SEPTEMBER I1, 1902] 
NATURE 
473 
and the experimenter to be able to reach within a few degrees 
of the zero, it is by no means certain that he would find the 
near approach of the death of matter sometimes pictured. Any 
forecast of the phenomena that would be seen must be based on 
the assumption that there is continuity between the processes 
studied at attainable temperatures and those which take place 
at still lower ones. Is such an assumption justified? It is true 
that many changes in the properties of substances have 
been found to vary steadily with the degree of cold to 
which they are exposed. But it would be rash to take 
for granted that the changes which have been traced in 
explored regions continue to the same extent and in the 
same direction in those which are as yet unexplored. Of 
such a breakdown low-temperature research has already 
yielded a direct proof at least in one case. A series of experi- 
ments with pure metals showed that their electrical resistance 
gradually decreases as they are cooled to lower and lower tem- 
peratures, in such ratio that it appeared probable that at the 
zero of absolute temperature they would have no resistance at 
all and would become perfect conductors of electricity. This 
was the inference that seemed justifiable by observations taken 
at depths of cold which can be obtained by means of liquid air 
and less powerful refrigerants. But with the advent of the more 
powerful refrigerant liquid hydrogen it became necessary to re- 
vise that conclusion. A discrepancy was first observed when a 
platinum resistance thermometer was used to ascertain the tem- 
perature of that liquid boiling under atmospheric and reduced 
pressure. All known liquids, when forced to evaporate quickly 
by being placed in the exhausted receiver of an air-pump, 
undergo a reduction in temperature, but when hydrogen was 
treated in this way it appeared to be an exception. The resist- 
ance thermometer showed no reduction as was expected, and 
it became a question whether it was the hydrogen or the 
thermometer that was behaving abnormally. Ultimately, 
by the adoption of other thermometrical appliances, the 
temperature of the hydrogen was proved to be lowered 
by exhaustion as theory indicated. Hence it was the 
platinum thermometer which had broken down; in other 
words. the electrical resistance of the metal employed in its con- 
struction was not, at temperatures about mzzvus 250° C., decreased 
by cold in the same proportion as at temperatures about mvs 
200°. This being the case, there is no longer any reason to 
suppose that at the absolute zero platinum would become a 
perfect conductor of electricity; and in view of the similarity 
between the behaviour of platinum and that of other pure 
metals in respect of temperature and conductivity, the presump- 
tion is that the same is true of them also. At any rate, the 
knowledge that in the case of at least one property of matter 
we have succeeded in attaining a depth of cold sufficient to 
bring about unexpected change in the law expressing the 
variation of that property with temperature, is sufficient to show 
the necessity for extreme caution in extending our inferences 
regarding the properties of matter near the zero of temperature. 
Lord Kelvin evidently anticipates the possibility of more 
remarkable electrical properties being met with in the metals 
near the zero. A theoretical investigation on the relation of 
““electrions”’ and atoms has led him to suggest a hypothetical 
metal having the following remarkable properties: below 1 degree 
absolute it is a perfect insulator of electricity, at 2 degrees it 
shows. noticeable conductivity, and at 6 degrees it possesses 
high conductivity. It may safely be predicted that liquid 
hydrogen will be the means by which many obscure problems 
of physics and chemistry will ultimately be solved, so that the 
liquefaction of the last of the old permanent gases is as pregnant 
now with future consequences of great scientific moment as | 
was the liquefaction of chlorine in the early years of the last 
century. ; 
The next step towards the absolute zero is to find another 
gas more volatile than hydrogen, and that we possess in the gas 
occurring in cleveite, identified by Ramsay as helium, a gas 
which is widely distributed, like hydrogen, in the sun, stars and 
nebule. A specimen of this gas was subjected by Olszewski to 
liquid air temperatures, combined with compression and sub- 
sequent expansion, following the Cailletet method, and resulted 
in his being unable to discover any appearance of liquefaction, 
even in the form of mist. His experiments led him to infer 
that the boiling-point of the substance is probably below 9 
degrees absolute. After Lord Rayleigh had found a new source 
of helium in the gases which are derived from the Bath springs, 
and liquid hydrogen became available as a cooling agent, a 
NO. 1715, VOL. 66] 
| would be about one-fourth that of liquid hydrogen. 
