odour of bitter almonds. It solidifies in the cold. In di-nitro-benzene,
the two nitro groups may be in the meta, ortho, or para position, the meta
position being the most general (see fig., page 4). By recrystallising
from alcohol, pure meta-di-nitro-benzene may be obtained in long
colourless needles. The ortho compound crystallises in tables, and the
para in needles. They are both colourless. When toluene is nitrated, the
para and ortho are chiefly formed, and a very little of the meta compound.

~Nitro Compounds of Benzene and Toluene.~--The preparation of the nitro
derivatives of the hydrocarbons of the benzene series is very simple. It
is only necessary to bring the hydrocarbon into contact with strong nitric
acid, when the reaction takes place, and one or more of the hydrogen atoms
of the hydrocarbon are replaced by the nitryl group (NO_{2}). Thus by the
action of nitric acid on benzene (or benzol), mono-nitro-benzene is
formed:--

C_{6}H_{6} + HNO_{3} = C_{6}H_{5}.NO_{2} +H_{2}O.
Mono-nitro-benzene.

By the action of another molecule of nitric acid, the di-nitro-benzene is
formed:--

C_{6}H_{5}.NO_{2} + HNO_{3} = C_{6}H_{4}(NO_{2})_{2} + H_{2}O.
Di-nitro-benzene.

These nitro bodies are not acids, nor are they ethereal salts of nitrous
acid, as nitro-glycerine is of glycerine. They are regarded as formed from
nitric acid by the replacement of hydroxyl by benzene radicals.

~Mono-nitro Benzene~ is made by treating benzene with concentrated nitric