substance that will fully prevent the passage of heat-impulses in either
direction. Hence one of the greatest tasks of the experimenters has
been to find a receptacle that would insulate a cooled substance even
partially from the incessant bombardment of heat-impulses from without.
It is obvious that unless such an insulating receptacle could be
provided none of the more resistent gases, such as oxygen, could be
long kept liquid, even when once brought to that condition, since an
environment of requisite frigidity could not practicably be provided.

But now another phase of the problem presents itself to the
experimenter. Oxygen has assumed the quiescent liquid state, to be
sure, but in so doing it has fallen below the temperature of its cooling
medium; hence it is now receiving from that medium more energy of
vibration than it gives, and unless this is prevented very soon its
particles will again have power to kick themselves apart and resume the
gaseous state. Something, then, must be done to insulate the liquefied
gas, else it will retain the liquid state for too short a time to be
much experimented with. How might such insulation be accomplished?

The most successful attack upon this important problem has been made by
Professor Dewar. He invented a receptacle for holding liquefied gases
which, while not fulfilling the ideal conditions referred to above, yet
accomplishes a very remarkable degree of heat insulation. In consists of
a glass vessel with double walls, the space between which is rendered
a vacuum of the highest practicable degree. This vacuum, containing
practically no particles of matter, cannot, of course, convey
heat-impulses to or from the matter in the receptacle with any degree
of rapidity. Thus one of the two possible means of heat transfer is shut
off and a degree of insulation afforded the liquefied substance. But
of course the other channel, ether radiation, remains. Even this may be
blocked to a large extent, however, by leaving a trace of mercury vapor
in the vacuum space, which will be deposited as a fine mirror on
the inner surface of the chamber. This mirror serves as an admirable
reflector of the heat-rays that traverse the vacuum, sending more
than half of them back again. So, by the combined action of vacuum and
mirror, the amount of heat that can penetrate to the interior of the
receptacle is reduced to about one-thirtieth of what would enter an
ordinary vessel. In other words, a quanti