FORMATION OF CRYSTALS. 



771 



glass slide during the solidification will depend the size and 

 arrangement of the crystals. Thus Santonine, when crystallizing 

 rapidly on a very hot plate, forms large crystals radiating from 

 centres without any undulations ; when the heat is less consider- 

 able, the crystals are smaller, and show concentric waves of very 



Fig. 410. 



Radiating Crystallization of Sulphate of Copper and Magnesia. 



decided form (Fig. 409), but when the slip of glass is cool, the 

 crystals are exceedingly minute. It would seem as if these last 

 results were due to interruptions in the formative process at cer- 

 tain points, consequent upon the hardening influence of cold, and 



petals on the plate. Starting-points may be made at pleasure, by touch- 

 ing the film with a fine needle to enable the moisture to get under it ; 

 but this treatment renders the centres imperfect. If allowed to go on, 

 the crystals would slowly cover the plate, or if breathed-on they form 

 immediately ; whereas if it is desired to preserve the flower-like forms 

 on a plain ground, as soon as they are large enough development is 

 suspended by again applying gentle heat ; the crystals are then covered 

 with pure Canada balsam and thin glass, to be finished off as usual. The 

 balsam must cover the edges of the film, or moisture will probably get 

 under it, and ciystallization go creeping on." 



