with nitric acid, and
then reducing the nitrobenzene, has already been referred to. For this
purpose we now heat the nitrobenzene with iron dust and a little
hydrochloric (muriatic) acid, and then distil over the aniline by means of
a current of steam blown through the still. By a similar process toluene
is converted into nitrotoluene, and the latter into toluidine.

The large quantity of aniline and toluidine now made has opened up a
channel for the use of the waste borings from cast-iron. These are ground
to a fine powder under heavy mill-stones, and constitute a most valuable
reducing agent, known technically as "iron swarf." The metallic iron
introduced in this form into the aniline still is converted into an oxide
of iron by the action of the nitrobenzene, and this oxide of iron is used
by the gas-maker for purifying the gas from sulphur as already described.
When the oxide of iron is exhausted, _i.e._ when it has taken up as much
sulphur as it can, it goes to the vitriol-maker to be burnt as a source of
this acid. Here we have a waste product of the aniline manufacture
utilized for the purification of coal-gas, and finally being made to give
up the sulphur, which it obtained primarily from the coal, for the
production of sulphuric acid, which is consumed in nearly every branch of
chemical industry.

Nitrotoluene and toluidine each exist in three distinct modifications, so
that it is more correct to speak of the nitrotoluenes and the toluidines;
but the explanation of these differences belongs to pure chemical theory,
and cannot now be attempted in detail. It must suffice to say that many
compounds having the same chemical composition differ in their properties,
and are said to be "isomeric," the isomerism being regarded as the result
of the different order of arrangement of the atoms within the molecule.

Consider a homely illustration. A child's box of bricks contains a certain
number of wooden blocks, by means of which different structures can be
built up. Supposing all the bricks to be employed for every structure
erected, the latter must in every case contain all the blocks, and yet the
result is different, because in each structure the blocks are arranged in
a different way. The bricks represent atoms, and the whole structure
represents a molecule; the structures all have the same ultimate
composition, and are therefore isomeric. This will serve as a rough
analogy, only it must not be understood that the dif