covery. It was Sir James Simpson who in 1848 first showed the value of
chloroform in surgical operations. A similar story can be told with
respect to these photographic developers.

Towards the middle of the last century a French chemist, the Count de la
Garaye, noticed a crystalline substance deposited from the extract of
Peruvian bark, then, as now, used in medicine. This substance was the lime
salt of an acid to which Vauquelin in 1806 gave the name of quinic acid
(_acide quinique_). In 1838 Woskresensky, by oxidizing quinic acid with
sulphuric acid and oxide of manganese, obtained a crystalline substance
which he called quinoyl. The name was changed to quinone by Woehler, and,
as we have already seen (p. 172), the term has now become generic,
indicating a group of similarly constituted oxygen derivatives of
hydrocarbons. Hydroquinone was obtained by Caventou and Pelletier by
heating quinic acid, but these chemists did not recognize its true nature.
It was the illustrious Woehler who in 1844 first prepared the compound in a
state of purity, and established its relationship to quinone. This
relationship, as the name given by Woehler indicates, is that of the nature
of a hydrogenised quinone. The compound is readily prepared by the action
of sulphurous acid or any other reducing agent on the quinone.

It has long been known in photography, that a developer must be of the
nature of a reducing agent, either inorganic or organic, and many
hydroxylic and amidic derivatives of hydrocarbons come under this
category. Thus, pyrogallol, which has already been referred to as a
trihydroxybenzene (p. 146), when dissolved in alkali rapidly absorbs
oxygen--it is a strong reducing agent, and is thus of value as a
developer. But although pyrogallol is a benzene derivative, and could if
necessary be prepared synthetically, it can hardly be claimed as a tar
product, as it is generally made from gallic acid. Now hydroquinone when
dissolved in alkali also acts as a reducing agent, and in this we have the
first application of a true coal-tar product as a photographic developer.
Its use for this purpose was suggested by Captain Abney in 1880, and it
was found to possess certain advantages which caused it to become
generally adopted.

As soon as a practical use is found for a chemical product its manufacture
follows as a matter of course. In the case of hydroquinone, the original
source, quinic acid, was obviously out of question, for econo