ness. If this is done it will change from a gas to a
liquid, but will occupy as much space as before, if it is condensed to a
pressure of six tons to the square inch.

Until twenty years ago it was supposed that oxygen and atmospheric air
(the latter a mixture of oxygen and nitrogen) were fixed gases and could
not be liquefied. In 1877, it is said that Raoul Pictet obtained the
first liquid oxygen, but only a few drops. About fifteen years later
Professor Dewar of the Royal Institution, London, succeeded in
liquefying not only oxygen but atmospheric air. And besides liquefying
the air he made ice of it.

In 1892 I visited London, where I met Professor Dewar, who invited me to
witness an exhibition of the manufacture of liquid oxygen--and
incidentally liquid air--at the Royal Institution. To me it was a most
wonderfully interesting event. I saw air, taken from the room, gradually
liquefy in a small glass test tube open at the top. When the tube was
withdrawn from the refrigerating chamber it boiled by the heat of the
room, and rapidly evaporated. We lighted a splinter of wood and blew it
out, leaving a live spark on the end of it, and held it over the mouth
of the tube, knowing that if anything like pure oxygen were evaporating
the splinter would relight and blaze (an old experiment with oxygen
gas). At first the splinter would not relight, because the evaporating
gases were a mixture of oxygen and nitrogen in the proportions to form
air. But owing to the fact that nitrogen evaporates sooner than oxygen,
a second trial was successful, for the splinter immediately began to
blaze, showing that the gas evaporating then was pure, or nearly pure,
oxygen.

When the liquid oxygen was poured into a saucer and brought into
proximity with the poles of a powerful magnet the liquid immediately
rushed out of the saucer and clung to the magnet poles; showing that
oxygen is magnetic.

Since that time other experimenters have succeeded in making liquid air
on a comparatively large scale, and the process is simple when we
consider some of the old methods.

Mr. Tripler of New York, who has made liquid air in great quantities,
does it substantially as follows: First, he compresses air to about 2500
pounds to the square inch. Of course the air is very hot when it is
first compressed because all the air in the tank has been reduced in
bulk about 166 times, and all the heat that was in the whole bulk of air
is concentrated into one-166t