16—5.
THE SORPTION ' PROCESSES
363
the adsorption is small, from 0.1 to 8.0 cc. per grain, This oHers
a method of isolating these gases when
others.
.
Dewar
.
'
found that considerable quantities of helium. were taken up.. by
charcoal cooled below liquid air temperatures, say 15° absolute.
'
Oxygen, once adsorbed, is only given up by prolonged heating at
"
high temperatures. In addition to the high adsorption capacity
of the large surface offered by the porous nature of charcoal, it also
serves to remove water vapor by capillary action.
-
’
Certain samples of palladium black have the property of
hydrogen to a very marked extent. For producing high
vacua, the palladium is heated-to 100° C. in the system which is
being evacuated. The
then cutoff andthe palladium
cooled with liquid air. Starting at 10—4 mm., the pressure will be
.
lowered to around 5 X 10”7 mm.
_
Under certain conditions nickel,
,
,
copper and iridium will also absorb appreciable quantities of
'
hydrogen.
_,
.
Thereare several methods for reducing the pressure after. the
vacuum chamber has been sealed off from the
In one of
'
these, zirconium, thorium or calcium is placed in the system before
evacuation.
After evacuating and sealing off, the substance is
.
volatilized by the external application of heat,— when all except the
rare gases such as argon, neon and kryptondisappear. , The
_
pressure must not exceeda :few millimeters if a high va'cuum, say
_
10—7 mm., is to be
1
_
,
_
…
'
: :
.
'
.
When an eZec;riçdl dz'.rclzàrge “passesthmugh a sealed GeiSsler° «tube, the
the pressure is so
'
-
low that the
ceases.t_ Th1s effect is observed
* in gas-lled
X—ray tubes, sothat1t150ccasmnallynecessary to .adrnit traces of
gas to PrevehŸ. the tube from
of
this
.
theelectrodesundœbœdlyp1ayan1mP@ftantpart
type of
smmbarmmcalcmmstmnüumaluminumandvafl@usmïXtuœs
_
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oralloysGftheseThegettefISvolat1hZ€dluStbeforethef‘ïbe
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