SEPTEMBER II, 1902] 
pressure and temperature until their critical points coincide. 
But on this plan the surfaces do not quite coincide, because the 
points where the three variables are respectively nothing are 
not corresponding points. Mme. Meyer’s plan is to bring all 
the critical points first into coincidence, and then to compress 
or extend all the representative surfaces parallel to the three 
axes of volume, pressure and temperature until the surfaces 
coincide. In this way, taking twenty-nine different substances, 
she completely verifies from experiment van der Waals’ law of 
correspondence. The theory of van der Waals has been one of 
the greatest importance in directing experimental investigation 
and in attacking the difficult problems of the liquefaction of the 
most permanent gases. One of its greatest triumphs has been 
the proof that the critical constants and the boiling-point of 
hydrogen theoretically deduced by Wroblewski from a study 
of the isothermals of the gas taken far above the tempera- 
ture of liquefaction are remarkably near the experimental 
values. We may safely infer, therefore, that if hereafter a gas 
be discovered in small. quantity even four times more volatile 
than liquid hydrogen, yet by a study of its isothermals at low 
temperature we shall succeed in finding its most important liquid 
constants, although the isolation of the real liquid may for the 
time be impossible. It is perhaps not too much to say that, as 
a prolific source of knowledge in the department dealing with 
the continuity of state in matter, it would be necessary to go 
back to Carnot’s cycle to find a proposition of greater importance 
than the theory of van der Waals and his development of the 
law of corresponding states. 
It will be apparent from what has just been said that, thanks 
to the labours of Andrews, van der Waals, and others, theory 
had again far outrun experiment. We could calculate the con- 
stants and predict some of the simple physical characteristics of 
liquid oxygen, hydrogen or nitrogen with a high degree of con- 
€idence long before any one of the three had been obtained in 
the static liquid condition permitting of the experimental veri- 
fication of the theory. This was the more tantalising, because, 
with whatever confidence the chemist may anticipate the sub- 
stantial corroboration of his theory, he also anticipates with 
almost equal conviction that, as he approaches more and more 
nearly to the zero of absolute temperature, he will encounter 
phenomena compelling modification, revision and refinement of 
formulas which fairly covered the facts previously known. Just 
as nearly seventy years ago chemists were waiting for some 
means of getting a temperature of 100 degrees below melting 
ice, so ten years. ago they were casting about for the means of 
going 100 degrees lower still. The difficulty, it need hardly be 
said, increases in a geometrical rather than in an arithmetical 
ratio. Its magnitude may be estimated from the fact that to 
produce liquid air in the atmosphere of an ordinary laboratory 
is a feat analogous to the production of liquid water starting 
from steam at a white heat, and working with all the imple- 
ments and surroundings at the same high temperature. The 
problem was not so much how to produce intense cold as how 
to save it when produced from being immediately levelled up 
by the relatively superheated surroundings. Ordinary non-con- 
ducting packings were inadmissible because they are both cum- 
brous and opaque, while in working near the limits of our 
resources it is essential that the product should be visible and 
readily handled. It was while puzzling over this mechanical 
and manipulative difficulty in 1892 that it occurred to me that 
the principle of an arrangement used nearly twenty years before 
in some calorimetric experiments, which was based upon the 
work of Dulong and Petit on radiation, might be employed 
with advantage as well to protect old substances from heat as 
hot ones from rapid cooling. I therefore tried the effect of 
keeping liquefied gases in vessels having a double wall, 
the annular space between being very highly exhausted. 
Experiments showed that liquid air evaporated at only 
one-fifth of the rate prevailing when it was placed in 
a similar unexhausted vessel, owing to the convective transfer- 
ence of heat by the gas particles being enormously reduced by 
the high vacuum. But, in addition, these vessels lend them- 
selves to an arrangement by which radiant heat can also be cut 
off. It was found that,when the inner walls were coated with 
a bright deposit of silver the influx of heat was diminished to 
one-sixth the amount entering without the metallic coating. 
The total effect of the high vacuum and the silvering is to reduce 
the ingoing heat to about 3 percent. The etficiency of such 
vessels depends upon getting as high a vacuum as possible, and 
cold is one of the best means of effecting the desired exhaustion. 
