(41) 
The substances. 
From the theory follows that some mixtures, when being condensed 
at a certain temperature will show the phenomenon, indicated as 
retrograde condensation second type. Mr. KurNeN had not been 
able to observe this phenomenon; therefore it was desirable to select 
substances, in which at least theoretically, the phenomenon of r. ¢. 
II was to be found. 
As Mr. KveNeN states in Phys. Soc. (13) 10, 1895, this is the 
case with some mixtures, if the component that has the higher 
vapour-pressures, has also the higher critical temperature. We settled 
therefore on HCl and C,Hg¢, because each of these substances has 
also a critical pressure which may be easily attained. 
HCl was obtained by adding drops of concentrated sulphurous 
acid to pure HCl, to which some Feg5O, had been added. When 
the gas obtained in this way, was dried, it was very pure, as appeared 
from the slight increase of pressure (at 12° about 0,2 atm.), when 
condensed and from the agreement of the critical data with those 
found by ANSDELL. 
In order to obtain ethane, acetate of sodium was subjected to 
electrolysis; the gas which was developed, was condensed at a low 
temperature (about —50°), the vapourphasis was removed, and from 
the liquid phasis a quantity of gas had been collected. Though this 
method is the same as had furnished good results to Mr. Kuenen, 
I have not succeeded in making the gas as pure; this C2 Hg pre- 
sented in being condensed at 21°, an increase of pressure of 1,4 
atm. As however the values for the critical data as found by me 
(crit. temp. 31°,88; crit. pressure 48,94 atm.) did not differ much 
from those found by Mr. KurNEN for his ethane (crit. temp. 31,95 
à 32,2; crit. press. 48,64 à 48,91) and as moreover it is difficult 
to obtain perfectly pure ethane, as clearly appears from the obser- 
vations of others (Dewar, OLSZEWSKY and HAENLEN found resp. 
for crit. temp. 35°, 34° and 34°,5 and for crit. press. 45,2; 50,2 
and 50 atm.) I resolved to continue the observations with the ethane 
I had obtained. 
The Method. 
The compressibility of the two substances mentioned and also that 
of four mixtures (prepared in a mixing-apparatus made completely 
of glass), was compared with that of dry air, free from carbonic acid, 
at temperatures which were the same for the two a and for 
