EFFECTS OF PRESSURE ON NITRATES. 023 



The diagram of KNO3 next demands attention. The phase I or 

 8 is the same as the phase 8 of RbNOs. It is most tempting- to call 

 the new high pressure phase III the same as 7 and IV the same as 6, 

 for this would mean that at high pressures, by a simple displacement 

 of the origin of pressure, the diagram of KNO3 takes its place with 

 those of higher molecular weight, RbNOs, CsNOs, TINO3. Further- 

 more, a shift of the pressure origin in this direction, corresponding to a 

 decrease in molecular weight, seems a not unnatural result of the 

 corresponding decrease of internal pressure. It is unfortunate that a 

 comparison of numerical results is not favorable to this identification. 

 The only data which we can use are for the line III-IV; we are to 

 inquire whether this can correspond to the line 7-6 of RbNOs, CsNOs, 

 and TINO3. The slope of the line III-IV is 0. 3, while that of 7-6 

 varies from 0.008 to 0.009 and the change of volume on the line III- 

 IV is 5.5% against 1.6, 1.3, and 1.3% respectively for the other three 

 salts. Another bit of presumptive evidence against the identification 

 of III with 7 is that the unstable form of KNO3 found below the transi- 

 tion I-II at atmospheric pressure is rhombohedric. It is plausible, 

 but not necessary, to suppose that this unstable form is the same as 

 III. But the form of 7 is cubic. The evidence is not conclusive 

 either way, however, and it would be worth while to make special 

 effort to find the system of KNO3 III; the triple point I-II-III is at 

 such a low pressure that it would not be out of the question to try for 

 a microscopic examination in a heavy glass capillary. 



In the case of NH4NO3 the probability is fairly good that the new 

 phase VI stable only at high pressures is the same as 6 of Rb, C's, and 

 Tl. This is indicated not only by its position with respect to 7 

 (NH4NO3, I) but also by the numerical values. The average slope of 

 7-VI is 0.0075 and the change of volume 1 .4% ; both of these are very 

 close to the corresponding values for the Rb, Cs, and Tl salts. The 

 probability is further much increased by the mixed crystal diagrams 

 of NH4NO3 with RbNOs and CsNOs. Mixed crystals of RbNOg or 

 CsNOs, 6, are stable over a very wide range of concentration, which 

 comes closest to pure NH4NO3 at the same temperature as that 

 indicated by an easy extrapolation of the line I-VI for NH4NOS to 

 atmospheric pressure. The same argument cannot be applied to the 

 mixed crystal diagram of NH4NOS with TINO3 because of the anomo- 

 lous behavior of NH4NO3, II. 



To make the correspondence of NH4NO3 with the heavier nitrates 

 complete, the phase IV of NH4NO3 should be identical with III of 

 TINO3. But both Groth and Wallerant treat these two phases as 



