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Physics. — “Jsothermals of di-atomic substances and their binary 
mixtures. XV. Vapour-pressures of oxygen and critical point 
of oxygen and nitrogen.” By H. KAMBRLINGH Onnes, C. DORSMAN, 
and G. Horst. (Communication N°. 1455 from the Physical 
Laboratory at Leiden). (Communicated by Prof. H KAMERLINGH 
ONNES,) 
(Communicated in the meeting of December 50, 1914). 
1. Apparatus and method. 
The vapour-pressures of oxygen at twelve different temperatures 
and the eritical points of oxygen and nitrogen were determined by 
means of the apparatus which was first used by KAMERLINGH ONNES 
and HYNDMAN for unpublished investigations of oxygen, of which 
those published on this occasion form a repetition, and later on by 
CROMMELIN for a determination of vapour-pressures of argon. A 
detailed description was given in Comm. Phys. Lab. N°. 115 (Meeting 
of April 1910). In the piezometer is an electromagnetic stirrer 
according to KUENEN. 
In order to make a study of the critical phenomena possible two 
narrow slits of the vacuum-vessel of the cryostat were left unsilvered. 
(Compare Comm. N°. 83. Meetings of Dec. 1902 and Febr. 1903. As 
mentioned in an erratum to Plate IV $ 6, the vessel 4’, is drawn 
18 cms longer than it was). Besides the piezometer there were two Pt- 
resistance thermometers in the cryostat which had both been directly 
compared with the hydrogen-thermometer. One of them (the other 
one Pt, broke afterwards) was Pt; of Comm. N’. 14la $6, the same 
which was also used in the measurements concerning the diameter. 
The pressures above 20 atm. were measured with the closed hydrogen- 
manometer described in Comm. N°. 97a § 3. At lower pressures the 
open gauge of KAMERLINGH ONNES (Comm. N°. 44) was used. 
In order, to determine the critical constants small portions of 
isothermals were studied in the neighbourhood of the critical point. 
At temperatures below the critical temperature starting from a 
suitably chosen volume and compressing by small amounts a con- 
dition can be found by sufficient compression in which the next 
compression produces condensation; at temperatures above the critical 
temperature condensation cannot occur. . 
The critical point having been approximately determined in this 
way, the observations were repeated within narrower limits of 
pressure, at temperatures ascending by about '/,,° C. Beginning with 
a pressure below the critical pressure and proceeding along an 
