472 
NATURE 
[SEPTEMBER 11, 1902 
and there is no reason to doubt that the yield will be still 
further improved. It is clear, therefore, that in the immediate 
future the production of liquid air and hydrogen will be effected 
most economically by the use of machines producing cold by the 
expenditure of mechanical work. 
Liquid Hydrogen and Helium. 
To the physicist the copious production of liquid air by the 
methods described was of peculiar interest and value as affording 
the means of attacking the far more difficult problem of the 
liquefaction of hydrogen, and even as encouraging the hope that 
liquid hydrogen might in time be employed for the liquefaction 
of yet more volatile elements, apart from the importance which 
its liquefaction must hold in the process of the steady advance 
towards the absolute zero. Hydrogen is an element of especial 
interest, because the study of its properties and chemical rela- 
tions led great chemists like Faraday, Dumas, Daniel, Graham 
and Andrews to entertain the view that if it could ever be 
brought into the state of liquid or solid it would reveal metallic 
characters. Looking to the special chemical relations of the 
combined hydrogen in water, alkaline oxides, acids and salts, 
together with the behaviour of these substances on electrolysis, 
we are forced to conclude that hydrogen behaves as the analogue 
of a metal. After the beautiful discovery of Graham that 
palladium can absorb some hundreds of times its own volume of 
hydrogen and still retain its lustre and general metallic character, 
the impression that hydrogen was probably a member of the 
metallic group became very general. The only chemist who 
adopted another view was my distinguished predecessor, Prof. 
Odling. In his ‘f Manual of Chemistry,’ published in 1861, he 
pointed out that hydrogen has chlorous as well as basic relations, 
and that they are as decided, important, and frequent as its other 
relations. From such considerations he arrived at the conclusion 
that hydrogen is essentially a neutral or intermediate body, and 
therefore we should not expect to find liquid or solid hydrogen 
possess the appearance of a metal. This extraordinary pre- 
vision, so characteristic of Odling, was proved to be correct 
some thirty-seven years after it was made. Another curious 
anticipation was made by Dumas in a letter addressed to Pictet, 
in which he says that the metal most analogous to hydrogen is 
magnesium and that probably both elements have the same 
atomic volume, so that the density of hydrogen, for this reason, 
would be about the value elicited by subsequent experiments. 
Later on, in 1872, when Newlands began to arrange the elements 
in periodic groups, he regarded hydrogen as the lowest member 
of the chlorine family ; but Mendeléeff in his later classification 
placed hydrogen in the group of the alkaline metals; on the 
other hand, Dr, Johnstone Stoney classes hydrogen with the 
alkaline earth metals and magnesium. From this speculative 
divergency it is clear no definite conclusion could be reached 
regarding the physical properties of liquid or solid hydrogen, 
and the only way to arrive at the truth was to prosecute low- 
temperature research until success attended the efforts to 
produce its liquefaction. This result I definitely obtained in 
1898, The case of liquid hydrogen is, in fact, an excellent 
illustration of the truth already referred to, that no theoretical 
forecast, however apparently justified by analogy, can be finally 
accepted as true until confirmed by actual experiment. Liquid 
hydrogen is a colourless, transparent body of extraordinary 
intrinsic interest. It has a clearly defined surface, is easily seen, 
drops well, in spite of the fact that its surface tension is only 
the thirty-fifth part of that of water, or about one-fifth that 
of liquid air, and can be poured easily from vessel to vessel. 
The liquid does not conduct electricity, and, if anything, 
is slightly diamagnetic. Compared with an equal volume 
of liquid air, it requires only one-fifth the quantity of 
heat for vaporisation; on the other hand, its specific 
heat is ten times that of liquid air or five times 
that of water. The coefficient of expansion of the fluid is 
remarkable, being about ten times that of gas ; it is by far the 
lightest liquid known to exist, its density being only one-four- 
teenth that of water ; the lightest liquid previously known was 
liquid marsh gas, which is six times heavier. The only solid 
which has so small density as to float upon its surface is a piece 
of pith wood. It is by far the coldest liquid known. At 
ordinary atmospheric pressure it boils at sznus 252°5 degrees or 
20°5 degrees absolute. The critical point of the liquid is about 
29 degrees absolute and the critical pressure not more than 
fifteen atmospheres. The vapour of the hydrogen arising from 
the liquid has nearly the density of air—that is, it is fourteen 
NO. 1715, VOL. 66] 
times that of the gas at the ordinary temperature. Reduction 
of the pressure by an air-pump brings down the temperature to 
minus 258 degrees, when the liquid becomes a solid resembling 
frozen foam, and this by further exhaustion is cooled to mdnus 
260 degrees, or 13 degrees absolute, which is the lowest steady 
temperature that has been reached. The solid may also be got 
in the form of a clear, transparent ice, melting at about 15 
degrees absolute, under a pressure of 55 mm., possessing the 
unique density of one-eleventh that of water. Such cold 
involves the solidification of every gaseous substance but one 
that is at present definitely known to the chemist, and so liquid 
hydrogen introduces the investigator to a world of solid bodies. 
