Physics. — ''On the measurement of very low tempeiritures. XXIX. 



Vapour- pressures of o.rygen and nitrogen for obtaining Jixed 



points on the temperature-scale beloio 0° C." Bj P. G. Cath. 



Communication N°. Ibid from the Physical Laboratory at 



Leiden. (Communicated by Prof. H. Kamerlingh Onnes). 



(Communicated in the meeting of Oct. 26, 1918). i) 



^ 1. Introduction. The measurements of Comm. N"^- i52ö and b 

 (1917) conducted in conjunction witli Prof. Kamkrlingh Onnes (these 

 Proc. XX pp. 991, 1155, 1160) and those of Comm. NM52c carried 

 out with Prof. Kamerlingh Onnes and J. M. Burgers (these Proc. XX 

 p. 1663) offered the welcome opportunity of making some determi- 

 nations of vapour-pressures of oxygen and nitrogen using the apparatus 

 and measuring instruments which had been fitted up and used for 

 obtaining and measuring constant temperatures. Kamerlingh Onnes and 

 BuAAK (1908) (Comm. NM07rï, these Proc. XI (1) p. 333) had added 

 to the available fixed j)oints on the temperature-scale the boiling point 

 of oxygen and a couple of lower points, determined under sufficient 

 guarantees as to accuracy. The determinations mentioned above were 

 intended to supplement the latter and might serve to obtain an 

 independent calibration of the oxygen-*) and nitrogen vapour-pressure 

 thermometers, founded directly *) on the temperature-scale of Kamerlingh 

 Onnes and his collaborators. 



Prof. Kamerlingh Onnes was good enough to invite me to avail 



1) This paper was originally presented in the meeting of June 1917; in being 

 prepared for the press it underwent some slight modifications on points of secon- 

 dary importance. 



2) The oxygen vapour-pressure thermometer is often used in the form given to 

 it by Stock and Nielsen (Ber. d. D. Chem. Ges. 2 (1906) p. 2066) and is then 

 usually simply called Stock's thermometer. For accurate measurements it is necessary 

 to resort to more suitable forms of the instrument, such as the one described in 

 this paper. The scale given by Stock and Nielsen is in need of very considerable 

 corrections (Comp. G. Holst, Comm. N". 148a; these Proc. XVIII (1) p. 829). 



S) Von Siemens (vid. G. Holst Comm. N'^. 148a) determined the tempe- 

 ratures at which he observed the vapour-pressures by means of a platinum- 

 thermometer, whose readings were reduced to those of Pti' of Kamerlingh Onnes 

 and his collaborators, chietly by means of four calibration points in such a manner 

 that the scales were made to coincide at the boiling point of oxygen as determined 

 by Kamerlingh Onnes and Braak. 



