951 
isothermal still smaller portions than before were taken by applying 
very small compressions. This process was continued until the critical 
pressure had been passed. After each compression some minutes 
were allowed to elapse, the substance being meanwhile thoroughly 
stirred with the electromagnetic stirrer, so that all traces of the heat 
developed by the adiabatic compression must have disappeared by 
the time it was ascertained whether condensation had taken place. 
After this the process was repeated in the reverse direction, gradually 
going down to lower pressures and ascertaining, what at each sub- 
sequent stage the effect was of a very small expansion, say of '/,, 
of an atmosphere. As before after each expansion some time was 
allowed to pass, until it could be assumed, that the gas had returned 
to the original temperature. If we proceed in this way along an 
isothermal immediately above 7% it is found that in the neighbourhood 
of the eritical condition ata small sudden, and therefore approximately 
adiabatic, expansion a liquid surface appears which disappears again 
with thorough stirring. If the isothermal lies below 7%, the meniscus 
produced by a very small expansion does not disappear on stirring. 
As the critical point we ultimately took the point at which on a 
small expansion the meniscus appeared half way up the tube and 
where conversely as the temperature re-established itself it disappeared 
again at the same level. By this procedure the critical temperature 
could easily be determined to about '/,,° C. On a future occasion 
this method will be published in greater detail with the addition of 
numerical results. 
It is worth mentioning that we did not succeed in observing the 
critical opalescence '), although in the experiments temperature and 
pressure were completely under control and the distinctness of 
observation of the phenomena in the piezometer-vessel left nothing 
to be desired as regards the permanent transparency of the walls. 
Our result, that the opalescence if existing must be very slight, 
derives its weight from the fact, that we looked for it with special 
attention, because on a former occasion one of us (H. K. O.) had 
been struck by its absence. 
§ 2. Preparation of the substances. The oxygen was prepared from 
1) These experiments were made in 1911. Looking through the literature of the 
subject, while writing this communication, we find the absence of the critical 
opalescence mentioned for the first time in a preliminary uote by E. Carposo, 
Eléments critiques et phases coéxisiantes des gaz permanents (II). Arch. d. Sc. 
phys. et nat. 15 Aout 1914 p. 137. 
CARDOSO adds a partial explanation based upon SMOLUCHOWSKI's theory in 
connection with the law of corresponding states. ' 
