16—23.
CATHODE SPUTTERING
381
in gure 16—14. Dry air from a calcium chloride (CaClg) tube is
bubbled through the mercury 4 in a long tube (around
meters)
for several hours.
A heavy scum will form on the surface which
contains practically all the impurities. The mercury is then
ltered through small holes punched in ltefpaper held in a funnel.
A good but more tedious method of cleaning mercury is to
lter it, and shake thoroughly in a bottle'wit‘h chromic acid, then
in water.
Then let it fall in a ne spray from a
through
several meters of dilute nitric acid, then through
of
distilled water and dry it by evaporation at around 3509 C. A
“drip” tube is shown in gure 16—15.
-
Filtered mercury may be distilled in a
with the
apparatus shown in gure 16—16 to obtain the cleanest mercury.
The mercury is gently vaporized by the heater at  and condenses
_
on the walls of the bulb B.
_
It then falls into the groove C' and runs
out at D.
About twenty pounds of mercury can be distilled in
'
eight hours of continuous operation,
-
16—23.
Cathode
the cathode dark space
in a discharge tube is approximately half way
to the walls of the
tube and the discharge is run for some time, the walls become
'
coated with a thin layer of the cathode metal. If a glass plate G,
'
r\‘\“r
—
‘
'
_
/l//M/,,,
[. _ _ _