y simply varying the pressure.
It is only when the pressure upon it is equal to the normal pressure of
the atmosphere at the sea level, as indicated by a barometric reading of
760 mm., that it boils at 100 deg..

~Preparation of liquid air.~ Attention has been called to the fact that
both oxygen and nitrogen can be obtained in the liquid state by strongly
cooling the gases and applying great pressure to them. Since air is
largely a mixture of these two gases, it can be liquefied by the same
methods.

The methods for liquefying air have been simplified greatly in
that the low temperature required is obtained by allowing a
portion of the compressed air to expand. The expansion of a gas
is always attended by the absorption of heat. In liquefying air
the apparatus is so constructed that the heat absorbed is
withdrawn from air already under great pressure. This process
is continued until the temperature is lowered to the point of
liquefaction.

[Illustration: Fig. 29]

~The Dewar bulb.~ It is not possible to preserve air in the liquid state
in a closed vessel, on account of the enormous pressure exerted by it in
its tendency to pass into the gaseous state. It may however be preserved
for some hours or even days before it will completely evaporate, by
simply placing it in an open vessel surrounded by a nonconducting
material. The most efficient vessel for this purpose is the _Dewar bulb_
shown in Fig. 29. The air is withdrawn from the space between the two
walls, thus making it nonconducting.

~Properties and uses of liquid air.~ When first prepared, liquid air is
cloudy because of the presence of particles of solid carbon dioxide.
These may be filtered off, leaving a liquid of slightly bluish color. It
begins to boil at about -190 deg., the nitrogen passing off first, gradually
followed by the oxygen, the last portions being nearly pure oxygen. To a
certain extent oxygen is now prepared in this way for commercial
purposes.

The extremely low temperature of liquid air may be inferred from the
fact that mercury when cooled by it is frozen to a mass so hard that it
may be used for driving nails.

Liquid air is used in the preparation of oxygen and as a cooling agent
in the study of the properties of matter at low temperatures. It has
thus been found that elements at extremely low temperatures largely lose
their chemical activity.

EXERCISES

1. When oxygen and nitrogen are