134 



BRIDGMAN. 



lower temperatures until at 50° the velocity is so low that a value of 

 Av too low was obtained, the reaction appearing to have stopped before 



it really had. At 25° an attempt was 

 made to avoid this source of error by 

 raising the temperature to 100° in the 

 middle of the run to ensure completion 

 of the transition, and then bringing it 

 back again. The attempt seems to have 

 been not entirely successful. At 0° the 

 pressure was low^ered to atmospheric 

 pressure for an hour to ensure comple- 

 tion, with apparently satisfactory results. 

 At the two upper points of the curve 

 there was a ver;s' slight rounding of the 

 corners noticeable at the last reading, 

 about 180 kgm. before the transition. 

 The behavior was not like that usually 

 found for an impure substance, and there 

 is therefore a slight possibility that there 

 may be a third phase, the transition line 

 splitting in the neighborhood of 160° into 

 two lines diverging at a very slight angle. 

 If such is the case, the coordinates above 

 are for the left hand of the two branches. 

 The amount of transgression of the 

 transition line possible before the transi- 

 tion starts is a matter of 100 or 200 kgm. 

 at the lower temperatures. A greater de- 

 gree of subpressure than of superpressure 

 may be supported. 

 The direct measurement of the difference of compressibility of the 

 two phases did not give results so accurate as are sometimes obtained, 

 but the conclusion is fairly safe that the difference is not greater than 

 =»=0.05l cm.^ per gm. per kgm. Using these limits for Aa, we find 

 fairly close limits for the c^uantities A/3 and ACp when calculated in the 

 usual way. Over the entire length of the curve both A/3 and ACp are 

 negative on -either assumption for Aa; that is, the high temperature 

 phase is less expansible and has the lower specific heat. At 100 kgm. 

 (—8°) A(3 lies within the limits — O.O426 to — O.O454 cm.^ per gm. per 

 degree Centigrade, and ACp between —0.023 and —0.026 cal. per gm. 

 At 2053 kgm. (190°) the limits of A/3 are -O.O58 to -O.O42I, and for 



2000 



Pressure, kgm./cm.^ 

 Ammonium Iodide 



Figure 22. Ammonium 

 Iodide. The observed equi- 

 librium pressures and tem- 

 peratures (circles) and the 

 observed differences of vol- 

 ume (crosses). 



