[Footnote 1: Translated from the original paper, by Prof. K.
Olszewski, in the Bulletin de l'Academie des Sciences de
Cracovie for June, 1896, "Ein Versuch, das Helium zu
verflunigen," by Morris Travers, and published in Nature.]
My experiments on the liquefaction of helium were carried out with a
sample of that gas, sent to me by Prof. Ramsay from London, in a
sealed glass tube holding about 140 c. cm. I take this opportunity of
rendering him my most sincere thanks. In his letter Prof. Ramsay
informed me that the gas had been obtained from the mineral clevite,
and that it was quite free from nitrogen and other impurities, which
could be removed by circulation over red hot magnesium, oxide of
copper, soda lime, and pentoxide of phosphorus. The density of the gas
was 2.133 and the ratio of its specific heats (Cp/Cv) 1.652, the
latter figure indicating that the molecule of helium was monatomic, as
had already been found to be the case with argon. Prof. Ramsay further
informed me that the gas was only very slightly soluble in water, 100
c. cm. of water dissolving scarcely 0.7 c. cm. of helium.
From the results of my earlier experiments I had been led to expect
that it would be only possible to liquefy helium at a very low
temperature; the small values obtained for the density and solubility
of the gas, together with the fact that its molecule is monatomic,
indicating a very low boiling point. For this reason I did not
consider it necessary to use liquid ethylene as a preliminary cooling
agent, but proceeded directly to conduct my experiments at the lowest
temperature that could be produced by means of liquid air. The
apparatus employed in these investigations is figured in the
accompanying diagram.
The helium was contained in the glass tube, c, of the Cailletet's
apparatus, C. The tube, c, reached to the bottom of a glass vessel, a,
which was intended to contain the liquid air. The vessel, a, was
surrounded by three glass cylinders, b, b' and b", closed at the
bottom and separated from one another. The outer vessel, b", was made
just large enough to fit into the brass collar, o, which supported the
lid, u, of the apparatus. The tube, a, fitted into an opening in the
center of the lid; the tube, t, connected with an apparatus delivering
liquid oxygen, passed through a hole on the right. The vessel, b, was
also connected with a mercury manometer and air pump by means of a T
tube, p, v, one arm of which passed through the third hole in the lid
of the apparatus. The tube, a, was closed by a stopper, through which
passed the tube, c, of the Cailletet's apparatus, a tube connected
with the drying apparatus, u, u', and one limb of a T tube, by means
of which the manometer and air pump could be put in connection with
the interior of the vessel. The lower part of the whole apparatus was
inclosed in a thick walled vessel, e, containing a layer of phosphorus
pentoxide.
By turning the valve, k, the vessel, b, could be partially filled with
liquid oxygen, which, under a pressure of 10 mm. of mercury, boiled at
about -210° C. Almost immediately the gaseous air began to condense
and collect in the tube, a; a supply of fresh air was constantly
maintained through the drying tubes, u and u', which were filled with
sulphuric acid and soda lime respectively. When the quantity of liquid
air ceased to increase, the tap on the U tube, u, was closed, the T
tube, p' v', was connected with the manometer and air pump, and the
liquid air was made to boil under a pressure of 10 mm. of mercury. In
order to protect the liquid air from its warmer surroundings, a very
thin, double wall tube, f, reaching to the level of the liquid in the
outer vessel, was placed inside the tube, a. When, as in some of my
experiments, liquid oxygen was used in the inner vessel, this part of
the apparatus was dispensed with.
[Illustration: Helium cooling apparatus]
Using the apparatus I have just described, I carried out two series of
experiments, in which liquid air and liquid oxygen were employed as
cooling agents. The tube of the Cailletet's apparatus was thoroughly
exhausted by means of a mercury pump, and then carefully filled with
dry helium. In the first series of experiments the helium, confined
under a pressure of 125 atmospheres, was cooled to the temperature of
oxygen boiling, first under atmospheric pressure (-182.5°), and then
under a pressure of 10 mm. of mercury (-210°). The helium did not
condense under these conditions, and even when, as in subsequent
experiments, I expanded the gas till the pressure fell to twenty
atmospheres, and in some cases to one atmosphere, I could not detect
the slightest indication that liquefaction had taken place. The first
time that I compressed the gas I had, indeed, noticed that a small
quantity of a white substance separated out and remained at the bottom
of the tube when the pressure was released. Possibly this may have
been due to the presence of a small trace of impurity in the helium,
but it could not have constituted more than 1 per cent. of the total
volume of the gas.
