(176) 
the critical pressure was read on the manometer tube attached to 
the experimental tube. 
For the triple-point pressure was found 35 c.m., and for the critical 
pressure 29 atmospheres. 
A bath of liquid neon would therefore prove of great advantage 
in investigations in the neighbourhood of the critical point of hydrogen. 
I hope to be in a position to make use of such a bath through the 
further kindness of Mr. Claude, to whom I now express my warmest 
gratitude for the help already given. 
§ 2. Critical pressure of helium. Lowest temperature reached. 
In the first liquefaction of helium I made use. of a very provi¬ 
sional estimation of the critical pressure giving a value of 7 atm. 
The observation of the vapour-pressure and the density of liquid 
helium showed that the critical pressure must be much lower and 
must lie rather between 2 and 3 atmospheres. 
If this value had been taken, then a lower value would also have 
been chosen for the pressure under which the gas to be liquefied 
was pumped through the regenerator spiral. That there was, in fact, 
occasion for this was shown by the circumstance that in the actual 
experiment the expansion pressure was lowered with successful result. 
Indeed, had account not been taken of a warming effect that was 
observed and of the knowledge that the Joule-Kelvin process can 
give a warming if the pressure is too high even when the tempera¬ 
ture is suitable for the production of liquid the experiment might 
perhaps have been unsuccessful; and the failure might have been 
attributed to a very special deviation of helium from the law of 
corresponding states, and hence to the expansion pressure not being 
sufficiently high, or to an error in the estimation of the critical 
temperature. 
When, therefore, a further liquefaction of helium was proceeded 
with *) (which had been delayed by various circumstances for a 
considerable time) the pressure in the regenerator spiral was from 
the beginning brought to a value of 50 atmospheres. With this the 
liquefaction proceeded as well as could be desired. To keep the 
circulation in action once the liquid was formed, it seemed advan¬ 
tageous to bring the pressure in the regenerator-spiral even to the 
0 In the first repetition of the experiment we did not succeed in preventing the 
formation of a slight deposit on the hydrogen glass. Had this been the ease ^ 
the same extent in the first experiment, then there had been every pro a ^ 
that, on account of the first observation of the liquid helium being muc mor^ 
difficult, the experiment bad been abandoned as unsuccessful, while as a ma e 
of fact, the apparatus contained liquid helium. 
