n the centre of low-temperature
research. By means of improved machinery and of ingenious devices for
shielding the substance operated on from the accession of heat, to which
reference will be made more in detail presently, Professor Dewar was
able to liquefy the gas fluorine, recently isolated by Moussan, and the
recently discovered gas helium in 1897. And in May, 1898, he was able to
announce that hydrogen also had yielded, and for the first time in
the history of science that* elusive substance, hitherto "permanently"
gaseous, was held as a tangible liquid in a cuplike receptacle; and this
closing scene of the long struggle was enacted in the same laboratory in
which Faraday performed the first liquefaction experiment with chlorine
just three-quarters of a century before.

It must be noted, however, that this final stage in the liquefaction
struggle was not effected through the use of the principle of
evaporating liquids which has just been referred to, but by the
application of a quite different principle and its elaboration into a
perfectly novel method. This principle is the one established long ago
by Joule and Thomson (Lord Kelvin), that compressed gases when allowed
to expand freely are lowered in temperature. In this well-known
principle the means was at hand greatly to simplify and improve the
method of liquefaction of gases, only for a long time no one recognized
the fact. Finally, however, the idea had occurred to two men almost
simultaneously and quite independently. One of these was Professor
Linde, the well-known German experimenter with refrigeration processes;
the other, Dr. William Hampson, a young English physician. Each of these
men conceived the idea--and ultimately elaborated it in practice--of
accumulating the cooling effect of an expanding gas by allowing the
expansion to take place through a small orifice into a chamber in which
the coil containing the compressed gas was held. In Dr. Hampson's words:

"The method consists in directing all the gas immediately after its
expansion over the coils which contain the compressed gas that is on its
way to the expansion-point. The cold developed by expansion in the first
expanded gas is thus communicated to the oncoming compressed gas, which
consequently expands from, and therefore to, a lower temperature
than the preceding portion. It communicates in the same way its own
intensified cold to the succeeding portion of compressed gas, which, in
its turn,