52 



it, so that the gas and liquid pliases in the Imll) could he slii-red 

 as required. ') 



^ 4. Observations and precautions. 



Before commencing the work at low temperatures, isotherms were 

 experimentally delermined for the two mixtures al 20°, the calcu- 

 lations being carried out on the lines of Comm. N°. 78. The values 

 of the expansion coellicients for the mixtures 



1 



".4 



dv 



A 



dt y^ 



required in the calculation of the isotherms were interpolated as 

 linear functions of the coniposilion from the coriesponding values 

 of the pure gases, the error involved being negligible. These values 

 were, in the case of oxvgen, those found bj Kamkulingh Onnks and 

 Hyndm.4N') and, in the case of nitrogen, calculated from isotherms 

 determined al 0° and 20° by one of us, which are not yet published. 

 These normal temperature isotherms were determined with small 

 reservoirs ± 5 cm' volume. For greater certainly a second series 

 of points were determined for the 75 "/o oxygen mixture using the 

 small reservoir of ± 1 cm' of the piezometer used in the critical 

 zone as a leak occurred during the first series, and consequently 

 onl} the normal volume delermined at liie end could be used in 

 the calculations. The agreement of this control is satisfactory. 

 Isotherms were made over a range starting about 5 degi'ees above 

 the temperatni'e of the critical point of contact, and extending as 

 low as the jiroportions of the piezometer allowed, i.e., 6 degrees 

 below that temperature for the 50° , mixtui-e, and 2| degrees for 

 the 757, mixture. The tem|)erature intervals were in general some 

 2 degrees, but, in the neighbourhooti of the zone, were i-educed to 

 Vso degree or less. All observations were made with rising pressure, 

 the importance of which fact is insisted on in Comm. 150A; and, 

 after finishing any series, the jjressure is completely I'eleased, and 

 gas in the piezometer well mixed by successively raising the pressure 

 to 10 atmospheres or so and lowering, before proceeding to a new 

 series. When only one phase was present, the pressure steps were 

 of the order 2 — 3 atmospheres, but, when two were present, and 

 near the critical zone, they were reduced to a few tenths of an 

 atmosphere and sometimes the raising was accovn pushed by even 



') A. VAN Eldik. Amsterdam Akad. Versl. Mei— Juni 1897. Leiden Comm. N". 39. 

 ') These Proc. IV pg. 7G1. (Maart 1902). Leiden Comm. N». 78. 



