1090 
It is therefore necessary in the experiments to lay down the 
general rule when partial condensation has once set in not again 
to increase the volume’) and, if the observations bave to be repeated 
or a new series has to be started at a different temperature, it is 
necessary — in order to obtain complete homogeneous mixing — 
several times in succession to lower the pressure in the apparatus 
to normal and recompress to the high pressure. Otherwise in conse- 
quence of the very slow diffusion in tbe capillary connecting tubes 
with the supply-tube of the piezometer a mixture of higher boiling 
point remains behind in the small experimental tube, so that the 
succeeding observations are bound to be incorrect and amongst 
others the condensation will set in too soon. The importance of all 
this was not sufficiently realized in our experiments to begin with, 
so that a great number of our earlier observations had to be rejected 
later on. 
The electro-magnetic stirring also involves greater difficulties at 
low temperature than otherwise, in consequence of the great width 
of the cryostat which brings with it a corresponding size of the 
electromagnet surrounding the cryostat. The observation-tube, capillary 
and stirrer were the same as used by CROMMELIN®) in his investiga- 
tion of argon. In this tube the difficulty just referred to is got over 
by the small piece of iron on which the electro-magnet acts being 
placed in an enlargement above the glass capillary of the experi- 
mental tube; to this piece of iron by means of a long glass thread 
running down through the capillary the stirrer inside the observation- 
tube is attached. By this means the stirrer can be moved up and 
down by means of a small hand-electromagnet. 
This arrangement involved the diffieulty for our purpose, that the 
“glasscapillary referred to has to be comparatively wide in order to 
leave room for the glassthread and that this is connected with 
greater danger of mutual diffusion between the different mixtures 
inside and outside the observation-tube and that greater uncertainties 
arise in the determinations of the densities. It also happened more 
than once that the stirrer got stuck in consequence of the flexibility 
of the glassthread and perhaps of microscopic deposits of solid 
substances. It goes without saying that the air had been freed as 
well as possible of water-vapour and carbon-dioxide: possibly the 
sticking of the glassthread indicates that traces of these substances 
had remained after all; in any case the disturbance usually became 
1) In the neighbourhood of the critical point, where the two phases have nearly 
the same composition, small expansions cannot be objected to. 
*) G. A. Crommeuin. Comm. 115 § 2. See also Comm. 83. Plate IV. 
