sulphuric acid.  The mixture is gradually introduced into the large cast-iron cylinder into which the benzene has been poured.  The outside of the cylinder is supplied with an arrangement by which fine jets of water can be made to play upon it in the early stages of the reaction which follows, and at the end of from eight to ten hours the contents are allowed to run off into a storage reservoir.  Here they arrange themselves into two layers, the top of which consists of the nitrobenzene which has been produced, together with some benzene which is still unacted upon.  The mixture is then freed from the latter by treatment with a current of steam.  Nitrobenzene presents itself as a yellowish oily liquid, with a peculiar taste as of bitter almonds.  It was formerly in great demand by perfumers, but its poisonous properties render it a dangerous substance to deal with.  In practice a given quantity of benzene will yield about 150 per cent of nitrobenzene.  Stated chemically, the reaction is shown by the following equation:—­

C_{6}H_{6}    +    HNO_{3}    =    C_{6}H_{5}NO_{2},    +  H_{2}O
(Benzene)       (Nitric acid)       (Nitrobenzene)         (Water).

The water which is thus formed in the process, by the freeing of one of the atoms of hydrogen in the benzene, is absorbed by the sulphuric acid present, although the latter takes no actual part in the reaction.

From the nitrobenzene thus obtained, the aniline which is now used so extensively is prepared.  The component atoms of a molecule of aniline are shown in the formula C_{6}H_{5}NH_{2}.  It is also known as phenylamine or amido-benzole, or commercially as aniline oil.  There are various methods of reducing nitrobenzene for aniline, the object being to replace the oxygen of the former by an equivalent number of atoms of hydrogen.  The process generally used is that known as Bechamp’s, with slight modifications.  Equal volumes of nitrobenzene and acetic acid, together with a quantity of iron-filings rather in excess of the weight of the nitrobenzene, are placed in a capacious retort.  A brisk effervescence ensues, and to moderate the increase of temperature which is caused by the reaction, it is found necessary to cool the retort.  Instead of acetic acid hydrochloric acid has been a good deal used, with, it is said, certain advantageous results.  From 60 to 65 per cent. of aniline on the quantity of nitrobenzene used, is yielded by Bechamp’s process.

Stated in a few words, the above is the process adopted on all hands for the production of commercial aniline, or aniline oil.  The details of the distillation and rectification of the oil are, however, as varied as they can well be, no two manufacturers adopting the same process.  Many of the aniline dyes depend entirely for their superiority, on the quality of the oil used, and for this reason it is subject to one or more processes of rectification.  This is performed by distilling, the distillates at the various temperatures being separately collected.