126 



BRIDGMAN. 



freezing point. In fact, this is the most impure substance, judged by 

 this criterion, for which I have ventured to pubUsh data. At 150° 

 it is entirely melted; at 139° it is mushy, perhaps jq melted, and even 

 as low as 110°, it is perceptibly moist with the remnants of the melt. 

 It may be of course that even the absolutely pure substance does 

 not behave at the melting point like a simple substance; it might be, 

 for example, that there is a reversible dissociation into something 

 like (NH4)2S04 and H2SO4, although this precise dissociation is not 

 likely. 



Partly as a consequence of the width of the domain of melting, and 

 partly as a result of the smallness of the change of volume, the data 

 obtained for this substance are unsatisfactory in many respects. The 



Pressure, kgm./cm.^ x 10^ 

 Ammonium Acid Sulfate 



Figure 17. Ammonium Acid Sulfate. Tlie observed equilibrium pres- 

 sures and temperatures. 



changes of volume are probal^ly in error by large amounts. These 

 changes as measured fall far short of satisfying the additive conditions 

 at the triple point I-II-III. Furthermore, the additive conditions 

 for AH are far from being satisfied at this point. The way in which 

 the observed data should be adjusted so as to satisfy this condition 

 does not readily suggest itself. I have preferred, therefore, not to try 

 at all to deduce the latent heats and the changes of energy from the 

 data, and in Table IX have tabulated only the equilibrium pressures 



