408 
ments would have been far greater for us bad we not frequently 
been obliged to aim at obtaining the same reading of the resistance 
thermometer instead of at the maintenance of a definite temperature. 
The investigation was carried out at three temperatures, approxi- 
mately — 252°.6 C., — 255°.5 C. and — 257°.3 C. A lower tempera- 
ture than — 257°.3 was not desirable as the smallest pressure to be 
measured at this temperature had already sunk as low as 5 em. and 
further progress in this direction would have necessitated another 
apparatus. 
For each isotherm the densities were so chosen that the ratio of 
the extreme densities was about two to one in each case. By this 
means it was brought about that in the solution of Ly from the 
two equations 
pra a + BA da, + Ca Cay 
(0A Aa Ba da, + Cada,” 
the coefficient of B4 was approximately 1. In this solution C4 was 
taken as a correction term from the equation of state VII. H,.3 
(formula (16) Comm. N°. 1094). 
Finally, for practical reasons, it was necessary to remain as far 
as possible away from the region of condensation, as a sudden fluc- 
tuation of the temperature could quite well occasion condensation to 
take place upon the walls of the piezometer, and particularly of the 
capillary, and this, in view of the excessively slow liberation of 
liquid and vapour from the glass, would render the measurements 
valueless. 
The measurements were made with a piezometer immersed in a 
bath of liquid hydrogen and connected through a capillary with the 
volumenometer studied in detail in Comm. N°. 127a (See Fig. 1, 
p. 407). The piezometer was first evacuated and a quantity of gas 
measured in the volumenometer; the valve between the two was 
then opened. Pressure equilibrium was then allowed to establish 
itself at the desired value, and then the pressure and the quantity 
of gas remaining in the volumenometer were determined. 
§ 2. Arrangement of experiments. Aurilary apparatus. 
The experimental arrangements are shown in fig. 1, p. 407"). One 
portion of the apparatus had already been utilised in the investiga- 
tion of the diameter for oxygen, and is described in Comm. N°. 117, 
Proc. Febr. 1911. The left hand part of fig. 1, p. 407 is an impro- 
) In the drawing some details of no importance are incorrectly represented, viz: 
the ice ought to cover the bottle R, the air-trap Zl is in reality much smaller, 
the safety-tube Y3 is of course not wholly filled with mercury. 
