The special object of M. Claude was to obtain oxygen in a practical manner by the actual distillation of liquid air.  Since nitrogen boils at -194 deg. and oxygen at -180.5 deg.  C., if liquid air be evaporated, the nitrogen escapes, especially at the commencement of the evaporation, while the oxygen concentrates 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 temperatures far below their fusion point, still retain a very appreciable conductivity, become, on the contrary, perfect insulators at low temperatures.  Their dielectric constants assume relatively high values.  MM.  Curie and Compan, who have studied this question from their own point of view, have noted, moreover, that the specific inductive capacity changes considerably with the temperature.