of Edinburgh, Session 1877-78. 733 
Hauer * states that mixed crystals of alumina and chrome alum 
grow in a solution of ammonia iron alum ; and, again, he states f 
that the more soluble isomorphous salt completely hinders the solu- 
tion of the less soluble, so that if solutions of common alum, chrome 
alum, and iron alum be mixed, precipitation of the less soluble 
alums will occur. 
The present paper is the result of some experiments made with 
the four alums — potash alumina, ammonia alumina, potash chrome, 
and ammonia iron. 
The solutions of the two first, potash alumina and ammonia 
alumina, were obtained by saturating water at 100° C. with the 
salts, and pouring off the mother-liquor from the crystals that de- 
posited on cooling. 
The chrome alum solution was obtained by saturating water with 
the alum at a temperature carefully kept below that at which the 
green modification of chrome alum is produced. 
Whilst the solution of iron alum was made by saturating water, 
acidulated with sulphuric acid, with the alum, at about 50° C. 
The following experiments were then made and observed : — 
a. When a crystal of potash chrome alum is placed in a solution 
of potash alumina alum exposed to the air, the chromium is turned 
out by the alumina, the interior of the crystal becomes granular, 
whilst a clear shell of alumina alum grows over it, the faces of 
which are finely striated. 
b. When potash chrome alum meal is added to a solution of 
potash alumina alum in a well-closed bottle, and kept at very nearly 
a constant temperature, the chrome alum dissolves, but no replace- 
ment takes place. 
c. When a crystal of potash chrome alum is placed in a solution 
of ammonia alumina alum exposed to the air, the ammonia alumina 
alum grows on it, there being only very slight replacement. 
d. Potash chrome alum meal added to solution of ammonia 
alumina alum in well-closed bottle, and kept at very nearly a con- 
stant temperature, no change takes place. 
e. Ammonia iron alum grows on crystals of potash alumina alum. 
/. Ammonia iron alum grows on crystals of potash chrome alum. 
* “ Sitzungsberichte,” Imperial Academy of Sciences, Vienna, 1860 , xxxix. 
+ “ Sitzungsberichte,” Imperial Academy of Sciences, Vienna, 1866 , liii. 
VOL. IX. 
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