Physics. — “The rectilinear diameter of hydrogen”. By E. Marnias, 
. C. A. CROMMELIN and H. KAMERLINGH Onnes. Communication 
N°. 1546 from the Physical Laboratory at Leiden. (Communi- 
cated by Prof. H. KAMERLINGH ONNms). 
(Communicated at the meeting of January 29, 1921). 
$ 1. Zntroduction. This communication forms the continuation of 
a series of contributions to the knowledge of the density-curves for 
liquid and saturated vapour and of the diameters, in the case of 
substances of low critical temperature and simple molecular struc- 
ture. The investigation by means of the dilatometer-method was 
started in the Leiden physical laboratory a considerable time ago 
and has dealt successively with oxygen *), argon’) and nitrogen *). 
Great importance was attached to the extension of the measurements 
to hydrogen for the knowledge of its equation of state, especially 
in connection with previous determinations of the liquid densities 
between boiling point and melting point‘), of the critical point °) 
and the various computations of the critical density °) °). 
The research could not be carried out, however, until the experi- 
mental difficulties had been overcome as regards the construction 
of a transparent bath of constant and uniform temperature between 
the critical point and the boiling point, i.e. between about — 240° 
C. and — 253° C. 
§ 2. The apparatus for the compression of hydrogen, the mea- 
surement of the liquid and vapour volumes, the determination of 
the vapour pressures and that of the volume of the gas under 
1) E. MATHIAS and H. KAMERLINGH ONNES, these Proceedings 13, p. 939, 
Leiden Comm. NO. 117. 
2) E. Matuias, H. KAMERLINGH ONNEs and C. A. CROMMELIN, these Pro- 
ceedings 15, p. 667, Leiden Comm. N°. 131a. 
5) E. Matuias, H. KAMERLINGH ONNES and C. A. CROMMELIN, these Pro- 
ceedings 17, p. 953, Leiden Comm. N°. 145c. 
4) H. KAMERLINGH ONNES and C. A. CROMMELIN, these Proceedings, 16, 
p. 245, Leiden Comm. N°. 137a. 
5) H. KAMERLINGH ONNES, C. A. CROMMELIN and P.G. Catu, these Procee- 
dings 20, p. 178, Leiden Comm. N°. 15lc. 
6) J. J. VAN LAAR, Chem. Weekblad 16 (1919), p. 1557. 
