622 BRIDGMAN. 



CsNOg and TINO3 a line 8-7 like that of RbNOg. Such a line has 

 not been found, however. Failure to find it for TINO3 is not strange ; 

 the transition 8-7 may well have risen above the melting point, which 

 is comparatively low, only 205°. But failure to find it for CsNOs 

 is not so easy to explain. If the transition point has risen above the 

 melting point, which for CsXOs is at 414°, the effect of increasing 

 molecular weight on the transitions 8-7 and 7-6 is very different in 

 going from RbNOa to CsNOa; the latter is depressed from 164° to 154°, 

 while the former would be raised from 219° to over 414°. It seems 

 quite possible that the transition for CsNOs may have been over- 

 looked, and that a careful search up to 414° would be worth while. 

 The mixed crystal diagrams of RbNOs with CsNOs and TINO3 suggest 

 that both the latter salts would show the modification 8 if it were not 

 for the interference of the liquidus line. 



With regard to the transition 6-2 of TINO3 there is certain indirect 

 evidence that such transitions exist for both CsNOs and RbNOs, 

 although their existence has not been directly proved. Wallerant 

 states that CsNOs passes to another modification on cooling in liquid 

 air. The crystalline form of this new modification he found to be 

 rhombohedric, approximately cubic. This is not the same as the 

 crystalline system of TINO3, 2, which is orthorhombic, but the diffi- 

 culty of crystallographic observation at very low temperatures does 

 not make it absurd to suppose that the system may have been in- 

 correctly determined. The existence of another modification is of 

 itself significant. With regard to RbNOs, Wallerant's observations on 

 mixed crystals of the system NH4NO3 - RbNOs make it almost certain 

 that at low temperatures RbNOs has another modification. In the 

 mixed crystal diagram there is a range of concentration, varying with 

 temperature, within which RbNOs and NH4NO3 crystallize together 

 in a form with all the characteristics of TINO3, 2, a form which has not 

 been found for either pure RbNOs or NH4NOS. An easy extrapola- 

 tion of the region of the mixed crystals indicates at low temperatures 

 the existence of the phase as pure RbNOs. I am not aware that 

 Wallerant made the same examination of RbNOs down to liquid air 

 that he did of CsNOs. If it should turn out that RbNOs and CsNOs 

 have the phase 2, the transition point to 6 would be quite differently 

 situated for these two salts than it is for TINO3. This need not be 

 surprising; the work of Tutton on the sulfates, for example, has 

 shown that although the Thallium salts are closely related to those of 

 the alkaline metals proper, yet a detailed analysis shows abrupt dis- 

 continuites in the numerical magnitudes. 



