RICHARDS. — TRANSITION TEMPERATURES OF SODIUM CHROMATE. 179 



To bring about the transition at 19.90°, it is necessary to superheat 

 the dekahydrate past the lower transition point, 19.53°. Not many 

 other instances of this sort have been carefully studied. Walker and 

 Fyffe^^ have examined the solubility of Ba(C2H302)2 • 3H2O (which un- 

 dergoes a transition into the monohydrate and solution at 24.7°) as far 

 as 26.1°, 1.4° above its transition point; and the solubility of a 

 Ni (103)2 -21120 has been studied by Meusser^^up to 50°, 13° above 

 the temperature of its transition into the anhydride and solution. ^^ 



The addition of a foreign substance not isomorphous with the deka- 

 hydrate lowers the transition point as usual.^*^ The addition of sodium 

 chloride was found some time since by C. A. Bigelow and one of us to 

 lower the transition temperatures (19.53) about the expected amount. 

 This result was not published at the time, but has since been confirmed 

 by J. L. R. Morgan. 14 



A much more interesting effect is observed with isomorphous hy- 

 drates. For example, successive additions of sodium sulphate to the 

 solution of chromate gave successive elevations of the transition tem- 

 perature, even when the crystals contained no sodium sulphate. ^^ 

 These elevations were roughly proportional to the amount added. The 

 experiments were not carried on with a view to determining the limits 

 of this change, but an elevation of about 4° was found in some in- 

 stances. The temperatures at first obtained were variable ; but on 

 vigorously stirring in an environment 0.5° warmer than the mixed 

 hydrates, equilibrium was apparently reached after five minutes, and 

 often remained undisturbed for fifteen minutes or longer. This raising 

 of the transition point involves an interesting extension of the range 

 of stability of the dekahydrate. 



So far as we can find this phenomenon of the raising of the transition 

 temperature of hydrated crystals by the presence of isomorphous hy- 

 drated crystals is here described for the first time. It deserves and 

 shall receive here in the near future more thorough investigation. 



" Walker and Fyffe, Journal Chem. Soc, 83, 180 (1903). 



" Meusser, Berichte, 34, 2440 (1901). 



*^ See also Berthoud, Sur I'impossibilit^ de surchauffer un solide. Journal 

 de chim. Phys., 8, 377 (1910). 



" Lowenherz, Zeit. phys. Chem., 4, 349 (1889). 



" Morgan, Zeit. anorg. Chem., 55, 263 (1907) ; 56, 168 (1907). 



^^ This fact was first observed by Churchill and one of us about ten 5''ear3 

 ago, but has not been published until now. The value 19.6° (which made no 

 pretensions to great accuracy) assigned to the lowest transition point of 

 sodium chromate at that time was undoubtedly too high from this cause, 

 because the salt was made from ordinary chromic anhydride. These Pro- 

 ceedings, 34, 277 (1899). 



