1178 
The first six experimental days were used for studying one definite 
temperature, — 243°10, in order to obtain as much routine and 
experience as possible, in the management of the eryostat (so dif- 
ferent in principle from the ordinary liquid baths), as also in the 
adjustment of the equilibrium and the taking of the readings. 
Special attention was given to checking tbe equilibrium between 
vapour and liquid. When this is reached, not only the hydrogen 
meniscus in the dilatometer but also the mercury in the tube of 
the compression cylinder remains at absolutely constant level, some- 
times after considerable intervals of waiting. Both the liquid density 
and that of the vapour were measured three times; moreover a 
number of measurements of the vapour pressure were made under 
various conditions (meniscus in appendix, bulb or tube). 
In the discussion of our results it will appear that in a special 
range of temperatures certain circumstances prevail which affect 
the measurements of the vapour density and require to be more 
fully investigated. 
For the reduction of the experiments the pressure in the dilato- 
meter has to be accurately known and for this purpose, as men- 
tioned, during the measurements a great number of pressures were 
determined, referring therefore to a meniscus sometimes in the 
appendix and sometimes in the tube; moreover a number of readings 
were taken with the hydrogen meniscus in the bulb of the dilato- 
meter, in which case a proper equilibrium between liquid and 
vapour can be better counted on. The measurements with the 
meniscus in the appendix and the tube, i.e. with little or much 
liquid, also serve to form an opinion as to possible deviations of 
the temperature low down and high up in the eryostat, since the 
absolute purity of the bydrogen could be assumed as certain. These 
pressure measurements we would rather not call ‘‘vapour pressure 
determinations’, since for determinations of that kind, when properly 
conducted, stirring is an absolute necessity. We propose shortly in 
the same cryostat to measure vapour pressures of hydrogen in a 
piezometer with electro-magnetic stirrer. 
The result of our pressure measurements was as follows: the 
readings in the bulb, when plotted, lie on a smooth curve, which 
as it seems to us cannot deviate appreciably from the true vapour 
pressure curve; the readings in the appendix are a little less regular, 
but they do not show any systematic deviations from the previous 
ones; the observations in the tube, on the other hand, at correspond- 
ing temperatures consistenly give slightly higher values. 
From this we have to infer, that the temperature must have been 
