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still lower value of 20 atmospheres. Just as has already been 
remarked, now that the conditions for the liquefaction of helium are 
known, the repetition of the experiment with the same apparatus 
and the keeping of 60 c.c. of liquid helium in it for several hours 
has become a work that offers no serious difficulty, although it 
necessitates much exact and careful preparation. Now we can attempt 
a further step and pour off the liquid helium into a cryostat in 
which various measuring apparatus can be immersed. 
In the new experiment I have been successful in still further 
lowering the temperature of the liquid helium. It was definitely 
known that in the first experiment the vapour pressure was lowered 
to 1 c.m., and that the helium was still liquid at that pressure; 
now I have successfully lowered the pressure to 2.2 m.m. Even at 
that pressure the helium remains a light mobile liquid. When lowered 
to this low temperature it shows considerable shrinkage, and, in 
correspondence with the greater density clearly shows a rise against 
the wall of the glass 1 ). The temperature which is reached by thus 
strongly cooling the liquid helium may be estimated to lie below 
2°.5 K., perhaps at 2° K. 
I have tried in the new experiment to measure these extremely 
low temperatures with the gas thermometer. It must, however, be 
remarked that when the temperature sinks so low, the thermometer 
may be used only so long as the pressure of the helium in the 
thermometer remains lower than the vapour pressure of the liquid 
helium, and when the latter is 2 m.m. the pressure in the thermo¬ 
meter at that temperature must be still lower, e.g. 1 m.m. The 
pressure at which the thermometer is filled at ordinary temperature 
is therefore taken not higher than 100 m.m. The difficulties of 
determinations with this very special gasthermometer in the immediate 
vicinity of the helium apparatus have not yet been quite overcome, 
and hence further information concerning the temperature cannot 
yet be given. 
We can, however, already announce a first experimental deter¬ 
mination of the critical pressure: it gave 2.75 atmospheres. While 
an estimate formed from the observations with liquid helium showed 
that the estimated value on which the liquefaction experiment was 
based was much -too high, it now seems to show a deviation in the 
other direction, viz. it gives too low a value for the critical pressure. 
~i) In the former communication it was said that the meniscus at the boiling 
point stands out against the wall as sharp as a knife. And, indeed, this reproduces 
in the best way the first impression which the meniscus makes. Proper illumination, 
however, even under these circumstances, succeeded in making visible an extremely 
slight rise of the liquid against the wall of the vessel. 