The contrast between this refrigerating substance and liquid air 
is most remarkable. On the removal of the loose plug of 
cotton-wool used to cover the mouth of the vacuum vessel in 
which it is stored, the action is followed by a miniature snow- 
storm of solid air, formed by the freezing of the atmosphere at 
the point where it comes into contact with the cold vapour 
rising from the liquid. This solid air falls into the vessel and 
accumulates as a white snow at the bottom of the liquid 
hydrogen. When the outside of an ordinary test-tube is cooled 
by immersion in the liquid, it is soon observed to fill up with 
solid air, and if the tube be now lifted out a double effect is 
visible, for liquid air is produced both in the inside and on the 
outside of the tube—in the one case by the melting of the solid, 
and in the other by condensation from the atmosphere. A tuft 
of cotton-wool soaked in the liquid and then held near the pole 
of a strong magnet is attracted, and it might be inferred therefrom 
that liquid hydrogen is a magnetic body. This, however, is not 
the case; the attraction is due neither to the cotton-wool nor to- 
the hydrogen—which indeed evaporates almost as soon as the 
tuft is taken out of the liquid—but to the oxygen of the air, 
which is well known to be a magnetic body, frozen in the wool 
by the extreme cold. 
The strong condensing powers of liquid hydrogen afford 
a simple means of producing vacua of very high tenuity. 
When one end of a sealed tube containing ordinary air is 
placed for a short time in the liquid, the contained air accumu- 
lates as a solid at the bottom, while the higher part is almost 
entirely deprived of particles of gas. So perfect is the vacuum 
thus formed, that the electric discharge can be made to pass only 
with the greatest difficulty. Another important application of 
liquid air, liquid hydrogen, &c., is as analytic agents. Thus, 
if a gaseous mixture be cooled by means of liquid oxygen, only 
those constituents will be left in the gaseous state which are less 
condensable than oxygen. Similarly, if this gaseous residue be 
in its turn cooled in liquid hydrogen, a still further separation 
will be effected, everything that is less volatile than hydrogen. 
being condensed to a liquid or solid. By proceeding in this 
fashion it has been found possible to isolate helium from a 
mixture in which it is present to the extent of only one part im 
one thousand. By the evaporation of solid hydrogen uhder the 
air-pump we can reach within 13 or 14 degrees of the zero, 
but there or thereabouts our progress is barred. This gap of 
13 degrees might seem at first sight insignificant in comparison 
with the hundreds that have already been conquered. But 
to win one degree low down the scale is quite a different 
matter from doing so at higher temperatures; in fact, to 
annihilate these few remaining degrees would be a far greater 
achievement than any so far accomplished in low-temperature 
research. For the difficulty is twofold, having to do partly 
with process and partly with material. The application of the 
methods used in the liquefaction of gases becomes continually 
harder and more troublesome as the working temperature is 
reduced ; thus, to pass from liquid air to liquid hydrogen—a 
difference of 60 degrees—is, from a thermodynamic point of 
view, as difficult as to bridge the gap of 150 degrees that 
separates liquid chlorine and liquid air. By the use of a new 
liquid gas exceeding hydrogen in volatility to the same extent 
as hydrogen does nitrogen, the investigator might get to within 
five degrees of the zero; but even a second hypothetical sub- 
stance, again exceeding the first one in volatility to an equal 
extent, would not suffice to bring him quite to the point of his 
ambition. That the zero will ever be reached by man is 
extremely improbable. A thermometer introduced into regions. 
outside the uttermost confines of the earth’s atmosphere might 
approach the absolute zero, provided that its parts were highly 
transparent to all kinds of radiation, otherwise it would be 
affected by the radiation of the sun, and would therefore 
become heated. But supposing all difficulties to be overcome, 
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