regards most substances are
profoundly modified when excessively cooled. Thus if a piece of any pure
metal is placed in an electric circuit and plunged into liquid air, its
resistance to the passage of the electricity steadily decreases as the
metal cools, until at the temperature of the liquid it is very trifling
indeed. The conclusion seems to be justified that if the metal could be
still further cooled until it reached the theoretical "absolute zero,"
or absolutely heatless condition, the electrical resistance would also
be nil. So it appears that the heat vibrations of the molecules of a
pure metal interfere with the electrical current. The thought suggests
itself that this may be because the ether waves set up by the vibrating
molecules conflict with the ether strain which is regarded by some
theorists as constituting the electrical "current." But this simple
explanation falters before further experiments which show, paradoxically
enough, that the electrical resistance of carbon exactly reverses what
has just been said of pure metals, becoming greater and greater as the
carbon is cooled. If an hypothesis were invented to cover this case
there would still remain a puzzle in the fact that alloys of metals
do not act at all like the pure metals themselves, the electrical
resistance of such alloys being, for the most part, unaffected by
changed temperature. On the whole, then, the facts of electrical
conduction at low temperatures are quite beyond the reach of present
explanation. They must await a fuller knowledge of molecular conditions
in general than is at present available--a knowledge to which the
low-temperature work itself seems one of the surest channels.

Even further beyond the reach of present explanation are the facts as to
magnetic conditions at low temperatures. Even as to the facts themselves
different experimenters have differed somewhat, but the final conclusion
of Professor Dewar is that, after a period of fluctuation, the power of
a magnet repeatedly subjected to a liquid-air bath becomes permanently
increased. Various substances not markedly magnetic at ordinary
temperatures become so when cooled. Among these, as Professor Dewar
discovered, is liquid oxygen itself. Thus if a portion of liquid air be
further cooled until it assumes a semi-solid condition, the oxygen may
be drawn from the mass by a magnet, leaving a pure nitrogen jelly. These
facts are curious enough, and full of suggestion, but like