Chemical affinity is almost put an end to; phosphorus and potassium
remain inert in liquid oxygen. It should, however, be noted, and this
remark has doubtless some interest for the theories of photographic
action, that photographic substances retain, even at the temperature
of liquid hydrogen, a very considerable part of their sensitiveness to
light.
Sir James Dewar has made some important applications of low
temperatures in chemical analysis; he also utilizes them to create a
vacuum. His researches have, in fact, proved that the pressure of air
congealed by liquid hydrogen cannot exceed the millionth of an
atmosphere. We have, then, in this process, an original and rapid
means of creating an excellent vacuum in apparatus of very different
kinds--a means which, in certain cases, may be particularly
convenient.[9]
[Footnote 9: Professor Soddy, in a paper read before the Royal Society
on the 15th November 1906, warns experimenters against vacua created
by charcoal cooled in liquid air (the method referred-to in the text),
unless as much of the air as possible is first removed with a pump and
replaced by some argon-free gas.
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