299 
rienced in experiments at liquid hydrogen temperatures and at which 
my measurements had to be made. 
Determinations made by Cuappuis') and by Rayueien, in each case 
with apparatus designed to attain a higher degree of accuracy than 
that of the Leiden volumenometer, also afford a basis of comparison. 
CHAPPUIS measured compressibilities at O° C. between 1.4 and 1.8 
atmospheres. His results give B49 = 0,00058, from which, using the 
figure given by KAMERLINGH ONNEs and BRAAK ®) for the difference 
between Baricco and B4oo we get 
Bazoo = 0,00064 and O—-C = —0,00003. 
The values deduced from the two single observations distant by 
about half the pressure difference from each other, in which the 
errors are increased, differ by 0,0001. 
Finally, Lord RArreicH’s *) measurements were made with an appa- 
ratus specially designed to give an accurate comparison between pu, 
at half an atmosphere and its value at double that pressure. From 
them we get B4jo.7 — 0,00054 from which, using again the KAMERLINGH 
Onnes—Braak result just given, we obtain 
Ba200 = 0,00057 and O—C = —0,00010. 
So that comparison between the results now given with those 
yielded by these different researches shows a satisfactory agreement. 
In the proposed determination of 4,4 at hydrogen temperatures 
circumstances will be much more favourable than at ordinary tem- 
perature, for Bada will then be 15 to 20 times greater at the same 
pressure. We may regard the value obtained for 6,4 in this way at 
—252°C. as accurate to within 2°/,, and to within 10 °/, at —259° C. 
Physics. — “Jsotherms of diatomic gases and of their binary 
mixtures. XI. On determinations with the volumenometer of 
the compressibility of gases under small pressures and at low 
temperatures.” By W. J. pr Haas. Communication N°. 127° 
from the Physical Laboratory at Leiden. (Communicated by 
Prof. H. KAMERLINGH ONNESs). 
(Communicated in the meeting of May 25, 1912). 
§ 1. Criticism of the pressure equilibrium between the piezometer 
and the volumenometer. In the investigation of the compressibility 
of hydrogen vapour with which a subsequent paper by Prof. Kamur- 
1) P. Cuappuis, Nouvelles études sur le thermométre à gaz. 
2) Comm. no. 1005, These Proceedings Dec. '07. 
8) Lord RAYLEIGH, Proc. Roy. Soc, 73 (1904). Ztsch. phys. Chem. 52 (1905), 
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