Sir James Dewar points out that Dalton's law demands that every gas composing the atmosphere should have, at all heights and temperatures, the same pressure as if it were alone, the pressure decreasing the less quickly, all things being equal, as its density becomes less.

It results from this that the temperature becoming gradually lower as we rise in the atmosphere, at a certain altitude there can no longer remain any traces of oxygen or nitrogen, which no doubt liquefy, and the atmosphere must be almost exclusively composed of the most volatile gases, including hydrogen, which M.A.Gautier has, like Lord Rayleigh and Sir William Ramsay, proved to exist in the air.

The spectrum of the _Aurora borealis_, in which are found the lines of those parts of the atmosphere which cannot be liquefied in liquid hydrogen, together with the lines of argon, crypton, and xenon, is quite in conformity with this point of view.

It is, however, singular that it should be the spectrum of crypton, that is to say, of the heaviest gas of the group, which appears most clearly in the upper regions of the atmosphere.
Among the gases most difficult to liquefy, hydrogen has been the object of particular research and of really quantitative experiments.
Its properties in a liquid state are now very clearly known.

Its boiling-point, measured with a helium thermometer which has been compared with thermometers of oxygen and hydrogen, is -252 deg.; its critical temperature is -241 deg.