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 dry ness. 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. 