specimen of helium cooled in liquid hydrogen showed the forma- 
tion of fluid, but this turned out to be owing to the presence of 
an unknown admixture of other gases. As a matter of fact, a 
year before the date of this experiment I had recorded indica- 
tions of the presence of unknown gases in the spectrum of 
helium derived from this source. When subsequently such con- 
densable constituents were removed, the purified helium showed 
no signs of liquefaction, even when compressed to 80 atmo- 
spheres, while the tube containing it was surrounded with solid 
hydrogen. Further, on suddenly expanding, no instantaneous 
mist appeared. Thus helium was definitely proved to be a 
much more volatile substance than hydrogen in either the liquid 
or solid condition. The inference to be drawn from the adia- 
batic expansion effected under the circumstances is that helium 
must have touched a temperature of from 9 to 10 degrees for a 
short time without showing any signs of liquefaction, and con- 
sequently that the critical pointmust be still lower. . This would 
force us to anticipate that the boiling-point of the liquid will 
be about 5 degrees absolute, or liquid helium will be four times 
more volatile than liquid hydrogen, just as liquid hydrogen is 
four times more volatile than liquid air. Although the lique- 
faction of the gas is a problem for the future, this does not pre- 
vent us from safely anticipating some of the properties of the 
fluid body. It would be twice as dense as liquid hydrogen, 
witha critical pressure of only 4 or 5 atmospheres. The liquid 
would possess a very feeble surface-tension, and its compressi- 
bility and expansibility would be about four times that of liquid 
hydrogen, while the heat required to vaporise the molecule 
Heating 
the liquid 1 degree above its boiling-point would raise the pres- 
sure by 1} atmospheres, which is more than four times the incre- 
ment for liquid hydrogen. The liquid would be only seventeen 
times denser than its vapour, whereas liquid hydrogen is sixty- 
five times denser than the gas it gives off. Only some 3 or 4 
degrees would separate the critical temperature from the boiling- 
point and the melting-point, whereas in liquid hydrogen the 
separation is respectively 10 and 15 degrees. As the liquid 
reiractivities for oxygen, nitrogen and hydrogen are closely 
proportional to the gaseous values, and as Lord Rayleigh 
has shown that helium has only one-fourth the refrac- 
tivity of hydrogen, although it is twice as dense, we must infer 
that the refractivity of liquid helium would also be about one- 
fourth that of liquid hydrogen. Now hydrogen has the smallest 
refractivity of any known liquid, and yet liquid helium will have 
only about one-fourth of this value—comparable, in fact, with 
liquid hydrogen just below its critical point. This means that 
the liquid will be quite exceptional in its optical properties, and 
very difficult to see. This may be the explanation of why no 
mist has been seen on its adiabatic expansion from the lowest 
temperatures. Taking all these remarkable properties of the 
liquid into consideration, one is afraid to predict that we are at 
present able to cope with the difficulties involved in its pro- 
duction and collection. Provided the critical point is, however, 
not below 8 degrees absolute, then from the knowledge of the 
conditions that are successful in producing a change of state in 
hydrogen through the use of liquid air, we may safely predict 
that helium can be liquefied by following similar methods. If, 
however, the critical point is as low as 6 degrees absolute, then 
it would be almost hopeless to anticipate success by adopting the 
process that works so well with hydrogen. The present an- 
ticipation is that the gas will succumb after being subjected to 
this process, only, instead of liquid air under exhaustion being 
| used as the primary cooling agent, liquid hydrogen evaporating 
under sirnilar circumstances must be employed. In this case, 
the resulting liquid would require to be collected in a vacuum 
vessel the outer walls of which are immersed in liquid hydrogen. 
The practical difficulties and the cost of the operation will be 
very great ; but, on the other hand, the descent to a temperature 
within 5 degrees of the zero would open out new vistas of 
scientific inquiry, which would add immensely to our knowledge 
of the properties of matter. To command in our laboratories a 
temperature which would be equivalent to that which a comet 
might reach at an infinite distance from the sun would indeed be 
a great triumph for science. If the present Royal Institution 
attack on helium should fail, then we must ultimately succeed by 
adopting a process based on the mechanical production of cold 
through the performance of external work. When a turbine can 
be worked by compressed helium, the whole of the mechanism 
and circuits being kept surrounded with liquid hydrogen, then 
we need hardly doubt that the liquefaction will be effected. In 
