The Chemistry of Powder and Explosives - Sciencemadness Dot Org

NITROGLYCERIN 207
because the boiling points in vacuum are considerably below the
ignition temperatures.
Nitroglycerin crystallizes in two forms, a stable form, dipyramidal
rhombic crystals, which melt or freeze at 13.2-13.5°, and a
labile form, glassy-appearing triclinic crystals, m.p. 1.9-2.2°. It
does not freeze readily or quickly. When cooled rapidly, it
becomes more and more viscous and finally assumes the state
of a hard glassy mass, but this is not true freezing, and the glassy
mass becomes a liquid again at a temperature distinctly below
the melting point of the crystalline substance. Nitroglycerin in
dynamite freezes in crystals if the explosive is stored for a considerable
length of time at law temperatures, the form in which
it solidifies being determined apparently by the nature of the
materials with which it is mixed." If liquid nitroglycerin is
cooled strongly, say to —20° or —60°, stirred with a glass rod,
and seeded with particles of one or the other form, then it crystallizes
in the form with which it has been seeded. If the solid is
melted by warming, but not warmed more than a few degrees
above its melting point, it will on being cooled solidify in the
form, whether labile or stable, from which it had been melted.
If, however, it is warmed for some time at 50°, it loses all preference
for crystallizing in one form rather than in the other, and
now shows the usual phenomena of supercooling when it is
chilled. Crystals of the labile form may be preserved sensibly
unchanged for a week or two, but gradually lose their transparency
and change over to the stable form. Crystals of the
stable form cannot be changed to the labile form except by melting,
warming above the melting point, and seeding with the
labile form.
Nitroglycerin is miscible in all proportions at ordinary temperatures
with methyl alcohol, acetone, ethyl ether, ethyl acetate,
glacial acetic acid, benzene, toluene, nitrobenzene, phenol, chloroform,
and ethylene chloride, and with homologous nitric esters
such as dinitroglycerin, dinitrochlorohydrin, nitroglycol, and trimethylcncglycol
dinitratc. Absolute ethyl, prupyl, isopropyl, and
amyl alcohols mix with nitroglycerin in all proportions if they
arc hot, but their solvent power falls (iff rapidly at lower temperatures.
One hundred grains of absolute ethyl alcohol dissolves
i^Hibbert, Z. gcs. Schicss- u. Sprengsloffw., 9, 83 (1914).