In the same way, magnetic properties have been studied.  A very interesting result is that found in oxygen:  the magnetic susceptibility of this body increases at the moment of liquefaction.  Nevertheless, this increase, which is enormous (since the susceptibility becomes sixteen hundred times greater than it was at first), if we take it in connection with equal volumes, is much less considerable if taken in equal masses.  It must be concluded from this fact that the magnetic properties apparently do not belong to the molecules themselves, but depend on their state of aggregation.

The mechanical properties of bodies also undergo important modifications.  In general, their cohesion is greatly increased, and the dilatation produced by slight changes of temperature is considerable.  Sir James Dewar has effected careful measurements of the dilatation of certain bodies at low temperatures:  for example, of ice.  Changes in colour occur, and vermilion and iodide of mercury pass into pale orange.  Phosphorescence becomes more intense, and most bodies of complex structure—­milk, eggs, feathers, cotton, and flowers—­become phosphorescent.  The same is the case with certain simple bodies, such as oxygen, which is transformed into ozone and emits a white light in the process.

Chemical affinity is almost put an end to; phosphorus and potassium remain inert in liquid oxygen.  It should, however, be noted, and this remark has doubtless some interest for the theories of photographic action, that photographic substances retain, even at the temperature of liquid hydrogen, a very considerable part of their sensitiveness to light.

Sir James Dewar has made some important applications of low temperatures in chemical analysis; he also utilizes them to create a vacuum.  His researches have, in fact, proved that the pressure of air congealed by liquid hydrogen cannot exceed the millionth of an atmosphere.  We have, then, in this process, an original and rapid means of creating an excellent vacuum in apparatus of very different kinds—­a means which, in certain cases, may be particularly convenient.[9]

[Footnote 9:  Professor Soddy, in a paper read before the Royal Society on the 15th November 1906, warns experimenters against vacua created by charcoal cooled in liquid air (the method referred-to in the text), unless as much of the air as possible is first removed with a pump and replaced by some argon-free gas.  According to him, neither helium nor argon is absorbed by charcoal.  By the use of electrically-heated calcium, he claims to have produced an almost perfect vacuum.—­ED.]