122 BRIDGMAN. 



The transition line I-II is somewhat unusual in that A F increases with 

 increasing temperature. 



The band of indifference shows interesting variations. On each of 

 the three curves, I-III, I-II, and II-III, the band decreases in width 

 with increasing temperature; but besides the general effect of tempera- 

 ture, there is a very marked specific effect of the nature of the reacting 

 phases. Both the bands I-II and II-III are broader at their lower 

 ends than I-III is at its upper end, although the temperature of the 

 lower end of the first two bands is the same as that of the upper end of 

 the I-III band. The actual widths are as follows. I-III is 200 kgm. 

 wide at 0°, and 60 kgm. wide at 25.2°; I-II is 250 kgm. wide at 35.3°, 

 75 kgm. at 50°, 20 kgm. at 58°; II-III is 60 kgm. wide at 35.3°, 20 

 kgm. at 50°, and 3 kgm. at 70.2°. Along with this change in the width 

 of the band there goes a parallel change in the reaction velocity. No 

 trouble was ever found in forcing the desired reaction to take place; 

 but no observations were made of the precise amount of subcooling 

 or superheating. It was observed once at 50° that I had to be car- 

 ried 650 kgm. across the line before II appeared. 



No other transitions were found to 12700 kgm. at either room tem- 

 perature or 153°. 



The efltect of pressure on the melting point of urethan has also been 

 measured by Tammann,^^ who did not find any other modifications. 

 Tammann's pressure range was to 2905 kgm. At the upper end of the 

 melting curve he must therefore, have had the modification II without 

 knowing it. Tammann remarks that below 1500 kgm. the melting 

 curve is parabolic, but that above 1500 it is nearly linear; this linear- 

 ity is evidently the effect of the overlooked transition. In fact, if one 

 plots Tammann's points, he will find some evidence for the change in 

 direction of the curve above the triple point, although the points are 

 rather irregular. Tammann also looked for other modifications of the 

 solid out to 3000 kgm. at 40°; if he had tried 60° instead, he would 

 probably have found II. The melting point at atmospheric pressure 

 was 48.14° against 47.90° above. The presumption from this alone 

 would be, therefore, that Tammann's specimen was purer than mine. 

 But at high pressures his curve drops below mine, being 1 .8° lower at 

 the upper end, and he also states that the reaction was unusually slow 

 for a melting reaction. Both of these points suggest impurity. In 

 the work above, the melting set in as rapidly as for any normal liquid. 

 It almost seems as if the design of Tammann's apparatus had been 



32 G. Tammann, "K. und S.," p. 239. 



