300 
LINGH Onnes and myself will deal, the volumenometer described in 
the previous Communication was used to measure the quantity of 
hydrogen contained under different pressures in a reservoir — the 
piezometer reservoir — which was immersed in liquid hydrogen and 
connected with the volumenometer by a capillary and tap. The 
pressure of the gas in the piezometer reservoir was then given for 
each measurement by the pressure of the gas in the volumenometer 
in pressure equilibrium with it. It was shown in Communications 
N°. 121¢ (Proc. May 1911) and N°. 127% (These Proc. p. 295) that the 
accuracy with which the pressure, volume and temperature of the 
quantity of gas contained in the volumenometer could be determined 
was sufficient to allow of the evaluation of the virial coefficients B 
at low temperatures for hydrogen vapour from determinations of 
the compressibility of that vapour. More particular attention must 
now be bestowed upon the question of pressure equilibrium between 
the volumenometer and the piezometer. 
In the course of the above experiments it was repeatedly necessary 
to adjust the mercury in the volumenometer to one of the lower 
necks (for instance, m,, m,, or m,. Cf. Comm. N°: 117, Pl. LE Proc 
Febr. 1911). The quantity of gas contained in the volumenometer 
was in those cases always less (though not many times) than that in 
the piezometer of 110 ce. capacity and at a temperature of —252° 
to — 258° U, so that the gas in the piezometer was of a density 
from 12 to 20 times greater than that in the volumenometer. On each 
side of the capillary, therefore, which had to be long on account 
of the construction of tbe cryostat and narrow on account of the 
uncertainty of the volume correction to be applied for it, there are 
relatively large quantities of gas. On account of friction in the 
capillary, pressure equilibrium will be but slowly attained. A preli- 
minary experiment had shown the desirability of a means to decide 
from the measurements themselves when exactly this pressure equi- 
librium had been attained. In order therefore to obtain the necessary 
data for this, the behaviour of the pressure in the volumenometer 
was systematically observed during the final experiments upon the 
compressibility of hydrogen vapour at low temperatures (June 23 
and 24, July 8, 14, and 18, 1911) on each occasion on which the 
meniscus was adjusted to one of the necks m,, m,, m,, m, — this 
of course only after satisfying the experimental conditions to be fulfilled 
for equilibrium (regulation of cryostat and of volumenometer thermo- 
stat, constancy of room temperature). At intervals, as a rule every 
5 minutes, the difference between the levels of the mercury in the 
„manometer and in the volumenometer was read and corrected, from 
