an
amido-derivative of a sulpho-acid of beta-naphthol, so that naphthalene is
the generating hydrocarbon of this substance.

The thio-derivative of toluidine described as "primuline" (p. 160), has
recently been found by its discoverer to possess a most remarkable
property which enables this compound to be used for the photographic
reproduction of designs in azo-colours. Diazotised primuline, as already
explained, combines in the usual way with amines and phenols to form
azo-dyes. Under the influence of light, however, the diazotised primuline
is decomposed with the loss of nitrogen, and the formation of a product
which does not possess the properties of a diazo-compound. The product of
photochemical decomposition no longer forms azo-colours with amines or
phenols. If, therefore, a fabric is dyed with primuline, then diazotised
by immersion in a nitrite bath, and exposed under a photographic negative,
those portions of the surface to which the light penetrates lose the power
of giving a colour with amines or phenols. The design can thus be
developed by dipping the fabric into a solution of naphthol,
naphthylamine, &c. By this discovery another point of contact has been
established between photography and coal-tar products. Nor is this the
only instance of its kind, for it has also been observed that a
diazo-sulpho-acid of one of the xylenes does not combine with phenols to
form azo-dyes excepting under the influence of light. A fabric can
therefore be impregnated with the mixture of diazo-sulpho-acid and
naphthol, and exposed under a design, when the azo-colour is developed
only on those portions of the surface which are acted upon by light.

The last indirect application of coal-tar colouring-matters to which
attention must be called is one of great importance in biology. The use of
these dyes as stains for sections of animal and vegetable tissue has long
been familiar to microscopists. Owing to the different affinities of the
va