sible, to
boil the mercury in situ, which involves making the wrapping of
asbestos, but, after all, we come back to the position I began by
taking up, viz. that the easiest and most reliable method is by
fusion of the glass--all the rest are unsuitable for work of real
precision.

I should be ungrateful, however, were I not to devote a few lines to
the great convenience and merit of so-called "centering cement." This
substance has two or three very valuable properties. It is very tough
and strong in itself, and it remains plastic on cooling for some time
before it really sets. If for any reason a small tube has to be
cemented into a larger one, which is a good deal larger, so that an
appreciable mass of cement is necessary, and particularly if the joint
requires to have great mechanical strength, this cement is invaluable.
I have even used a plug of it instead of a cork for making the joint
between a gas delivery tube and a calcium chloride tower. (Why are
these affairs made with such abominable tubulures?)

The joint in question has never allowed the tube to sag though it
projects horizontally to a distance of 6 inches, and has had to
withstand nearly two years of Sydney temperature. The cement consists
of a mixture of shellac and 10 per cent of oil of cassia.

The shellac is first melted in an iron ladle, and the oil of cassia
quickly added and stirred in, to an extent of about 10 per cent, but
the exact proportions are not of importance. Great care must be taken
not to overheat the shellac.

APPENDIX TO CHAPTER I

ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
ROENTGEN'S RADIATION

[Footnote: Written in May 1896.]

WHEN Professor Roentgen's discovery was first announced at the end of
1895 much difficulty was experienced in obtaining radiation of the
requisite intensity for the repetition of his experiments. The
following notes on the production of vacuum tubes of the required
quality may therefore be of use to those who desire to prepare their
own apparatus. It appears that flint glass is much more opaque to
Roentgen's radiation than soda glass, and consequently the vacuum
tubes require to be prepared from the latter material.

Fig. 39.

A form of vacuum tube which has proved very successful in the
author's hands is sketched in Fig. 38. It is most easily constructed
as follows. A bit of tubing about 2 centimetres diameter, 15
centimetres long, and 1.5 millimetre wall thick