1059 
in the neighbourhood of 45° K. sufticiently constant to make reliable 
measurements of the critical temperature. Since in the hydrogen- 
vapour cryostat’) we have obtained an apparatus by which it is 
possible to govern the temperatures in the range between the melting 
point of oxygen and the boiling point of hydrogen, this difficulty 
has disappeared and we could now attempt the long-desired deter- 
mination of the critical condition of neon with every chance of 
success. The reason why our results must still be looked upon as 
preliminary ones is not due to a want of constancy in the tempe- 
rature of observation or to other defects in the method adopted, 
but to the fact that the neon on which we have experimented was 
not absolutely pure. Small as the admixtures were, their influence 
showed itself very clearly in a gradual increase of pressure during 
condensation.®) The difference between initial and final pressures in 
the vapour-pressure measurements immediately below the critical 
point amounted to .2 of an atmosphere.*) In the determination of the 
vapour-pressure of hydrogen in the immediate vicinity of the critical 
point which was carried out with the same apparatus (comp. the 
next Communication N°. 151c) where, on account of the purification 
of hydrogen by distillation, the purity of the experimental gas was 
completely guaranteed, differences of that kind did not occur. 
If the pressure rises during condensation, the determination of 
the critical data becomes uncertain.*) Our result for the critical 
temperature may therefore differ from the true value by a few. 
tenths of a degree; a similar uncertainty applies to the critical 
pressure. The circumstance, that observations on the critical tempe- 
rature of neon are so far completely lacking and that it will take 
some time before the more accurate measurements’) aimed at will 
be completed, justify sufficiently the publication of our present 
results. 
1) Comp. the preceding Comm. N°. 151a. 
2) Previous investigations, in the first place by Kuenen (Comm. N°. 8 Meeting of 
Oct. 1893 and Comm. N°. 11 Meeting of May and June 1894), have sufficiently shown 
the great influence which even very small admixtures produce on the phenomena 
in the critical region. 
3) In the table the pressure at the beginning of condensation is given as the 
vapour pressure. 
4) Instead of the critical temperature of the pure substance the experiment gives 
the plaitpoint temperature of the mixture. 
5) In these determinations we hope to be able to utilize a visual method by a 
modification of the hydrogen vapour eryostat (comp. Comm, N°. 151a) which will 
allow us to follow the phenomena inside the experimental chamber by eye, 
