above the mercury-a method used for securing
air-tightness in "mercury joints" by Mr. Gimingham, Proc. R. S.
1874.

Further protection may be attained for many purposes by coating the
platinum wire with a sheath of glass, say half an inch long, fused to
the platinum wire to a depth of one-twentieth of an inch all round.

In some cases the electrodes must be expected to get very hot, for
instance, when it is desired to platinise mirrors by the device of
Professor Wright of Yale. In this and similar cases I have met with
great success by using "barometer" tubes of about one-twelfth of an
inch bore, and with walls, say, one-tenth of an inch thick.
[Footnote: "Barometer" tube is merely very thick-walled glass tubing,
and makes particularly bad barometers, which are sold as weather
glasses.]

This tube is drawn down to a long point--say an inch long by
one-eighth of an inch external diameter, and the wire is fused in for
a length, say, of three-quarters of an inch, but only in the narrow
drawn--down part of the tube. At different times I have tried four
such seals, and though the electrodes were red hot for hours, I have
never had an accident--of course they were well annealed.

Fig. 37.

For directions as to the making of high vacuum tubes, see the section
dealing with that matter.

Sec. 50. As economy of platinum is often of importance, the following
little art will save money and trouble. Platinum is easily caused to
join most firmly to copper--with which, I presume, it alloys--by the
following method. Hold the platinum wire against the copper wire, end
to end, at the tip of the reducing flame of a typical blowpipe--or
anywhere--preferably in the "reducing" part of the oxygas flame; in
a moment the metals will fuse together at the point of contact, when
they may be withdrawn.

Such a joint is very strong and wholly satisfactory, much better than
a soldered joint. If the work is not carried out successfully so that
a considerable drop of copper-platinum alloy accumulates, cut it off
and start again. The essence of success is speed, so that the copper
does not get "burned." If any considerable quantity of alloy is formed
it dissolves the copper, and weakens it, so that we have first the
platinum wire, then a bead of alloy, and then a copper wire fused into
the bead, but so thin just outside the latter that the joint has no
mechanical strength.

Sec. 51. The Art of making Air-light Joints.

Lamp-man