295 
Physics. — “Zsotherms of diatomic gases and of their binary 
mixtures. X. Control measurements with the volumenometer of 
the compressibility of hydrogen at 20° C. By W. J. pr Haas. 
Communication N°. 127a from the Physical Laboratory at Leiden. 
(Communicated by Prof. H. KAMERLINGH ONNps). 
(Communicated in the meeting of April 26, 1912). 
X. The compressibility of hydrogen at 20° C. 
§ 1. Introduction. In Communication IX. “Control measurements 
with the volumenometer”, (Comm. N°. 121a, Proc. May 1911) a 
discussion based upon experimental data was given of the degree of 
accuracy attainable in determinations with the volumenometer. A 
determination of the compressibility of hydrogen at ordinary tem- 
perature has now given an additional desirable test of the accuracy 
with which the various experimental conditions in their mutual 
relationships have been fulfilled. 
The investigation is based upon the Leiden measurements of the 
compressibility of hydrogen at pressures up to 60 atm. The accurate 
piezometers (Comm. N°. 50) and the sectional open manometer (Comm. 
N°. 44) specially designed by Kameriincn Onnzs for that investigation 
rendered a very high accuracy attainable in those measurements. 
Considering this degree of accuracy, we may therefore take ScHALK- 
WIJk’s measurements with those apparatus at 20° C. to be quite 
accurate, and ascribe the small difference between his formula and 
that deduced from Amagear’s results to a lower degree of accuracy 
in one or other of AMAGAT’s measurements (perhaps in his determination 
of the normal volume, which can be done more accurately by 
KAMERLINGH OnNEs’s method). This conclusion is also supported by 
the fact that ScHALKWIJK’s formula is confirmed by the results obtained 
by KAMERLINGH Onnes and HYNDMAN (Comm. N°. 78). 
We may therefore write at 20°C. 
pva = 1.07258 + 0,000667 d4 + 0,00000099 d4? 
in which p is the pressure, v4 the volume in terms of the normal 
volume and d4 is the reciprocal of v4. On account of the small 
densities which occur in measurements made with the volumenometer 
(in which d4 is at the most 1.1) the d4* term may be neglected. 
The compressibility at 20°C. is then given by 
