and Laboratory Methods. 15:>1 



ever, not as satisfactory as the sulphate, particularly 

 in the hands of beginners, for cesium chloride crystal- 

 lizes in the isometric system, thus sometimes leading 

 to confusion. Cesium sulphate, on the contrary, 

 crystallizes in the orthorhombic system. An examina- 

 tion of a preparation with the latter salt, between 

 crossed nicols, will therefore permit of an easy differ- 

 entiation between crystals of cesium sulphate and 

 those of cesium alum. 



Cesium sulphate is not found in the list of reagents 

 heretofore given. It is made from the chloride as 

 follows : 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 sulphur 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 primary cesium sulphate that is formed, and not the 

 normal sulphate as indicated in the reaction given above. 



Test drops containing cesium alum have a great tendency to remain in a 

 state of supersaturation. Often a single large crystal only will appear. In such 

 an event, crushing the crystal and drawing its fragments through the drop will 

 almost invariably yield a large crop of well formed crystals. 



Testing for aluminum with cesium sulphate leaves little to be desired as to 

 accuracy and elegance, but requires a little practice to learn just the proper con- 

 centration. Too dilute a test drop requires very long waiting. Spontaneous 

 evaporation leads almost invariably to supersaturation. Evaporation over the 

 "micro" flame is very unsatisfactory. On the other hand, the addition of the 

 reagent to too concentrated a test drop gives rise to the immediate formation of 

 dendritic masses and skeleton crystals. It is true that the experienced worker 

 ^'ill usually at once recognize these dendrites as due to the presence of aluminum, 

 but in view of the fact that beautiful and far more characteristic crystals can be 

 obtained, the worker should not be satisfied with an unsightly preparation. 



It is because of the difficulties just mentioned that the method of first 

 precipitating the aluminum as hydroxide has been suggested. By this method 

 the operator always knows the concentration of the test drop and the probable 

 amount of free sulphuric acid. Moreover, all other free acids have been removed 

 as well as many objectionable salts, a matter of not a little importance. 



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 sulphate and the cesium alum. It is very seldom that the 

 cesium magnesium double sulphate separates; when it does its crystals are to be 

 referred to the monoclinic system. 



It is of course obvious that in the case of simple substances it is merely 

 necessary to acidify with sulphuric acid and add the reagent. Excellent results 

 can be thus obtained. But this method of procedure requires (1) just the proper 

 concentration, (2) the absence of much free sulphuric acid, (3) the absence of 

 free acids other than sulphuric. 



