is made colder, both before and after expansion, than any
that had gone before. This intensification of cooling goes on until the
expansion-temperature is far lower than it was at starting; and if
the apparatus be well arranged the effect is so powerful that even the
smaller amount of cooling due to the free expansion of gas through a
throttle-valve, though pronounced by Siemens and Coleman incapable
of being utilized, may be made to liquefy air without using other
refrigerants."

So well is this principle carried out in Dr. Hamp-son's apparatus for
liquefying air that compressed air passing into the coil at ordinary
temperature without other means of refrigeration begins to liquefy in
about six minutes--a result that seems almost miraculous when it is
understood that the essential mechanism by which this is brought about
is contained in a cylinder only eighteen inches long and seven inches in
diameter.

As has been said, it was by adopting this principle of self-intensive
refrigeration that Professor Dewar was able to liquefy hydrogen. More
recently the same result has been attained through use of the same
principle by Professor Ramsay and Dr. Travers at University College,
London, who are to be credited also with first publishing a detailed
account of the various stages of the process. It appears that the use of
the self-intensification principle alone is not sufficient with hydrogen
as it is with the less volatile gases, including air, for the reason
that at all ordinary temperatures hydrogen does not cool in expanding,
but actually becomes warmer. It is only after the compressed hydrogen
has been cooled by immersion in refrigerating media of very low
temperature that this gas becomes amenable to the law of cooling on
expansion. In the apparatus used at University College the coil of
compressed hydrogen is passed successively through (1) a jar containing
alcohol and solid carbonic acid at a temperature of--80 deg. Centigrade; (2)
a chamber containing liquid air at atmospheric pressure, and (3)
liquid air boiling in a vacuum bringing the temperature to perhaps 2050
Centigrade before entering the Hampson coil, in which expansion and
the self-intensive refrigeration lead to actual liquefaction. With this
apparatus Dr. Travers succeeded in producing an abundant quantity of
liquid hydrogen for use in the experiments on the new gases that were
first discovered in the same laboratory through the experiments on
liquid air-