FORMATION OF CRYSTALS. 



?6«J 



natural specimens which the Mineral kingdom affords as Micro- 

 scopic objects ; these should be viewed by reflected light, under a 

 very low power : — 



Antimony, sulphuret 



Asbestos 



Aventurine 



Ditto, artificial 

 Cupper, 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. 



Thin sections of Granite and other rocks of the more or less 

 regularly-crystalline structure adverted to in the preceding para- 

 graph-, also of Agate, Arragonite, Tremolite, Zeolite, and other 

 Minerals, are very beautiful objects for the Polariscope. 



598. 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, pre- Fig. 408. 



viously warmed, a satu- 

 rated 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 crystal- 

 lization will speedily begin 

 at the upper edge, where 

 the proportion of liquid 

 to solid is most quickly 

 reduced by evaporation, 

 and will gradually extend 

 downwards. If 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 

 solidified may be re- dis- 

 solved, after which the 

 process will recommence with increased rapidity. — This interest- 

 ing 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, ai~e 

 made to stand out bright upon a dark ground, by the use of the 



3 D 



Crystallized Silver. 



