MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 343 



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. 



Cesium alum is one of a group of double sulphates known as 

 " alums," having the general formula M 2 (SO4) 3 N2SO4 24 H 2 O, 

 where M can be Al, Cr, Mn, Fe, In, Ga, Tl; and N- 

 Na, K, Rb, Cs, NH 4 , 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 crystallizable 

 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 crystallizable. Sodium, 

 potassium and ammonium sulphates readily unite to form more 

 or less crystallizable 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 

 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 



