ii2 MOLECULAR STRUCTURE 



to 27 the saturation curve for the tartrate mixture 

 (i) holds; at 27 formation of racemate occurs, and if the 

 two tartrates are present in equal amounts, we obtain 

 the former racemate curve. If one or other tartrate is 

 present in excess, we come in the one case to the curve of 

 saturation for racemate and dextro- tartrate (2), in the other 

 to that for racemate and laevo-tartrate. Schematically : 



A^) Kacemate 4- (^Tartrate 

 (^Tartrate + ^Tartrate (i) / 



27 MS) Racemate -f^Tartrate 



The results of such measurements of solubility may best 

 be represented by a model, in which the quantities of the 

 two salts are measured along two perpendicular planes, 

 whose line of intersection constitutes the axis of tempera- 

 ture. The solubility curves then lie between the two 

 planes, and any projection of them can be obtained, such as 

 Fig. 21. 



The essential difference between the formation of a 

 double salt and a racemate now appears in the symmetry 

 -possessed by the latter case, due to the exactly equal 

 solubilities of the optical antipodes, whereas the components 

 of a double salt have in general different solubilities. The 

 number of measurements needed to prepare the model is 

 for this reason smaller in the case of the racemate, and it 

 is only necessary to experiment with the racemate, or 

 inactive mixture, and one of the antipodes. This is the 

 case with rubidium racemate : only here the phenomenon 

 is reversed as compared with double-salt formation, for the 

 formation of racemate takes place at lower temperatures, 

 and in the equation 



<Rb 2 C 4 6 H 4 ) 2 . 4 H 2 ^ QKb 2 C 4 6 H 4 + 



the left-hand side represents the form stable at low tem- 

 peratures (below 40-4). Thus, what in the preceding 

 equation came on the left, the tartrate mixture (i), comes 



