590 BRIDGMAN. 



pressure, 145° for the transition temperature by Wallerant.^ The 

 value found above for the transition temperature was 153.7°. In 

 view of the exceptional purity of the sample used above, it would 

 seem that 153.7° should be given the preference. 



Direct measurements were made of the difference of thermal ex- 

 pansion of the two phases at low pressures (77 kgm.), and of the 

 difference of compressibility at higher pressures. The measurements 

 are better than usual, and give more consistent results. Three 

 measurements of the difference of compressibility at high pressures 

 were made; one of these was somewhat too large, but the other two 

 agreed within 5%. The values follow. 



At 77 kgm. 



Aa = 0.0649 

 A/3 - O.O433 



At 5000 kgm. 



ACp= 1.0 



Aa = O.OeSl 

 A/S = O.O4I5 

 ACn= 0.28 



At 77 kgm., A|3 was the observed value, and at 5000, Aa. We see 

 that at both pressures the high temperature phase is the more com- 

 pressible, has the greater expansion, and the greater specific heat. 

 The differences between the two phases become less at higher pres- 

 sures. The decrease of Aa with rising pressure was unmistakably 

 shown by the direct measurements. This behavior of CsNOs is what 

 one would perhaps be inclined to call entirely normal. The difference 

 of compressibility between the two phases is of the order of the com- 

 pressibility of platinum. 



Especially careful search was made for other modifications, since 

 others were expected in analogy with the other nitrates. None were 

 found to 12000 kgm. however, at 20°, 177°, or 200°. 



Rubidium Nitrate. I have to thank the kindness of Professor 

 T. W. Richards for this rare substance. He personally prepared the 

 samples by repeated crystallization. The purification was continued 

 until the flame test, very sensitive in these cases, showed no trace of 

 any of the other alkali metals. The salt as finally provided by Pro- 

 fessor Richards was crystallized from acid solution, and gave an acid 

 reaction. It was unfortunate that I did not succeed in removing all 



6 F. Wallerant, Bull. soc. fr. min. (190,5) 311-374. 



