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. 
Fic. 53.—CRysTALs oF HypRous AND ANHYDROUS FERROUS CHLORIDE. 
(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 cesium 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 cesium salt 
(Fig. 54). 
Calcium is best tested for by precipitation with sul- 
