acid of the magenta-still has its use. The arsenious
acid resulting from the reduction of this arsenic acid is generally
obtained in the form of a lime salt after the removal of the magenta by
the purifying processes to which the crude product is submitted. From the
arsenical waste arsenious acid can be recovered, and converted back into
arsenic acid by the action of nitric acid. Quite recently the arsenical
residue has been used with considerable success in America as an
insecticide for the destruction of pests injurious to agricultural crops.

Concurrently with these technical developments of coal-tar products, the
scientific chemist was carrying on his investigations. The compounds which
science had given to commerce were made on a scale that enabled the
investigator to obtain his materials in quantities that appeared fabulous
in the early days when aniline was regarded as a laboratory curiosity, and
magenta had been seen by only a few chemists.

The fundamental problem which the modern chemist seeks to solve is in the
first place the composition of a compound, _i.e._ the number of the atoms
of the different elements which form the molecule, and in the next place
the way in which these atoms are combined in the molecule. Reverting to
our former analogy, the first thing to be found is how many different
blocks enter into the composition of the structure, and the next thing is
to ascertain how the blocks are arranged. When this is done, we are said
to know the "constitution" or "structure" of the molecule, and in many
cases when this is known we can build up or synthesise the compound by
combining its different groups of atoms by suitable methods. The coal-tar
industry abounds with such triumphs of chemical synthesis; a few of these
achievements will be brought to light in the course of the remaining
portions of this work.

The chemical investigation of magenta was commenced by Hofmann, whose name
is inseparably connected with the scientific development of the coal-tar
colour industry. In 1862 he showed that magenta was the salt of a base
which he isolated, analysed, and named rosaniline. He established the
composition of this base and of the violet and blue colouring-matters
obtained from it by the processes already described. In 1864 he made the
interesting discovery that magenta is not formed by the oxidation of
_pure_ aniline, but that a mixture of aniline and toluidine is essential
for the production of this colo