20 STRUCTURE OF MINERALS. 



up, as that for our tables, still the grains were crystalline in 

 their origin and are crystalline in structure. 



This subject may be turther illustrated by many other sub- 

 stances. A hot solution of sugar set away to cool, will form 

 crystals upon the bottom, or upon any thread or stick in the 

 vessel; and these crystals will continue increasing till a 

 large part of the sugar has become crystals. It is a com- 

 mon and instructive experiment to place a delicate frame- 

 work of a basket or some other object, in a solution of su- 

 gar or alum ; after a while it becomes a basket of finished 

 gems, the crystals glistening with their many polished facets. 

 Again, if a quantity of sulphur be melted, it will crystallize 

 on cooling. To obtain distinct crystals, the surface crust 

 should be bjsoken as soon as formed, and the liquid part 

 within be poured out ; the cavity, when cold, will be found 

 to be sludded with delicate needles. The crust in this case 

 is as truly crystallized as the needles, although but faint tra- 

 ces of a crystalline texture are apparent on breaking it. 

 This was owing to too rapid cooling. Melted lead and bis- 

 muth will crystallize in the same manner. There is a sub- 

 stance, iodine, which when heated passes int© the state of a 

 vapor ; on cooling again, the glass vessel containing the 

 vapor is covered with complex crystals, as brilliant as pol- 

 ished steel. During the cold of winter, the vapors constitu- 

 ting clouds, often become changed to snow ; this is a similar 

 process of crystallization, for every flake of snow is a con* 

 geries of crystals, and often they present the forms of regu- 

 lar six-sided stars. So also, our streams become covered 

 with ice ; and this is another form of the crystallization of 

 water. 



The power which solidifies, and the power which crystal- 

 lizes, are thus one and the same. Crystallography, there- 

 fore, is not merely a science treating of certain regular so- 

 lids in Mineralogy; it is the science of solidification in 

 general. 



Modes of Crystallization. In the above examples we 

 have presented three different modes of crystallization. In 

 one case, the substance is in solution in water, (or -some sol- 

 vent ;) the particles are thus free to move, and as the solvent 

 passes off by evaporation, they unite and form the crystal- 



Explain the case of sulphur. Give instances or crystals forming from 

 vapor. What does the science of crystallography embrace ? What 

 are the modes of crydaljization alluded to in the examples given ? 



