e to do good work with that particular sample of
glass; although the devitrification may sometimes be remedied by heating
the devitrified glass to a higher temperature. The presence of aluminum
compounds appears to have some influence on the tendency of the glass to
resist devitrification. Soda-glass, as a rule, is more liable to crack
by sudden heating than lead-glass, and articles made from soda-glass
often tend to crack spontaneously if badly made or, in the case of
heavier and thicker articles, if insufficiently annealed.

_Combustion-Glass._--Is usually a glass containing more calcium silicate
and potassium silicate than the ordinary "soft" soda-glass. It is much
less fusible than ordinary soda-glass, and passes through a longer
intermediate or viscous stage when heated. Such a glass is not very
suitable for use with the blowpipe owing to the difficulty experienced
in obtaining a sufficiently high temperature. If, however, a certain
amount of oxygen is mixed with the air used in producing the blowpipe
flame this difficulty is minimised.

_Resistance-Glass._--May contain zinc, magnesium, and other substances.
As a rule it is harder than ordinary soda-glass, and less suitable for
working in the blowpipe flame. It should have very little tendency to
dissolve in water, and hence is used when traces of alkali or silicates
would prove injurious in the solutions for which the glass vessels are
to be used.

_Lead-Glass._--This, or "flint" glass as it is often called from the
fact that silica in the form of crushed and calcined flint was often
used in making the English lead-glasses, contains a considerable
proportion of lead silicate. Such a glass has, usually, a particularly
bright appearance, a high refractive index, and is specially suitable
for the production of the heavy "cut-glass" ware.

Lead-glass tubing is easy to work in the blowpipe flame, melts easily,
but does not become fluid quite so suddenly as most soda-glasses;
articles made from it are remarkably stable and free from tendency to
spontaneous cracking, although, as is essential for all the heavy or
"glass-house" work, the massive articles need annealing in the oven.

The two chief disadvantages of lead-glass for laboratory work are that
it is blackened by the reducing gases if held too near to the blue cone
of the blowpipe flame, and that it is rather easily attacked by chemical
reagents; thus ammonium sulphide will cause blackening.

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