144 



ELEMENTS OF APPLIED MICROSCOPY. 



from a warm, aqueous solution, large, tabular, mono- 

 symmetric (rhombic) crystals form. If, now, the slide 

 be quickly heated by the application of a small flame, 

 the large plates fall to pieces, and minute crystals of 

 the anhydrous salt appear in their place (Fig. 53). 

 On cooling, the small crystals take up water and run 

 together in their original form. 



Fig. 53. — Crystals of Hydrous and Anhydrous Ferrous CHLORroE. 

 (After Lehmann.) 



Behrens and other authors give a large number of 

 specific microchemical tests, some of which the student 

 should make in order to gain practice in the recognition 

 of typical crystalline forms. Aluminium, for example, 

 may be easily detected in a solution by evaporating with 

 a small drop of sulphuric acid, dissolving the residue in 

 water, and adding a grain of caesium chloride. Large, 

 colorless, isometric octahedra of caesium alum are pro- 

 duced, or, if more than 1% of aluminium sulphate be pres- 

 ent, rectangular dendrites sprout from the caesium salt 



(Fig. 54). 

 Calcium is best tested for by precipitation with sul- 



