208 



case of spontaneous fission is completely analogous to that of the 

 formation and decomposition of many double-salts, there being 

 a definite transition-temperature, above which the racemic compound 

 is stable, while on the contrary at a temperature below it, the equi- 

 molecular mixture of both tartrates is the more stable solid phase 

 in equilibrium with the optically inactive, saturated solution. 



It was found that an equimolecular mixture of the dextro-, and 

 laevogyratory tartrates at 27, 2 C. was transformed into Scacchi's 

 racemate, while three quarters of the water of crystallisation was set 

 free. This racemate, however, appears only to exist between 27,2 C. 

 and 36 C., because above 36 C. it is changed into a mixture of 

 sodium-racemate and ammonium-racemate ; both these salts could 

 be obtained from a solution at 40 C. The transition-temperature 

 of a mixture of the right-, and left-handed salts into the two 

 mentioned racemates, lies at about 29, 2 C. 



In this case the transition-temperature was a minimum tempera- 

 ture for the field of existence of the racemate; however, this need 

 not always be the case. Thus, while e. g. potassium-sodium-racemate 

 (-J- 3H 2 0) with its transition-temperature of 6 C., is quite 

 analogous to Scacchi's racemate in this respect 1 ), the rubidium- 

 racemate (+ 2 H 2 0) was found 2 ) to have a transition-temperature 

 of 40, 4 C., this, however, being for it a maximum temperature; so 

 that at temperatures above 40, 4 C. the spontaneous fission into the 

 optically active components occurs, while all the water of crystal- 

 lisation it set free. Evidently the occurrence of such a minimum 

 .or maximum transition-temperature for a racemate, is intimately 

 connected with the algebraic sign of the heat-effect accompanying 

 its formation, - - a fact completely in agreement with the law of 

 mobile equilibrium. In the cases mentioned, the heat-effect is of 

 course related also to the setting free of, or to the combination with, 

 some molecules of water of crystallisation ; but also when this compli- 

 cation does not occur, the explanation as given here, must hold. 



Thus, from an optically inactive solution of the right- and 

 left-handed methylmannosides (mpt: 193 C.), either a mixture of 

 the two active forms, or the racemic compound may be obtained, 

 according to the crystals being deposited below 8 C. or above 



J ) J. H. Van 'tHoff, loco cit. 



2 ) J. H. Van 't Hoff, Vorlesungen it. theov. und phys. Chemie, II. (1899), 

 p. 104; J. H. Van 't Hoff and W. Miiller, Ber. d. d. Chem. Ges., 31, 2206. (1898). 



