EFFECTS OF PRESSURE ON NITRATES. 



()21 



with any of them. AgNOa may be disregarded in any comparative 

 study, therefore, and in the following only the five alkaline nitrates 

 are considered. 



A comparison of the phase diagrams is more significant when we 

 combine with it a discussion of the mixed crystal relations. Wallerant 

 has given in his book a most elaborate account of the mixed crystal 

 situation, and I shall draw my data from him. 



One conclusion to be drawn from this discussion will be that simi- 

 larity of phase diagrams is evidence of the most complete similarity 

 of structure, more complete even than the ability to form continuous 

 series of mixed crystals. Judged by this latter test, the five alkaline 

 nitrates are very similar. Wallerant has proved that every one of the 

 five can form mixed crystals in limited proportions with every one of 

 the atmospheric polymorphic forms of all the other nitrates. The 

 phase diagrams are not the same, however, for all five nitrates; those 

 of Rubidium, Caesium, and Thallium are most similar. Each of 

 these three diagrams contains a transition line between the cubic 

 form (7) and the orthorhombic, pseudo-hexagonal, form (6). The 

 use of the same numerals for these phases in the different diagrams is 

 justified by the fact that within the proper temperature range the 

 phase 7 or 6 of any of the three salts will form a continuous series 

 of mixed crystals with the phase 7 or 6 of any of the others. We may 

 call the line 7-6 the "same" line in the various diagrams. This line 

 preserves various of its characteristics with very little change from 

 substance to substance, as is shown by the summary in Table VIII. 



The lines 7-6 correspond so closely that we expect further similarity 

 of the phase diagrams of these three salts. We should expect for 



