474 
all probability gases other than helium will be discovered of 
greater volatility than hydrogen. It was at the British Asso- 
ciation Meeting in 1896 that I made the first suggestion of the 
probable existence of an unknown element which would be 
found to fill up the gap between argon and helium, and this 
anticipation was soon taken up by others and ultimately con- 
firmed. Later, in the Bakerian Lecture for 1901, I was led to 
infer that another member of the helium group might exist 
havirg the atomic weight about 2, and this would give us a gas 
still more volatile, with which the absolute zero might be still 
more nearly approached. It is to be hoped that some such 
element or elements may yet be isolated and identified as 
coronium or nebulium. If amongst the unknown gases possessing 
a very low critical point some have a high critical pressure instead 
of a low one, which ordinary experience would lead us to antici- 
pate, then such difficultly liquefiable gases would produce fluids 
having different physical properties from any of those with which 
we are acquainted. Again, gases may exist having smaller 
atomic weights and densities than hydrogen, yet all such gases 
must, according to our present views of the gaseous state, be 
capable of liquefaction before the zero of temperature is reached. 
The chemists of the future will find ample scope for investiga- 
tion within the apparently limited range of temperature which 
separates solid hydrogen from the zero. Indeed, great as is the 
sentimental interest attached to the liquefaction of these refrac- 
tory gases, the importance of the achievement lies rather in the 
fact that it opens out new fields of research and enormously 
widens the horizon of physical science, enabling the natural 
philosopher to study the properties and behaviour of matter 
under entirely novel conditions. This department of inquiry is 
as yet only in its infancy, but speedy and extensive develop- 
ments may be looked for, since within recent years several 
special cryogenic laboratories have been established for the 
prosecution of such researches, and a liquid-air plant is becoming 
a common adjunct to the equipment of the ordinary laboratory. 
The Upper Air and Auroras. 
The present liquid ocean, neglecting everything for the mo- 
ment but the water, was at a previous period of the earth’s 
history part of the atmosphere, and its condensation has been 
brought about by the gradual cooling of the earth’s surface. 
This resulting ocean is subjected to the pressure of the remaining 
uncondensed gases, andas these are slightly soluble they dissolve 
to some extent in the fluid. The gases in solution can be taken 
out by distillation or by exhausting the water, and if we compare 
their volume with the volume of water as steam, we should find 
about 1 volume of air in 60,000 volumes of steam. This would 
then be about the rough proportion of the relatively permanent 
gas to condensable gas which existed in the case of the vaporised 
ocean. Now let us assume the surface of the earth gradually 
cooled to some 200 degrees below the freezing-point ; then, after 
all the present ocean was frozen, and the climate became three 
times more intense than any Arctic frost, a new ocean of liquid 
air would appear, covering the entire surface of the frozen globe 
about thirty-five feet deep. We may now apply the same 
reasoning to the liquid air ocean that we formerly did to the water 
one, and this would lead us to anticipate that it might contain in 
solution some gases that may be far less condensable than the 
chief constituents of the fluid. Inorder to separate them we 
must imitate the method of taking the gases out of water. 
Assume a sample of liquid air cooled to the low temperature 
that can be reached by its own evaporation, connected by a 
pipe to a condenser cooled in liquid hydrogen ; then any volatile 
gases present in solution will distil over with the first portions 
of the air, and can be pumped off, being uncondensable 
at the temperature of the condenser, In this way, a gas 
mixture, containing, of the known gases, free hydrogen, 
helium and neon, has been separated from liquid air. It is 
interesting to note in passing that the relative volatilities of 
water and oxygen are in the same ratio as those of liquid air 
and hydrogen, so that the analogy between the ocean of water 
and that of liquid air has another suggestive parallel. The 
total uncondensable gas separated in this way amounts to about 
one fifty-thousandth of the volume of the air, which is about the 
same proportion as the air dissolved in water. That free 
hydrogen exists in air in small amount is conclusively proved, 
but the actual proportion found by the process is very much 
smaller than Gautier has estimated by the combustion method. 
The recent experiments of Lord Rayleigh show that Gautier, 
who estimated the hydrogen present as one five-thousandth, 
NO. 1715, VOL. 66] 
NATURE 
[SEPTEMBER II, 1902 
has in some way produced more hydrogen than he can manage 
to extract from pure air by a repetition of the same process. 
