TRANSITION OF STEREO-ISOMERS 109 



The transition of Glauber salt appears as a melting point, 

 and can be determined either as the temperature at which 

 that salt melts, or that at which a mixture of anhydrous 

 sodium sulphate and water freezes. Beckmann's apparatus 

 for exact determination of melting points is admirably 

 adapted to the observation. 



The same can be done for the formation and decomposi- 

 tion of racemates, and in that way, e. g., the temperature 

 (40) at which rubidium racemate splits up into the optical 

 antipodes has been determined 1 . 



Since, however, a somewhat large amount of material 

 is needed to determine transition temperatures in this way, 

 the dilatometric ".process described in Part I, p. 25, in which 

 the change of volume accompanying transition is used 

 as indication, is often to be preferred. A dilatometer filled 

 with the tartrate mixture, e. g., gave, as indication of the 

 transition of the sodium-ammonium tartrates into racemate, 

 the following readings of the level of the liquid : 



16 7 350 mm. l6 per ^ 



26 7 5io > 



27-7 3 6 72 - * 



3t-7 727 I H I D 



The conversion, therefore, took place between 26-7 and 

 27-7, with an expansion equivalent to a rise of the liquid 

 by 162 15=147 mm. 



As in the case of Glauber salt, so in that of the racemate, 

 the transition is connected with a singularity in the solu- 

 bility curve. This for Glauber salt shows a break at 33, 

 due to the intersection at that temperature of two solu- 

 bility curves, one referring to the hydrate, the other to the 

 anhydride (Part I, p. 66). The following numbers, taken 

 from Lowel, will serve to illustrate this ; they give the 

 weight of anhydrous sodium sulphate per 100 parts of 

 water in the saturated solution : 



Saturation for 31-84 32-65 34 



N^SOi.ioH.O 40 U9-78) 55 



J,2. ..: Na 2 S0 4 49.91 U9-78) 49-56 



1 Van 't Hoff, Muller, Berl. Ber. 31. 2206. 



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