In the second series of experiments I employed liquid air, boiling
under a pressure of 10 mm. of mercury. The helium was first confined
under a pressure of 140 atmospheres, and then allowed to expand till
the pressure fell to twenty atmospheres, or, in some cases, to one
atmosphere. The results of these experiments were also negative, the
gas remained perfectly clear during the expansion, and not the
slightest trace of liquid could be detected. The boiling point of
liquid air was taken, from my previous determination, to be -220° C.
(Comptes Rendus, 1885, p. 238). This number cannot, however, be taken
as a constant, as the liquid air, boiling under reduced pressure,
becomes gradually poorer in nitrogen. Further, the quantity of
nitrogen lost by the liquid air on partial evaporation varies not only
with the rate of boiling, but even according to the manner in which it
has been liquefied.
If air, under high pressure, be cooled first to the temperature of
boiling ethylene, and then to -150° C., it liquefies, and, on reducing
the pressure slowly, liquid air is obtained boiling under atmospheric
pressure. During the process a considerable quantity of the liquid air
evaporates, and the proportion of nitrogen to oxygen in the remaining
liquid is less than in air liquefied under high pressure. If the
liquid air obtained by this process be made to boil under a pressure
of 10 mm. of mercury, the proportion of nitrogen in the mixture
continues to decrease, but, on account of the large quantity of oxygen
present, the liquid does not solidify, although its temperature is
some six degrees below the freezing point of nitrogen. When, as in
some of my former experiments, the air was liquefied under normal
pressure by means of liquid oxygen boiling under a pressure of 10 mm.
of mercury, the ratio of nitrogen to the oxygen in the liquid air was
the same as in the gaseous air from which it had been produced. The
liquid air, obtained by direct condensation at normal pressure,
appeared to lose oxygen and nitrogen with about equal rapidity, and at
the end of the experiment a considerable quantity of liquid nitrogen
remained behind in the apparatus. On reducing the pressure to 10 mm.
of mercury the nitrogen solidified. Prof. Dewar has stated that liquid
air solidifies as such, the solid product containing a slightly
smaller percentage of nitrogen than is present in the atmosphere. My
experiments have proved this statement to be incorrect; liquid oxygen
does not solidify even when boiling under a pressure of 2 mm. of
mercury.
After carrying these experiments to a successful conclusion, I found
that it was yet necessary to prove that, on reducing the vapor
pressure of boiling oxygen, to a minimum, no corresponding fall of
temperature takes place. The vessel, e, was partially filled with
liquid oxygen, and, by means of a small siphon, a small quantity of
the liquid was allowed to flow into the tube, a. The inner vessel, a,
was then connected with the air pump and manometer, and the pressure
was reduced to 2 mm. of mercury. The oxygen remained liquid and quite
clear. In a second experiment the temperature of the liquid oxygen,
boiling under 2 mm. of mercury pressure, was measured by means of a
thermometer. The temperature indicated lay above -220° C., a
temperature easily arrived at by means of liquid air. I, therefore,
concluded that liquid air was a much more efficient cooling agent than
liquid oxygen, and that it would be quite unnecessary to make further
experiments on the liquefaction of helium.
In every single instance I have obtained negative results, and, as far
as my experiments go, helium remains a permanent gas, and apparently
much more difficult to liquefy than even hydrogen. The small quantity
of the gas at my disposal, and, indeed, the extreme rarity of the
minerals from which it is obtained, compelled me to carry out my
investigation on a very small scale. Using a larger apparatus, and
working at a much higher pressure, I could have submitted the gas to
greater expansion. Further, I should have been able to measure the
temperature of the gas at the moment of expansion by means of a
platinum thermometer, as I did when working with hydrogen; but to make
such experiments I should have required 10, if not 100, liters of the
gas. As I was unable to determine the temperatures to which I cooled
the gas, by any experimental means, I have been obliged to calculate
them from Laplace's and Poisson's formula for the change of
temperature in a gas during adiabatic expansion.