138 



BRIDGMAN. 



the microscope, in which temperature control is not so easy, and was 

 not an equilibrium measurement between two phases simultaneously 

 present. I have already stated that the character of the transition 

 rules out the possibility of there having been any effect exerted by 

 impurities on the data given here. 



Ammonium Chloride. — This was Kahlbaum's purest, and was 

 dried in vacuum at 100° before using. Its behavior is very similar to 

 that of NH4Br. The transition line is also steep, and the transition 

 temperature at atmospheric pressure is higher than that of XH4Br. 

 All the measurements were made in the low pressure apparatus, and 

 only three points were determined, up to 203° and 275 kgm. The 

 reaction is just as sharp as for NH4Br, and the pressure limits no wider; 

 in fact it would be difficult to imagine two substances more alike in the 

 character of the transition. 



lo 8 



I. 



Q. 



b- "^0 100 200 300 ^ 

 Pressure, kgm. /cm. ^ > 

 Ammonium Chloride*^ 



E a 



. 100 200 300 

 E Pressure, kgm. /cm.* 

 ^Ammonium Chloride 



Fig. 26. Fig. 27. 



Figure 26. Ammonium Chloride. The observed equilibrium pressures 

 and temperatures (circles) and the observed differences of volume (crosses). 



Figure 27. Ammonium Chloride. The calculated latent heat and the 

 change of internal energy. 



The observed equilibrium pressures and temperatures and the 

 changes of volume are shown in Figure 26, the computed values of the 

 latent heat and the change of internal energy are shown in Figure 27, 

 and the numerical values are collected in Table XIII. The transition 

 curve is distinctly convex do-wmward, as it is also for NH4I and XH4Br. 

 The Av curve is concave downward. Such is also the case for XH4Br, 

 but in the case of XH4CI the Ad curve has not yet reached the maximum 

 in the range shown, whereas the corresponding curve for XELjBr does 

 pass through a maximum in the range of the experiment. 



It has been known previously that XH4CI also has a second form at 

 high temperature which has a greater volume than the low temperature 

 form. Wallace ^'^ gives 159 ° for the transition point, against 184.3° 

 above. There is no previous measurement of AV. 



