benzene, C_{6}H_{6}, prepared from the light oil obtained from coal-tar,
when nitrated forms nitro-benzenes, such as mono-nitro-benzene,
C_{6}H_{5}NO_{2}, and di-nitro-benzene, C_{6}H_{4}(NO_{2})_{2}, in which
one and two atoms are replaced by the NO_{2} group. The latter of these
compounds is used as an explosive, and enters into the composition of such
well-known explosives as roburite, &c. The presence of nitro groups in a
substance increases the difficulty of further nitration, and in any case
not more than three nitro groups can be introduced into an aromatic
compound, or the phenols. All aromatic compounds with the general formula,
C_{6}H_{4}X_{2}, give, however, three series. They are called ortho, meta,
or para compounds, depending upon the position of NO_{2} groups
introduced.

Certain regularities have been observed in the formation of nitro-
compounds. If, for example, a substance contains alkyl or hydroxyl groups,
large quantities of the para compound are obtained, and very little of the
ortho. The substitution takes place, however, almost entirely in the meta
position, if a nitro, carboxyl, or aldehyde group be present. Ordinary
phenol, C_{6}H_{5}.OH, gives para- and ortho-nitro-phenol; toluene gives
para- and ortho-nitro-toluene; but nitro-benzene forms meta-di-nitro-
benzene and benzoic acid, meta-nitro-benzoic acid.[A]

[Footnote A: "Organic Chemistry," Prof. Hjelt. Translated by J.B. Tingle,
Ph.D.]

If the graphic formula of benzene be represented thus (No. 1), then the
positions 1 and 2 represent the ortho, 1 and 3 the meta, and 1 and 4 the
para compounds. When the body phenol, C_{6}H_{5}.OH, is nitrated, a
compound is formed known as tri-nitro-phenol, or picric acid,
C_{6}H_{2}(NO_{2})_{3}OH, which is used very extensively as an explosive,