ncentrates in the residual liquid, which finally
consists of pure oxygen, while at the same time the temperature rises
to the boiling-point (-180.5 deg. C.) of oxygen. But liquid air is costly,
and if one were content to evaporate it for the purpose of collecting
a part of the oxygen in the residuum, the process would have a very
poor result from the commercial point of view. As early as 1892, Mr
Parkinson thought of improving the output by recovering the cold
produced by liquid air during its evaporation; but an incorrect idea,
which seems to have resulted from certain experiments of Dewar--the
idea that the phenomenon of the liquefaction of air would not be,
owing to certain peculiarities, the exact converse of that of
vaporization--led to the employment of very imperfect apparatus. M.
Claude, however, by making use of a method which he calls the
reversal[8] method, obtains a complete rectification in a remarkably
simple manner and under extremely advantageous economic conditions.
Apparatus, of surprisingly reduced dimensions but of great efficiency,
is now in daily work, which easily enables more than a thousand cubic
metres of oxygen to be obtained at the rate, per horse-power, of more
than a cubic metre per hour.

[Footnote 8: Methode avec retour en arriere.--ED]

It is in England, thanks to the skill of Sir James Dewar and his
pupils--thanks also, it must be said, to the generosity of the Royal
Institution, which has devoted considerable sums to these costly
experiments--that the most numerous and systematic researches have
been effected on the production of intense cold. I shall here note
only the more important results, especially those relating to the
properties of bodies at low temperatures.

Their electrical properties, in particular, undergo some interesting
modifications. The order which metals assume in point of conductivity
is no longer the same as at ordinary temperatures. Thus at -200 deg. C.
copper is a better conductor than silver. The resistance diminishes
with the temperature, and, down to about -200 deg., this diminution is
almost linear, and it would seem that the resistance tends towards
zero when the temperature approaches the absolute zero. But, after
-200 deg., the pattern of the curves changes, and it is easy to foresee
that at absolute zero the resistivities of all metals would still
have, contrary to what was formerly supposed, a notable value.
Solidified electrolytes which, at temperatu