be regarded as the limit to which the
temperature falls. It is true that if the air is exhausted to the utmost
possible extent, the temperature may fall still lower, but when the
substance has become solid, a further diminution of the pressure in the
space is of little advantage. At any rate, as a solid body evaporates
only on the surface, and solid gases are bad conductors of heat, further
cooling will only take place very slowly, and will scarcely neutralize
the influx of heat. If the pressure p3 is very small, it is perhaps
practically impossible to reach T3; if so, T3 in the following lines
will represent the temperature practically attainable. There is thus for
every gas a limit below which it is not to be cooled further, at least
not in this way. If, however, we can find another gas for which the
critical temperature is sufficiently above T3 of the first chosen gas,
and if it is converted into a liquid by cooling with the first gas, and
then treated in the same way as the first gas, it may in its turn be
cooled down to (T3)2. Going on in this way, continually lower
temperatures may be attained, and it would be possible to condense all
gases, provided the difference of the successive critical temperatures
of two gases fulfils certain conditions. If the ratio of the absolute
critical temperatures for two gases, which succeed one another in the
series, should be sensibly greater than 2, the value of T3 for the first
gas is not, or not sufficiently, below the Tc of the second gas. This is
the case when one of the gases is nitrogen, on which hydrogen would
follow as second gas. Generally, however, we shall take atmospheric air
instead of nitrogen. Though this mixture of N2 and O2 will show other
critical phenomena than a simple substance, yet we shall continue to
speak of a Tc for air, which is given at -140 deg. C., and for which,
therefore, Tc amounts to 133 deg. absolute. The lowest T which may be
expected for air in a highly rarefied space may be evaluated at 60 deg.
absolute--a value which is higher than the Tc for hydrogen. Without new
contrivances it would, accordingly, not be possible to reach the
critical temperature of H2. The method by which we try to obtain
successively lower temperatures by making use of successive gases is
called the "cascade method." It is not self-evident that by sufficiently
diminishing the pressure on a liquid it may be cooled to such a degree
that the temperature will be lowered to T3