THE NOBLE GASES 357tion to form a cage-like structure inside which one noble gas atom
is imprisoned. This has been called a "clathrate' compound (Latin,
clathri = lattice), but there are no chemical forces between the noble
gas atom and the atoms of the cage, so such a substance is not really a
compound of the noble gas.
USES
Helium has been used in quantity as a substitute for hydrogen
in filling airships. A mixture of 80% helium and 20% oxygen is
used instead of air in diving apparatus because helium, unlike
nitrogen, is not appreciably soluble in blood even under pressure.
(The liberation of dissolved nitrogen from the blood, when the
pressure is released, gives rise to "caisson disease' or "the bends'.)
A similar helium-oxygen mixture has been used to assist breathing
in cases of asthma and other respiratory diseases.
Helium has two important scientific uses. First, liquid helium
is used to realise very low temperatures, in order to study peculiar
phenomena which occur near the absolute zero—cryogenics. Some
metals attain enormously high electrical conductivity when cooled
down to near absolute zero, and hence powerful electro-magnets
can be made using very small coils cooled in liquid helium. Secondly,
it is used in gas thermometers for low temperature measurement.
Further, any of the rare gases may be used to give an inert atmo-
sphere for handling very reactive metals; for example an atmosphere
of argon is used in the preparation of titanium and in metallurgical
processes, involving this metal, because it is attacked at red heat by
both oxygen and nitrogen.
Electric discharge tubes are filled with neon (which causes the
familiar red glow) and ordinary electric filament lamps with argon.
The higher the temperature of the filament in such a lamp, the
greater is its efficiency of illumination, but the greater also is its
loss of metal by evaporation; metal vapour condenses on the glass
bulb, blackening it, and the filament soon evaporates. To permit
the use of a high temperature filament without evaporation, a
gas is used to fill the lamp; and the greater the molecular weight
of this gas, the less tendency there is for metal atoms to diffuse
through it. Hence argon (40) is better than nitrogen (28) for this
purpose, and of course, krypton and xenon are better still, though
more expensive to use.
Radon, sealed in small capsules called "seeds', has been used as a
radioactive substance in medicine, but is being superseded by more
convenient artificially-produced radioisotopes.