MICROCHEMICAL REACTIONS OF ALUMINUM 
389 
In the presence of magnesium sulphate there is formed a double 
sulphate of magnesium and cesium; hence in dealing with such 
cases it is necessary to add a sufficient amount of cesium sulphate 
to permit of the formation of both the cesium magnesium sul¬ 
phate and the cesium alum. It is very seldom that the cesium 
magnesium sulphate separates; when it does the crystals are to 
be referred to the monoclinic system. 
Manganous sulphate will likewise form a double sulphate with 
cesium sulphate separating in monoclinic crystals. 
Double sulphates of cesium may also form in the presence 
of sulphates of Cu, Cd, Zn, Ni, Co and Mg, in very con¬ 
centrated solutions; but in all cases the crystals are aniso¬ 
tropic prisms which cannot be confused with the crystals of 
cesium alum. 
Cesium alum is one of a group of double sulphates known as 
‘'alums,” having the general formula M2(S04)3 • N2SO4 • 24 H2O, 
where —M— can be Al, Cr, Mn, Fe, In, Ga, Tl; and —N— 
Na, K, Rb, Cs, NH4, Ag, or Tl. All alums are isomorphous, 
and are to be referred to the isometric system. Theoretically, 
therefore, one would be led to expect that the presence of ele¬ 
ments capable of taking the place of aluminum in alums would be 
liable to interfere with the test for aluminum. But in addition to 
their property of being able to replace aluminum in these double 
sulphates, we must consider the crystallizing power of the com¬ 
pounds formed. It is herein that lies the explanation of the 
value of cesium sulphate over and above that of any other of the 
sulphates we might be inclined to select. Of the above listed 
alum-forming elements, aluminum is the only one which unites 
with cesium or rubidium sulphates to form easily crystallizahle 
alums. The other elements unite with these two sulphates only 
with difficulty, and the alums formed can be regarded, from a 
microchemical standpoint, as difficultly crystallizahle. Sodium, 
potassium and ammonium sulphates readily unite to form more 
or less crystallizahle alums with the other alum-forming elements 
as well as with aluminum. 
Not infrequently it will be found that cesium alum has a 
marked tendency to adsorb various substances which may be 
