55 



retrograde condensation, allhough the latter was limited to a range 

 of 0.13 degree at most. 



As an illnstratioii of tlie perfection to which the regulation of 

 pressure and temperature has been brought in the Leiden Laboratory, 

 the 50*/, mixture was maintained under liie plait-point conditions 

 tor over an hour, tiie blue opalescence being continually there, 

 while an indefinite meniscus alternately appeared and disappeared 

 in the middle of the bulb on stirring. From the results a p, v^ 

 graph for each mi.xture is constructed, and the points of beginning 

 and end condensation determined as previously stated : the border 

 curve is drawn through these. It was found that the two-phase line 

 during the period of observation is to all intents and purposes a 

 straight line, although, in the case of the 50 7, mixture, the first 

 points determined aftei' condensation lie below this line on every 

 isotherm. This can hardly be explained by any delay in the appea- 

 rance of the liquid phase (which would give the reverse effect), and 

 the deviation is far greater than any eiror of observation. 



The accuracy of the pressure determination is at least i in 5000'); 

 that of the temperature reading within 0.02 of a degree, while the 

 probable observation error of the volumes is not greater than 1 in 

 2000 when one phase was present, and 1 in 200 when two phases 

 were present — apart from a possible constant calibration error of 

 1 in 500. To eliminate the last error it would have been necessary 

 to measure a few points of the isotherm of 20° C. of hydrogen 

 with this piezometei-, and to compare the results with the accurate 

 isotherm of Schalkwyk. But as such accuracy was of little impor-, 

 tance in our case, this was not done. 



The results for the two mixtures are given below with: 



p := pressure in atmospheres. 



I'A = volume, expressed in the normal volume. 



V/: = volume of liquid, expressed in volume of the small reservoir. 



^ =z temperature on the provisional intern. Kelvin scale, reduced 

 by 273.09. 



The condensation |)oiiUs, as found from the p.v.1 graphs, are 

 plotted on a p,t (t = 8) graph ; the results of Kcenkx and Clark 

 being included on the same graph. (Fig. 4). The vapour pressures 

 of pure oxygen') and nitrogen') are also plotted, and the critical 



M C. A. Grommelin and Mej. E. J. Smid. These Proc. XVlti (1 1 pg. 47-'. Leiden. 

 Goinm. Leiden. N". 146c. 



') H. Kamerlingh On.nes, G. Dorsman and G. Holst. I. c. 



») C. A. Grommeun. TheseProc. XVtl (2) 959 (Dec. 1914.; Leiden Gomm.N". Uorf. 



