composition shown by
the formula C_{27}H_{24}N_{4}. It is produced from the sulphate of
aniline by mixing it with a cold saturated solution of bichromate of
potash, and allowing the mixture to stand for ten or twelve hours. A
blue-black precipitate is then formed, which, after undergoing a process
of purification, is dissolved in alcohol and evaporated to dryness. A
metallic-looking powder is then obtained, which constitutes this
all-important base. Mauve forms with acids a series of well-defined salts
and is capable of expelling ammonia from its combinations. Mauve was the
first aniline dye which was produced on a large scale, this being
accomplished by Perkin in 1856.

The substance known as carbolic acid is so useful a product of a piece of
coal that a description of the method of its production must necessarily
have a place here. It is one of the most powerful antiseptic agents with
which we are acquainted, and has strong anaesthetic qualities. Some
useful dyes are also obtained from it. It is obtained in quantities from
coal-tar, that portion of the distillate known as the light oils being
its immediate source. The tar oil is mixed with a solution of caustic
soda, and the mixture is violently agitated. This results in the caustic
soda dissolving out the carbolic acid, whilst the undissolved oils
collect upon the surface, allowing the alkaline solution to be drawn from
beneath. The soda in the solution is then neutralised by the addition of
a suitable quantity of sulphuric acid, and the salt so formed sinks while
the carbolic acid rises to the surface.

Purification of the product is afterwards carried out by a process of
fractional distillation. There are various other methods of preparing
carbolic acid.

Carbolic acid is known chemically as C_{6}H_{5}(HO). When pure it appears
as colourless needle-like crystals, and is exceedingly poisonous. It has
been used with marked success in staying the course of disease, such as
cholera and cattle plague. It is of a very volatile nature, and its
efficacy lies in its power of destroying germs as they float in the
atmosphere. Modern science tells us that all diseases have their origin
in certain germs which are everywhere present and which seek only a
suitable _nidus_ in which to propagate and flourish. Unlike mere
deodorisers which simply remove noxious gases or odours; unlike
disinfectants which prevent the spread of infection, carbolic acid
strikes at the very root and