composition of all dynamites, and several
smokeless powders; and the second includes gun-cotton, collodion-cotton,
nitrated wood, and the majority of the smokeless powders, which consist
generally of nitro-cotton, nitro-lignin, nitro-jute, &c. &c., together
with metallic nitrates, or nitro-glycerine.

The nitro-explosives consist generally of some organic substance in which
the NO_{2} group, known as nitryl, has been substituted in place of
hydrogen.

Thus in glycerine,

|OH
C_{3}H_{5}|OH,
|OH

which is a tri-hydric alcohol, and which occurs very widely distributed as
the alcoholic or basic constituent of fats, the hydrogen atoms are
replaced by the NO_{2} group, to form the highly explosive compound,
nitro-glycerine. If one atom only is thus displaced, the mono-nitrate is
formed thus,

|ONO_{2}
C_{3}H_{5}|OH;
|OH

and if the three atoms are displaced, C_{3}H_{5}(ONO_{2})_{3}, or the tri-
nitrate, is formed, which is commercial nitro-glycerine.

Another class, the nitro-celluloses, are formed from cellulose,
C_{6}H_{10}O_{5}, which forms the groundwork of all vegetable tissues.
Cellulose has some of the properties of the alcohols, and forms ethereal
salts when treated with nitric and sulphuric acids. The hexa-nitrate, or
gun-cotton, has the formula, C_{12}H_{14}O_{4}(ONO_{2})_{6}; and
collodion-cotton, pyroxylin, &c., form the lower nitrates, i.e., the
tetra- and penta-nitrates. These last are soluble in various solvents,
such as ether-alcohol and nitro-glycerine, in which the hexa-nitrate is
insoluble. They all dissolve, however, in acetone and acetic ether.

The solution of the soluble varieties in ether-alcohol is known as
collodion, which finds many applications in the arts. The hydrocarbon
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 s