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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. [sothermal Evaporation.—Much practical importance attaches 
to the consideration of this subject, ze. 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.? To Pasteur, crystallisation of a solution of optically 
inactive sodium-ammonium racemate yielded hemihedral crystals 
‘Van’t Hoff, Die Bildung und Spaltung von Dopfelsalzen, p. 8 et seg. ; 
heel The Phase Rule, chap. XV. 
® Pasteur, Researches in Molecular Amends ‘y, Alembic Club Reprints No. 14 
Pp. 20, 
