mercury.
Move it from point to point throughout the length of the tube, and note
its length in each position. If in one part it occupies a length of tube
corresponding to eight degrees, and at another only seven degrees, then
at the former point the value of each division is only seven-eighths of
those at the latter position.

From the results obtained, a table of corrections for the thermometer
should be prepared.

It is sometimes necessary to join soda glass to lead glass. In this case
the edge of the lead glass tube may be bordered with white enamel before
making the joint. Enough enamel must be used to prevent the lead and
soda glasses from mingling at any point. The enamel is easily reduced,
and must be heated in the oxidising flame. Dr. Ebert recommends _Verre
d'urane_ for this purpose. It is supplied by Herr Goetze of Leipzig
(Liebigstrasse).

CHAPTER VI.

_GLASS TUBING._

The diagrams given below show the sizes and thickness of the glass tubes
most frequently required. In ordering, the numbers of these diagrams may
be quoted, or the exact dimensions desired may be stated.

Glass tubes are usually sold by weight, and therefore the weight of tube
of each size that is wished for should be indicated, and also whether it
is to be of lead or soda glass.

[Illustration]

[Illustration]

CHAPTER VII.

_VITREOUS SILICA._

=Introductory.=--Vitreous Silica was made in fine threads by M. Gaudin
in 1839,[22] and small tubes of it were made in 1869 by M. A. Gautier,
but its remarkable qualities were not really recognised till 1889, when
Professor C. V. Boys rediscovered the process of making small pieces of
apparatus of this substance, and used the torsion of "quartz fibres" for
measuring small forces. More recently the author of this book has
devised a process for preventing the "splintering" of quartz which gave
so much trouble to the earlier workers, and jointly with Mr. H. G.
Lacell, has produced a variety of apparatus of much larger dimensions
than had been attempted =previously=. At the time of writing we can
produce by the processes described in the following pages tubes 1 to 1.5
cm. in diameter and about 750 cm. in length, globes or flasks capable of
containing about 50 cc., masses of vitreous silica weighing 100 grams or
more, and a variety of other apparatus.

[22] A brief summary of the history of this subject will be found in
_Nature_, Vol. 62, and in the Proceedings of the Royal I