SOLUBILITY OF POLYMORPHIC BODIES 137 



Bellati and Romanese on ammonium nitrate 1 . This suit- 

 stance freezes at 168 in the regular system; becomes 

 rhomboheclric at i 24, with an evolution of n-86 calories 

 per kilogram ; at 82-5 becomes rhombic, with an evolution 

 of 5-33 calories; and at 31 rhombic with a different axial 

 ratio, with evolution of 5-02 calories. 



E. Polymorphic Modifications have a Constant Ratio of 

 Solubilities, proportional to the Differential Co- 

 efficients of the Saturation Pressures, in Solvents 

 which take up so little of the Substance, that the 

 Laws of Dilute Solutions are applicable. 



This relation follows from the equation (Part I, p. 36) 



dlogO _ _Q_ 

 ctT 



which unites the concentration of the solution or vapour 

 with the heat Q absorbed when one kilogram-molecule is 

 dissolved or evaporated (without doing external work). 

 For the two polymorphic bodies we have 



_ 



dT <zT* 

 and 



dlOgCu _ QlT 



dT = 2T 2 ' 

 whence 



(Hog (Cn/CQ _ Qn-Q, 

 dT 



where R is the heat of transition. 

 Integrated, this gives : 



The constant is given by the fact that at the transition 



* Lehmann, Molekularphysik, p. 159. 



