presence of water. It was but a step from the laboratory into the
factory in this case, and at the present time the aldehyde is made on a
large scale by chlorinating boiling toluene beyond the stage of benzyl
chloride, and heating the mixture of benzal chloride and benzotrichloride
with lime and water under pressure. By this means the first compound is
transformed into benzoic aldehyde, and the second into benzoic acid. This
last substance is also required by the colour-maker, as it is used in the
manufacture of blue by the action of aniline on rosaniline; without some
such organic acid the transformation of rosaniline into the blue is very
imperfect.

Benzoic acid, like the aldehyde, is a natural product which has long been
known. It was obtained from gum benzoin at the beginning of the
seventeenth century, and its preparation from this source was described by
Scheele in 1755. The same chemist afterwards found it in urine, and from
these two sources, the one vegetable and the other animal, the acid was
formerly prepared. Its relationship to benzene has already been alluded to
in connection with the history of that hydrocarbon. It will be remembered
that by heating this acid with lime Mitscherlich obtained benzene in
1834. In one operation, therefore, setting out from toluene, we make these
two natural products, the aldehyde and acid, which are easily separable by
technical processes. The wants of the technologist have been met, and he
has been enabled to compete successfully with Nature, for he can
manufacture these products much more cheaply than when he had to depend
upon bitter almonds or gum benzoin. The synthetical bitter-almond oil is
chemically identical with that from the plant. Besides its use for the
manufacture of colouring-matters, it is employed for flavouring purposes
and in perfumery, this being the first instance of a coal-tar perfume
which we have had occasion to mention. The odour in this case, it must be
remembered, is that of the actual compound which imparts the
characteristic taste and smell to the almond; it is not the result of
substituting a substance which has a particular odour for another having a
similar odour, as is the case with nitrobenzene, which, as already
mentioned, is used in large quantities under the name of "essence of
mirbane," for imparting an almond-like smell to soap.

The introduction of malachite green marks another epoch in the history of
the technology of the triphenylme