152 SCIENCE PROGRESS 
of two kinds, which when separated by a process of picking 
gave two sodium-ammonium tartrates, identical in all respects, 
except that whilst one was dextro-rotatory like the salt of 
ordinary tartaric acid, the other was levo-rotatory to exactly 
the same amount. The failure of subsequent investigators to 
produce in crystallisation anything but the double racemate led 
to a thorough investigation of the whole subject,! and to the 
establishment of the fact that above 28° the double racemate 
crystallises out, whilst crystallisation below 28° yields a mixture 
of the two tartrates. Hence it has been said that Pasteur owed 
his discovery to the happy chance? that he carried out his 
evaporation below 28°. 
5. The Carnallite Problem—This is a case of technical 
importance. The potassium chloride used as the starting-point 
in the manufacture of the many potassium compounds of com- 
mercial importance (carbonate, hydrate, cyanide, nitrate, etc., 
etc.) is found native as the mineral sy/vite, but by far the larger 
amount of that required is obtained from the mineral carnallite, 
KCl. MgCl,.6H,O. This salt has its transition point at 168°, 
when it breaks up into KCl, MgCl,.2H,O and 4H,O. Owing 
to the great difference in solubility between the constituent 
chlorides, the transition interval is so extended that at ordinary 
pressure there is no temperature at which carnallite can be 
treated with water without partial decomposition and separation 
of the less soluble KCl as a solid phase. This is the principle 
underlying the old empirical process of the separation of the 
KCl, which is carried out as follows: 
Treatment with water until a solution is obtained, which at 
25° is saturated with carnallite, results in the separation of 
about five-sixths of the KCl in the solid form. The solution 
saturated for the double salt is evaporated, when carnallite 
crystallises out, and a solution is produced which for 1,000 
molecules of H,O contains 105 of MgCl, and 2 of KCl, and 
which therefore, for practical purposes, may be considered a 
solution of pure magnesium chloride. This solution is disposed 
1 Van'’t Hoff, loc. cit., p. 82. 
* Considering the popular meaning of the term “chance,” one would wish 
to protest against its use in a connection such as the above, and to add some 
such parenthesis as: “chance, which in the matter of great scientific discoveries 
with unfailing discrimination and unbroken regularity selects for the recipient of 
its gifts only the truly great.” 
—— 
