PRESIDENT’S ADDRESS. 30 
Nitrogen can be liquefied and solidified, its freezing-point 
being —214°. 
When air is solidified it appears to be a jelly-like sub- 
stance, the liquid oxygen being held, as it were, in a spongy 
mass of nitrogen. By the use of glass vessels with double 
walls, with an insulating vacuum between them, Dewar has 
succeededin restraining the evaporation of oxygen and similar 
liquefied gases to such an extent that they can be retained 
in open vessels for a comparatively long time, as when once 
the glass vessel is cooled down the liquid oxygen merely 
evaporates slowly from the surface ; hence the manipulation 
is much simplified. 
Hydrogen was found to be much more difficult to liquefy 
than the other gases, and was first brought about in 1894, 
its critical temperature being —240°. Dewar succeeded in 
solidifying oxygen by means of a jet of liquid hydrogen.. 
On account of its rapid evaporation it cannot be collected 
and handled like oxygen.* 
It is only within the last few years that fluorine has been 
obtained in the pure condition, simply because it destroyed 
the material of the apparatus in which it was prepared ; but. 
that difficulty has now been overcome—from the readiness 
with which fluorine, under ordinary conditions, acts upon 
glass and other substances. One of the early names given 
to it was Phthor, or the Destroyer. 
Fluorime has hitherto resisted all attempts to liquefy 
it, but Professors Moissan and Dewar succeeded in April 
last by using liquid oxygen as the refrigorant at a little 
under the ordimary atmospheric pressure. It is a clear 
pale yellowish fluid, which does not attack glass, although 
its vapour does so with the greatest energy ; at low tem- 
perature it can be stored and manipulated in glass vessels, 
and at such low temperatures its chemical activity is so 
much reduced that it has no action upon substances upon 
* Since the above was written Professor Dewar has succeeded in liquefying 
large quantities of hydrogen. 
Cc 
