390 
ELEMENTARY CHEMICAL MICROSCOPY 
present, leading to a modification of the crystal form or to 
colored solid solutions. 
Precautions. 
Although it is obvious that in the case of simple compounds 
converted into sulphates it is merely necessary to add the reagent 
and allow the preparation to crystallize, it is essential that due 
regard be paid to (i) just the right concentration, (2) the absence 
of much free sulphuric acid, (3) the absence of other free mineral 
or organic acids, (4) the absence of colloidal substances. 
To avoid most of these difficulties it is always advisable to 
proceed as follows: To the drop to be tested add ammonium 
hydroxide in slight excess, decant the solution and wash the 
gelatinous precipitate with water. Then add a drop of water 
and follow it with a very little dilute sulphuric acid, only just 
enough to dissolve the aluminum hydroxide. Warm gently; 
cool, and to the drop add a fragment of the reagent. After a few 
seconds, beautiful large crystals of cesium alum separate. 
Cesium chloride can be employed as reagent, providing that 
the solution to be tested contains a little free sulphuric acid. The 
chloride is, however, not as satisfactory as the sulphate, particu¬ 
larly in the hands of beginners, for cesium chloride crystallizes 
in the isometric system, thus sometimes leading to confusion. 
Cesium sulphate, on the contrary, crystallizes in the ortho¬ 
rhombic system. An examination of a preparation containing 
the latter salt, between crossed nicols, will therefore permit of an 
easy differentiation between crystals of cesium sulphate and 
those of cesium alum. 
If cesium sulphate is not at hand it may be prepared from the 
chloride in this manner: Place a drop of sulphuric acid at the 
corner of a slide or on platinum foil; add a small crystal of 
cesium chloride and evaporate to dryness. If no fumes of sul¬ 
phur trioxide escape, add another drop of acid and heat again. 
It is evident, that by this method of treatment, in the majority 
of cases, it is in reality primary cesium sulphate that is formed, 
and not the normal sulphate as implied above. Care must there¬ 
fore be exercised in its use. 
