DOUBLE SALTS 151 



salt, whilst in others it is the mixture of simple salts. Again, 

 some double salts, such as astrakanite and the double racemate, 

 above mentioned, are formed with the splitting off of water, 

 whilst for others the opposite is the case. Now the splitting 

 off of water of crystallisation is generally a concomitant of 

 heat absorption, the taking up of such water of heat evolution. 

 And hence, in the case of the examples quoted — and it is the 

 same in all others investigated hitherto — whenever the double 

 salt is formed on raising the temperature, the transformation 

 is accompanied by absorption of heat and splitting off of water, 

 and vice versa, relations in perfect agreement with Le Chatelier's 

 theorem. The heat evolution or heat absorption accompanying 

 the formation of the double salt in each case acts in the contrary 

 direction to the temperature change impressed on the system 

 from without. 



4. Isothermal Evaporation. — Much practical importance attaches 

 to the consideration of this subject, i.e. to the effect produced 

 by changing at constant temperature the concentration of a 

 solution which contains two components capable of existing 

 either as two simple salts or as a double salt. Study of the 

 isothermal curves for temperatures without or within the trans- 

 formation interval, makes it possible to foretell all the possible 

 cases, and to arrive at rules which prove valuable guides in the 

 practical processes of crystallisation. 1 To follow out this part 

 of the study of double-salt formation, even in its general outlines, 

 is beyond the scope of this paper ; but one special case calls 

 for mention, because of its great historical interest in another 

 connection. From the point of view of the theory of double salts, 

 this case is an instance of the difference in the results of an 

 isothermal evaporation according as to whether this is carried 

 out below or above the transition point. Pasteur's preparation 

 of / rotatory tartaric acid is justly looked upon as the starting- 

 point of the doctrine of stereo-isomerism, and his own highly 

 dramatic account of the circumstances of this discovery con- 

 stitutes one of the most attractive incidents in the history of 

 science. 2 To Pasteur, crystallisation of a solution of optically 

 inactive sodium-ammonium racemate yielded hemihedral crystals 



1 Van't Hoff, Die Bildung unci Spaltung von Dopftelsalzen, p. 8 et seq. ; 

 Finlay, The Phase Rule, chap. xvi. 



2 Pasteur, Researches in Molecular Asymtnetry, Alembic Club Reprints No. 14 

 p. 20. 



