598 BRIDGMAN. 



respect to each other; AH and AE rising with increasing pressure on 

 the I-II hne, and faUing on the II-III Une. The numerical results 

 are collected in Table V. 



There are not many values for comparison. For the transition 

 temperature I-II van Eyk ^ found 142.5°, Gossner '^ 151°, and Wal- 

 lerant ^ 125°. For the transition temperature II-III Gossner gives 

 80° and Wallerant 80°. Van Eyk entirely overlooked this transition 

 in his first paper; in a later paper ^ he corrected his earlier results 

 and gives 72.8° by a dilatometric, and 79°-80° by an optical method. 

 The values which I found above for the transition points were 144.6° 

 and 75°. Although these were obtained by extrapolation, the range 

 was small and the presumption is that they are at least as accurate 

 as the other values. My values are means between limits reached 

 by the reaction when running in opposite directions, both phases 

 being present, whereas certainly Gossner and Wallerant did not 

 shut their equilibrium point within values from the two sides, and 

 sometimes give as the transition temperature merely the temperature 

 of appearance of the new phase on heating. There seem to be no 

 pre\ious measurements of the thermal effects of the transition, and 

 the only measurement of change of volume is by van Eyk ^ who 

 found 0.0004 ± cm.^/gm. for the transition II-III. 



The difference of compressibility of the phases may be found in 

 the usual way from the difference of the slope of the isothermals above 

 and below the transition. For this purpose only those runs were 

 used which gave good values for Av. There are nine such good points 

 for the change I-II. Of these, seven show no difference of compressi- 

 bility and two show a slightly greater compressil^ility for II. The 

 inference seems justified that within the limits of error the compressi- 

 bility of I and II are the same. There are six good points for II-III, 

 and all of these indicate that III is the more compressible. Further- 

 more, these six points are all self consistent and show a decreasing 

 difference of compressibility with rising pressure and temperature. 

 At 75°, the compressibility of III, measured in cm.''' per gm. per kgm. 

 per sq. cm., is 0.065 greater than that of II, at 105° it is O.OeS greater, 

 and at 140°, 0.0615 greater. It is remarkable that the two phases 

 with the smaller difference of volume should have the greater differ- 

 ence of compressibility. Combining the difference of compressibility 

 with the change of volume along the transition line, the difference 



8 C. van Eyk, ZS. phvs. Cliem. 30, 4,30-4.59 (1899). 



9 C. van Eyk, ZS. phys. Chem. 51, 721-731 (1905). 