The spectroscopic examination of these gases throws new light 
upon the question of the aurora and the nature of the upper 
air. On passing electric discharges through the tubes containing 
the most volatile of the atmospheric gases, they glow with a 
bright orange light, which is especially marked at the negative 
pole. The spectroscope shows that this light consists, in the 
visible part of the spectrum, chiefly of a succession of strong 
rays in the red, orange and yellow, attributed to hydrogen, 
helium and neon. Besides these, a vast number of 
rays, generally less brilliant, are distributed through the 
whole length of the visible spectrum. The greater part of 
these rays are of, as yet, unknown origin. The violet and ultra- 
violet part of the spectrum rivals in strength that of the red and 
yellow rays. As these gases probably include some of the 
gases that pervade interplanetary space, search was made fer 
the prominent nebular, coronal and auroral lines. No definite 
lines agreeing with the nebular spectrum could be found, but 
many lines occurred closely coincident with the coronal and 
auroral spectrum. But before discussing the spectroscopic 
problem it will be necessary to consider the nature and condition 
of the upper air. 
According to the old law of Dalton, supported by the modern 
dynamical theory of gases, each constituent of the atmosphere 
while acted upon by the force of gravity forms a separate atmo- 
sphere, completely independent, except as to temperature, of the 
others, and the relations between the common temperature and 
the pressure and altitude for each specific atmosphere can be 
definitely expressed. If we assume the altitude and tempera- 
ture known, then the pressure can be ascertained for the same 
height in the case of each of the gaseous constituents, and in 
this way the percentage composition of the atmosphere at that 
place may be deduced. Suppose we start with a surface atmo- 
sphere having the composition of our air, only containing 
two ten-thousandths of hydrogen, then at thirty-seven miles, 
if a sample could be procured for analysis, we believe that it 
would be found to contain 12 per cent. of hydrogen and 
only 10 per cent. of oxygen. The carbonic acid practically 
disappears ; and by the time we reach forty-seven miles, where 
the temperature is 7222s 132 degrees, assuming a gradient of 
32 degrees per mile, the nitrogen and oxygen have so thinned 
out that the only constituent of the upper air which is left is 
hydrogen. If the gradient of temperature were doubled, the 
elimination of the nitrogen and oxygen would take place by the 
time thirty-seven miles was reached, with a temperature of 
minus 220 degrees. The permanence of the composition of the 
air at the highest altitudes, as deduced from the basis of the 
dynamical theory of gases, has been discussed by Stoney, Bryan, 
and others. It would appear that there is a consensus of opinion 
that the rate at which gases like hydrogen and helium could 
escape from the earth’s atmosphere would be excessively slow. 
Considering that to compensate any such loss the same gases are 
being supplied by actions taking place in the crust of the earth, 
we may safely regard them as necessarily permanent constituents 
of the upper air. The temperature at the elevations we 
have been discussing would not be sufficient to cause any 
liquefaction of the nitrogen and oxygen, the pressure being 
so low. If we assume the mean temperature as about 
the boiling-point of oxygen at atmospheric pressure, then 
a considerable amount of the carbonic acid must solidify as 
a mist, if the air from a lower level be cooled to this tempera- 
ture ; and the same result might take place with other gases 
of relatively small volatility which occur in air. This would 
explain the clouds that have been seen at an elevation of fifty 
miles, without assuming the possibility of water vapour being 
carried up sohigh. The temperature of the upper air must be 
above that on the vapour pressure curve corresponding to the 
barometric pressure at the locality, otherwise liquid condensation 
must take place. In other words, the temperature must be 
above the dew-point of air at that place. At higher elevations, 
on any reasonable assumption of temperature distribution, we 
inevitably reach a temperature where the air would condense, 
just as Fourier and Poisson supposed it would, unless the tem- 
perature is arrested in some way from approaching the zero. 
Both ultra-violet absorption and the prevalence of electric 
storms may have something to do with the maintenance of a 
higher mean temperature. The whole mass of the air above 
forty miles is not more than one seven-hundredth part of the 
total mass of the atmosphere, so that any rain or snow of liquid 
