1095 
stance that the condensation-loop in the p—7’ figure turned out to 
be comparatively narrow. The plait-point was found at 
t — — 140°.73 p = 87.25 alm. 
the critical point of contact at 
t= — 140°.638 # p = 37.17 atm. 
The special kind of condensation which is characteristic of mixtures 
is thus confined to a range of .1°, and it is therefore necessary to 
make the compression proceed extremely slowly, if the observation 
is to succeed. High demands as regards constancy during a long 
period are thus made on the temperature and the success of our 
endeavours was no doubt due to the excellent appliances and arran- 
gements which are available for this purpose in the eryogenic laboratory. 
In order not to extend this paper unduly we shall not describe 
our observations in all detail. An exception may be made for a 
phenomenon which was observed at — 140°.64 and a pressure of 
37.26 atmospheres and which shows very clearly the extreme sensi- 
tiveness of the substance in the critical region to changes of tempe- 
rature and pressure. At the above pressure the surface between liquid 
and vapour was just no longer visible; by lifting the stirrer 
in other corresponding states in this temperature-range — a mist 
was produced which, however, in this instance did not simply dis- 
appear, but automatically disappeared and reappeared again a few 
times in succession with a period of about one second. Evidently 
by the upward motion of the stirrer, the air under it becomes slightly 
expanded and thus cooled, while at the same time some gas is driven 
into the higher, warmer part of the capillary. The increase of pressure 
produced thereby assists in driving the gas back into the observation- 
tube, by which motion the mist becomes dissolved. The conditions 
must have been such this time, that the phenomenon repeated itself 
a few times in succession. The slow periodic time makes it probable, 
that a movement up and down of the mereury-surface in the wide 
compression-tube was at the bottom of the phenomenon. *) 
It is worth mentioning, that more than once a blue opalescence 
as 
1) A vibration of the air in the small tube under the influence of its own elas- 
ticity would take place much faster and is, moreover, improbable, because the air 
is cooled during the compression and warmed during the expansion, whereas the 
condition for a vibration being sustained by supply and withdrawal of heat is 
exactly opposite. The closed reservoir at the end of the tube should be the warm 
part of it, as in the case of the well-known singing which sometimes takes place 
in blowing a bulb at the end of a tube. 
