refully described on a label pasted on to the rack, and tubes
from different lots should not be used for critical welds. This
remark is more important in the case of soda than of lead glass.

In the case of very fine thermometer tubes it will be advisable to
cover the ends with a little melted shellac, or, in special cases, to
obtain the tubes sealed from the works. Soda glass can generally be
got in rather longer lengths than lead glass; the longer the lengths
are the better, for the waste is less.

It is useful to be able to distinguish the different kinds of glass by
the colour. This is best observed by looking towards a bright surface
along the whole length of the tube and through the glass. Lead glass
is yellow, soda glass is green, and hard glass purple in the samples
in my laboratory, and I expect this is practically true of most
samples. [Footnote: Some new lead glass I have is also almost purple
in hue. If any doubt exists as to the kind of glass, it may be tested
at once in the blow-pipe flame, or by a mixture of oils of different
refractive indices, as will be explained later.]

Sec. 12. The question of the solubility of glass in reagents is one of
great importance in accurate work, though it does not always meet with
the attention it deserves. It is impossible here to go into the
matter with sufficient detail, and the reader is therefore referred to
the Abstracts of the Chemical Society, particularly for the years 1889
and 1892. The memoir by F. Kohlrausch, Wied. Ann. xliv, should be
consulted in the original. The following points may be noted. A
method of testing the quality of glass is given by Mylius (C. S. J.
Abstracts, 1889, p. 549), and it is stated that the resistance of
glass to the action of water can generally be much increased by
leaving it in contact with cold water for several days, and then
heating it to 300 deg. to 400 deg. C. This improvement seems to be due to the
formation of a layer of moist silica on the surface, and its
subsequent condensation into a resisting layer by the heating. Mylius
(C. S. J. Abstracts, 1892, p. 411), and Weber, and Sauer (C. S.
J. Abstracts, 1892, p. 410) have also shown that the best glass for
general chemical purposes consists of:

Silica, 7 to 8 parts

Lime, 1 part

Alkali, 1.5 to 1.1 parts.

This is practically "Bohemian" tube glass.

The exact results are given in the Berichte of the German Chemical
Society, vol. xxv. An