Physics. — "Isotherms of di-atomic substances and their binary 

 muetures. XX. The critical curve of oxygen-nitrogen mixtures, 

 the critical phenomena, and some isotherms of la>o mixtures 

 loith 507o "'"^ 757, by oolume of oxygen in the neighbourhood 

 of the critical point." H_v J. P. Kl'knkn f, T. Verschoyi.e and 

 A. Th. van Ukk. Coniniimicatioii No. 161 fioni the Physical 

 Laboratory at. Leiden. (Dr. Kameki.ingh Onnks, holding his 

 deeply regretted friend in aftectionale. memory, is glad to 

 perform the hononral)le task of [ire.senting for the Pi'oeeedings 

 a paper by the lale Dr. Kuenen which was made almost 

 ready for the press), 



(Communicated at the meeting of November 25th. 1922). 



^1. Introduction. 



Thi.s work is a conliiination of tiiat of Koenen and Clark'), the 

 investigations, however, being carried ont in snch a way as to allow 

 of the constrnction of complete isotherms, which involved a slight 

 modification of the apparatus (hen used. The nii.ictnres on which 

 measurements \^e^e made, contained res|)ectively 50 "/, and 75 °/„ 

 by volume of oxygen, and it was found that these gave sufficient 

 data for the construction of the critical curve. This was found to 

 be almost a straight line, while both critical constants proved to 

 be an almost linear function of the composition. 



\ 2, Preparation of the mixtures. 



A simple mixing apparatus was employed, consisting essentially 

 of a measuring-bulb of about a litre capacity, surrounded by a 

 water-jacket and connected with an open manometer tube, in 

 which the gases to be mixed were measured, and of a mixing bulb 

 of some 2 litres capacity. Through 3-way taps the bulbs could be 

 put in connection with each other, the source of gas, the piezometer 

 to be filled, and a vacuum pump, as might be desired. In measuring 

 the relative quantities of the gases to be mixed, the mercury wa,s 



1) J. P. Koenen and A. L. Clark. These Proc. XIX (2) pg. 1088. (Febr. 1917.) 

 Leiden Comm. N". 1506. 



4 

 Proceedings Royal Acad. Amsterdam. Vol. XXVI. 