NO. 1715, VOL. 66] 
NATURE 
470 
All that is necessary is to fill completely the space that has to 
be exhausted with an easily condensable vapour, and then to 
freeze it out in a receptacle attached to the primary vessel that 
can be sealed off. The advantage of this method is that no 
air-pump is required, and that theoretically there is no limit to 
the degree of exhaustion that can be obtained. The action is 
rapid, provided liquid air is the cooling agent, and vapours like 
mercury, water or benzol are employed. It is obvious that 
when we have to deal with such an exceptionally volatile liquid 
as hydrogen, the vapour filling may be omitted because air 
itself is now an easily condensable vapour. In other words, 
liquid hydrogen, collected in such vessels with the annular space 
full of air, immediately solidifies the air and thereby surrounds 
itself with a high vacuum. In the same way, when it shall be 
possible to collect a liquid boiling on the absolute scale at about 
5 degrees, as compared with the 20 degrees of hydrogen, then 
you might have the annular space filled with the latter gas to 
begin with, and yet get directly a very high vacuum, owing to 
the solidification of the hydrogen. Many combinations of 
vacuum vessels can be arranged, and the lower the temperature 
at which we have to operate the more useful they become. 
Vessels of this kind are now in general use, and in them liquid 
air has crossed the American continent. Of the various forms, 
that variety is of special importance which has a spiral tube 
joining the bottom part of the walls, so that any liquid gas may 
be drawn off from the interior of such a vessel. In the working 
of regenerative coils such a device becomes all-important, and 
such special vessels cannot be dispensed with for the liquefaction 
of hydrogen. 
In the early experiments of Pictet and Cailletet, cooling 
was produced by the sudden expansion of the highly com- 
pressed gas preferably at a low temperature, the former 
using a jet that lasted for some time, the latter an instantaneous 
adiabatic expansion in a strong glass tube. Neither process 
was practicable as a mode of producing liquid gases, but 
both gave valuable indications of partial change into the 
liquid state by the production of a temporary mist. Linde, how- 
ever, saw that the continuous use of a jet of highly compressed 
gas, combined with regenerative cooling, must lead to liquefac- 
tion on account of what is called the Kelvin-Joule effect ; and he 
succeeded in making a machine, based on this principle, capable 
of producing liquid air for industrial purposes. These experi- 
mentershad proved that, owing to molecularattraction, compressed 
gases passing through a porous plug or small aperture were 
lowered in temperature by an amount depending on the dif- 
ference of pressure, and inversely as the square of the absolute 
temperature. This means that for a steady difference of pres- 
sure the cooling is greater the lower the temperature. . The only 
gas that did not show cooling under such conditions was 
hydrogen. Instead of being cooled it became actually hotter. 
The reason for this apparent anomaly in the Kelvin-Joule effect 
is that every gas has a thermometric point of inversion above 
which it is heated and below whichit is cooled. This inversion 
point, according to van der Waals, is six and three-quarter times 
the critical point. The efficiency of the Linde process depend 
on working with highly compressed gas well below the inversion 
temperature, and in this respect this point may be said to take 
the place of the critical one, when in the ordinary way direct 
liquefaction is being effected by the use of specific liquid cooling 
agents. The success of both processes depends upon working 
within a certain temperature range, only the Linde method 
gives us a much wider range of temperature within which lique- 
faction can be effected. This is not the case if, instead of 
depending on getting cooling by the internal work done by the 
attraction of the gas molecules, we force the compressed gas to 
do external work as in the well-known air machines of Kirk 
and Coleman, Both these inventors have pointed out that there 
is no limit of temperature, short of liquefaction of the gas in 
use in the circuit, that such machines are not capable of giving. 
While it is theoretically clear that such machines ought to be 
capable of maintaining the lowest. temperatures, and that with 
the least expenditure of power, it is a very different matter to 
overcome the practical difficulties of working such machines 
under the conditions. Coleman kept a machine delivering air 
at minus 83 degrees for hours, but he did not carry his experi- 
ments any further. Recently Monsieur Claude, of Paris, has, 
however, succeeded in working a machine of this type so 
efficiently that he has managed to produce one litre of liquid air 
per horse power expendec per hour in the running of the engine. 
This output is twice as good as that given by the Linde machine, 
