Crystallization. 357 



the crystal. Figure 155 shows dendritic or tree-like crystallizations, due 

 to polar action in the placing of molecules. The dendrite on the left is 

 a natural formation from Painted Rock, Montana. The rock is full of 

 the slightest of checks or cracks made by unequal settling or tilting of 

 the strata, and into these, water holding minerals in solution has infil- 

 trated and the particles of the minerals have become arranged according 

 to their polarities into the most beautiful forms. The silver trees on 

 the right of the fig. are formed by artificial electric currents in the pro- 

 cess of electrolysis. The same effects are seen in the frost foliage 

 formed on our windows in winter, and in the formation of leaves on veg- 

 etation in summer. 



FIG. 155. 

 FIG. 155. Metallic Crystallization. 



a. Natural Dendrite from rocks near Painted Rock station, Montana. 



b. Silver tree formed in a weak solution of silver nitrate by means of an electric cur- 

 rent (with platinum poles). 



c.Same formed in a solution a little stronger. 



The " trees " start at the negative pole of the battery (d) and grow toward the positive (e). 



Tin trees can be raised the same way from a solution of tin chloride ; and lead trees 

 from a solution of lead acetate. Crystalline dendritic forms are also produced from 

 chloride of ammonium, &c. 



"A solution saturated at a high temperature may, under certain cir- 

 cumstances, be cooled down several degrees without depositing crystals, 

 but the introduction of a crystal of the substance causes the whole to 

 solidify instantly into a crystalline mass. The phenomenon is easily ex- 

 hibited with Glauber's-salt. " That this difference is only one of degree, 

 however, is shown by the fact that crystallization will take place ' ' spon- 

 taneously " if the temperature be still further lowered. Crystals formed 

 at one particular temperature and then exposed to another temperature 

 at which crystals of another kind are produced, often lose their trans- 

 parency and without alteration of external form become changed into an 

 aggregate of small crystals of the latter kind. Examples of this altera- 

 tion of structure are afforded by sulphur, carbonate of calcium, mercuric 

 iodide, and many other bodies. " Mercuric iodide when sublimated at a 

 gentle' heat forms in scarlet tables belonging to the dimetric system. 

 But at a higher temperature its sublimate is in yellow rhombic tables of 

 the monoclinic system. The red crystals turn yellow when heated and 

 resume their red tint on cooling. The yellow crystals retain their color 



