he liquid state. In
other words, the fact of a so-called "critical temperature," varying
for different substances, above which a given substance is always a gas,
regardless of pressure, was definitively discovered. It became clear,
then, that before the resistant gases would be liquefied means of
reaching extremely low temperatures must be discovered. And for this,
what was needed was not so much new principles as elaborate and
costly machinery for the application of a principle long familiar--the
principle, namely, that an evaporating liquid reduces the temperature of
its immediate surroundings, including its own substance.

Ingenious means of applying this principle, in connection with the means
previously employed, were developed independently by Pictet in Geneva
and Cailletet in Paris, and a little later by the Cracow professors
Wroblewski and Olzewski, also working independently. Pictet, working on
a commercial scale, employed a series of liquefied gases to gain lower
and lower temperatures by successive stages. Evaporating sulphurous acid
liquefied carbonic acid, and this in evaporating brought oxygen under
pressure to near its liquefaction point; and, the pressure being
suddenly released (a method employed in Faraday's earliest experiments),
the rapid expansion of the compressed oxygen liquefies a portion of
its substance. This result was obtained in 1877 by Pictet and Cailletet
almost simultaneously. Cailletet had also liquefied the newly discovered
acetylene gas. Five years later Wroblewski liquefied marsh gas, and the
following year nitrogen; while carbonic oxide and nitrous oxide yielded
to Olzewski in 1884. Thus forty years of effort had been required to
conquer five of Faraday's refractory gases, and the sixth, hydrogen,
still remains resistant. Hydrogen had, indeed, been seen to assume the
form of visible vapor, but it had not been reduced to the so-called
static state--that is, the droplets had not been collected in an
appreciable quantity, as water is collected in a cup. Until this should
be done, the final problem of the liquefaction of hydrogen could not be
regarded as satisfactorily solved.

More than another decade was required to make this final step in the
completion, of Faraday's work. And, oddly enough, yet very fittingly,
it was reserved for Faraday's successor in the chair at the Royal
Institution to effect this culmination. Since 1884 Professor Dewar's
work has made the Royal Institution agai