CRYSTALLIZATION OF SALTS. 



645 



Antimony, sulphuret. 



Asbestos. 



Aventurine. 



ditto, artificial. 

 Copper, native. 



arseniate. 



malachite-ore. 



peacock-ore. 



pyrites (sulphuret). 



ruby-ore. 



Iron, ilvaite or Elba-ore. 



pyrites (sulphuret). 

 Lapis lazuli. 

 Lead, oxide (minium). 



sulphuret (galena). 

 Silver, crystallized. 

 Tin, crystallized. 



oxide. 



sulphuret. 

 Zinc, crystallized. 



455. The actual process of the Formation of Crystals may be 

 watched under the microscope with the greatest facility ; all that 

 is necessary being to lay on a slip 

 of glass, previously warmed, a FIG. 345. 



saturated solution of the salt, and 

 to incline the stage in a slight 

 degree, so that the drop shall be 

 thicker at its lower than at its 

 upper edge. The crystallization 

 will speedily begin at the upper 

 edge, where the proportion ot 

 liquid to solid is more speedily re- 

 duced by evaporation, and will 

 gradually extend downwards. It 

 it should go on too slowly, or 

 should cease altogether, whilst yet 

 a large proportion of the liquid 

 remains, the slide may be again 



warmed, and the part already soli- crystallized saver. 



dified may be redissolved; after 



which the process will recommence with increased rapidity. 

 This interesting spectacle may be watched under any microscope ; 

 and the works of Adams and others among the older observers, 

 testify to the great interest which it had for them. It becomes 

 far more striking, however, when the crystals, as they come into 

 being, are made to stand out bright upon a dark ground, by the 

 use of the spotted lens, the paraboloid, or any other form of 

 black-ground illumination ; still more beautiful is the spectacle 

 when the Polarizing apparatus is employed, so as to invest the 

 crystals with the most gorgeous variety of hues. The following 

 list specifies the salts and other mineral substances, whose crys- 

 talline forms are most interesting. When these are viewed with 

 polarized light, some of them exhibit a beautiful variety of colors 

 of their own, whilst others require the interposition of the 

 selenite plate for the development of color. 



Acetate of Copper. 



of Manganese. 



of Soda. 



of Zinc. 



Agate (transparent sections). 

 Alum. 



Arragonite (transparent sections). 

 Arseniate of Potass. 

 Bicarbonate of Potass. 

 Bichromate of Potass. 

 Bichloride of Mercury. 

 Boracic acid. 



