182 METAL-AMMONIA COMPOUNDS.
directions, optical isomers as they are called.^1 According to van't Hoff,
optical isomerism is caused by the fact that the atoms are arranged in the
molecules so that one compound is the mirror-image of the other, just as the
right hand is opposite to the left in the arrangement of the fingers; one cannot
be superimposed upon the other. Only in such a case can there be optical
isomerism. When, therefore, optical isomerism is detected there must exist
a corresponding spacial grouping of the atoms. The following drawings show
how this can be with a complex molecule containing a cobalt atom around
which the other atoms are grouped
X,Co / /CoA. Both Salts are Tetrammine Compounds. Dissolve some of
each salt separately at the laboratory temperature in concentrated
sulphuric acid and allow the deep-violet solutions to stand for 12
hours. Then add to each, while cooling with ice, somewhat more
than an equal volume of concentrated hydrochloric acid, drop by
drop, until the further addition causes merely a slight effervescence.
Two days later decant the liquid from the small, glistening, green
crystals, shake the latter with alcohol, drain them on a hardened
filter and wash them with more alcohol. Both preparations are
identical and consist of dichlorotetramminecobaltic acid sulphate
(praseo salt). To establish the identity, transform each portion
either into chloropentamminecobaltic chloride (1) or into chloro-
aquotetramminecobaltic chloride (2).- Dissolve small portions of each preparation in dilute ammo-
nia by heating gently. Add concentrated hydrochloric acid little
by little to the purplish-red solutions and then heat for half an hour
on the water-bath. This brings about an almost quantitative sep-
aration of chloropentamminecobaltic chloride in the form of minute
crystals which can be identified by the reactions given in No. 131. - Dissolve a small portion of each of the praseosulphate prep-
arations in water by heating gently, and when the solutions have
turned a deep, violet-red, which takes place after standing for some
time in the cold or more quickly if warm, add an equal volume of
alcohol and some concentrated hydrochloric acid. Let stand for
(^1) A. Werner, Bcr. 44, 1887 (1911) and subsequent papers.