TRANSITION OF STEREO-ISOMERS in 



Saturation with 25 35 40-4 40-7 54 



Tartrate mixture . 13-03 13-46 13 83 (Line A'B Fig. 20) 



Racemate . . . 10-91 12-63 13-48 . (Line AB ,, 



Whilst, therefore, comparison with a simple transition 

 between two hydrates (as of Glauber salt) brings out 

 evident relationships, there is a difference to be noted, 

 for the formation or decomposition of a racemate is not 

 a simple conversion of one salt into another, but of a salt 

 mixture (tartrates) into a single salt (racemate) or vice versa. 

 Remembering this, it is clearly the formation or decomposi- 

 tion of double salts that should be quoted as analogy, and 

 we will therefore describe what is essential in the relations 

 of solubility at the transition temperature in such cases. 

 (See Part I. p. 81.) Let us take the formation of astrakan- 

 ite, the sodium-magnesium sulphate, Na 2 Mg(SO 4 ) 2 . 4 H 2 O, 

 from its components, Na 2 SO 4 . 10 H 2 O and MgSO 4 . 7 H 2 O, 

 which takes .place at 22 according to the equation 



Na 2 S0 4 . ioH 2 O + MgSO 4 .7H 2 O = Na 2 Mg(S0 4 ) 2 .4H 2 O 



Here, again, we have an apparent fusion that may be 

 followed by the thermometer or dilatometer. 



We now find, in the behaviour with regard to solution, 

 new relations to the formation of a racemate. These 

 appear in the fact that at 22 three solubility curves inter- 

 sect. From low temperatures up, we have the solution 

 saturated for the two sulphates (i); at 22 formation of 

 astrakanite sets in, and according as there is excess of the 

 sodium or magnesium salt, we reach either the curve of 

 saturation for astrakanite and sodium sulphate (2), or that 

 of astrakanite and magnesium sulphate. Schematically : 



Na 2 S0 4 . ioH 2 , 2 ) Astr . + Na go . 1O H 2 O 



+ MgS0 4 . 7 H.O (i) ( 



22 \(3) Astr. + MgS0 4 . 7 H 2 O 



The same is to be expected for the formation of racemate. 

 Thus, for the sodium-ammonium salt from low temperature 



