very differently in this respect, and minute impurities prevent or delay
crystallisation. Solid nitro-glycerol[A] melts at about 12° C., but
requires to be exposed to this temperature for some time before melting.
The specific gravity of the solid form is 1.735 at +10° C.; it contracts
one-twelfth of its volume in solidifying. Beckerheim[B] gives the specific
heat as 0.4248 between the temperatures of 9.5° and 9.8° C., and L. de
Bruyn gives the boiling point as above 200°.

[Footnote A: Di-nitro-mono chlorhydrin, when added to nitro-glycerine up
to 20 per cent., is said to prevent its freezing.]

[Footnote B: _Isb., Chem. Tech._, 22, 481-487. 1876.]

Nitro-glycerine has a sweet taste, and causes great depression and
vertigo. It is soluble in ether, chloroform, benzene, glacial acetic acid,
and nitro-benzene, in 1.75 part of methylated spirit, very nearly
insoluble in water, and practically insoluble in carbon bisulphide. Its
formula is C_{3}H_{5}(NO_{3})_{3}, and molecular weight 227. When pure, it
may be kept any length of time without decomposition. Berthelot kept a
sample for ten years, and Mr G. M'Roberts, of the Ardeer Factory, for nine
years, without their showing signs of decomposition; but if it should
contain the smallest trace of free acid, decomposition is certain to be
started before long. This will generally show itself by the formation of
little green spots in the gelatine compounds, or a green ring upon the
surface of liquid nitro-glycerine. Sunlight will often cause it to
explode; in fact, a bucket containing some water that had been used to
wash nitro-glycerine, and had been left standing in the sun, has in our
experience been known to explode with considerable force. Nitro-glycerine
when pure is quite stable at ordinary temperatures, and samples have been
kept for years without any trace of decomposition. It is very susceptible