ts of modern chemistry in connection with
coal-tar products do not end with the formation of colouring-matters,
medicines, and perfumes. The introduction of the beautiful dyes has had an
influence in other directions, and has led to results quite unsuspected
until the restless spirit of investigation opened out new fields for their
application. A few of these secondary uses are sufficiently important to
be chronicled here. In sanitary engineering, for example, the intense
colouring power of fluorescein is frequently made use of to test the
soundness of drains, or to find out whether a well receives drainage from
insanitary sources. In photography also coal-tar colouring-matters are
playing an important part by virtue of a certain property which some of
these compounds possess.

The ordinary photographic plate is, as is well known, much more sensitive
to blue and violet than to yellow or red, so that in photographing
coloured objects the picture gives a false impression of colour intensity,
the violets and blues impressing themselves too strongly, and the yellows
and reds too feebly. It was discovered by Dr. H. W. Vogel in 1873 that if
the sensitive film is slightly tinted with certain colouring-matters, the
sensitiveness for yellow and red can be much increased, so that the
picture is a more natural representation of the object. Plates thus dyed
are said to be "isochromatic" or "orthochromatic," and by their use
paintings or other coloured objects can be photographed with much better
results than by the use of ordinary plates. The boon thus conferred upon
photographic art is therefore to be attributed to coal-tar chemistry.
Among the numerous colouring-matters which have been experimented with,
the most effective special sensitizers are erythrosin, one of the
phthaleins, quinoline red, a compound related to the same group, and
cyanin, a fugitive blue colouring-matter obtained from quinoline in 1860
by Greville Williams.

In yet another way has photography become indebted to the tar chemist. Two
important developers now in common use are coal-tar products, viz.
hydroquinone and eikonogen. The history of these compounds is worthy of
narration as showing how a product when once given by chemistry to the
world may become applicable in quite unexpected directions. Chloroform is
a case in point. This compound was discovered by Liebig in 1831, but its
use as an anaesthetic did not come about till seventeen years after its
dis