and, as nearly always happens, the practical consequences derived from
them have also been most important. It is owing to the more complete
knowledge of the general properties of fluids that immense progress
has been made these last few years in the methods of liquefying gases.

From a theoretical point of view the new processes of liquefaction can
be classed in two categories. Linde's machine and those resembling it
utilize, as is known, expansion without any notable production of
external work. This expansion, nevertheless, causes a fall in the
temperature, because the gas in the experiment is not a perfect gas,
and, by an ingenious process, the refrigerations produced are made
cumulative.

Several physicists have proposed to employ a method whereby
liquefaction should be obtained by expansion with recuperable external
work. This method, proposed as long ago as 1860 by Siemens, would
offer considerable advantages. Theoretically, the liquefaction would
be more rapid, and obtained much more economically; but unfortunately
in the experiment serious obstacles are met with, especially from the
difficulty of obtaining a suitable lubricant under intense cold for
those parts of the machine which have to be in movement if the
apparatus is to work.

M. Claude has recently made great progress on this point by the use,
during the running of the machine, of the ether of petrol, which is
uncongealable, and a good lubricant for the moving parts. When once
the desired region of cold is reached, air itself is used, which
moistens the metals but does not completely avoid friction; so that
the results would have remained only middling, had not this ingenious
physicist devised a new improvement which has some analogy with
superheating of steam in steam engines. He slightly varies the initial
temperature of the compressed air on the verge of liquefaction so as
to avoid a zone of deep perturbations in the properties of fluids,
which would make the work of expansion very feeble and the cold
produced consequently slight. This improvement, simple as it is in
appearance, presents several other advantages which immediately treble
the output.

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 co