770 



FORMATION OF CRYSTALS. 



Spotted Lens, the Paraboloid, or any other form of Black-ground 

 illumination ; still more beautiful is the spectacle when the Polar- 

 izing apparatus is employed, so as to invest the crystals with the 

 most gorgeous variety of hues. Very interesting results may often 

 be obtained from a mixture of two or more Salts, and some of the 



Fig. 409. 



Radiating Crystallization of Santonine. 



Double Salts give forms of peculiar beauty. * A further variety 

 may be produced by fusing the film of the substance which has 

 crystallized from its solution ; since on the temperature of the 



* The following directions have been given by Mr. Davies ("Quart. 

 .Toum. of Microsc. Science," N.S., Vol. ii., 1862, p. 128, and Vol. v., p. 205) 

 for obtaining these. " He makes a nearly saturated solution, say of the 

 double Sulphate of Copper and Magnesia ; he drives rapidly a portion on 

 a glass slide, allowing it to become hot so as to fuse the salt in its water 

 of crystallization ; there then remains an amorphous film on the hot glass. 

 On allowing the slide to cool slowly, the particles of the salt will absorb 

 moisture from the atmosphere, and begin to arrange themselves on the 

 glass, commencing from points. If then placed under the Microscope, 

 the points will be seen starting up here and there ; and from those centres 

 the crystals may be watched as they burst into blossom and spread their 



