CRYSTALLIZATIONS. 109 



When the snow descends in a calm atmosphere, the constituent crystals of 

 the flake are perfectly developed, but any agitation of the air, or an increase in 

 moisture or temperature, destroys their delicate structure. The single crystals 

 always unite at angles of thirty, sixty, and one hundred and twenty degrees, but 

 by their different modes of union, give rise to several hundred distinct varieties. 

 Scoresby, a celebrated Arctic navigator, has enumerated no less than six hundred 

 kinds, and delineated ninety-six ; and Kaemtz, a German meteorologist, has ob- 

 served twenty more, not figured by Scoresby. 



Although the varieties are so numerous, they are all comprised under 

 five principal classes, which are distinguished as follows : First crystals in 

 the form of plates, very thin, transparent, and of a delicate structure. This 

 class includes many remarkable varieties, which are represented by the first 

 twenty-five forms in cut 164. Secondly flakes either possessing a spherical 

 nucleus, or a plane form studded with needle-shaped crystals, like the 26th 

 figure in the cut. Thirdly slender, prismatic crystals, usually six-sided, but 

 sometimes having only three sides. Fourthly pyramids with six sides, as 

 shown in figure 27. Fifthly prismatic crystals having, perpendicular to their 

 length, both at the ends and in the middle, thin six-sided plates as delineated 

 in figures 28, 29, and 30. The last two classes are extremely rare, Scoresby 

 having observed the fifth but twice, and the fourth only once in all his voyages. 



The crystallization of aqueous vapor is beautifully displayed when a thin film 

 of moisture is frozen upon a window pane. Then, in addition to single, star-like 

 crystals, exquisite branching configurations are seen, extending their glittering 

 lines in all directions. When water, in a body, begins to freeze, similar results 

 occur, and at such times, along the edge of a rivulet, long, needle-shaped crys- 

 tals will be seen, darting from the ice that fringes the bank towards the centre 

 of the stream, and which, rapidly interlacing with each other, soon unite into 

 one compact mass. Often, upon raising a thin sheet of ice from the water, the 

 under surface will be observed covered with a network of crystals. The snow, 

 on account of its light and branching crystallization, descends softly upon the 

 earth, clothing its surface with a fleecy mantle, which effectually shields the 

 tender plants from the inclemency of the wintry season. If it had been ordered 

 otherwise, and all the moisture, that now forms the snow, had fallen in solid 

 masses of ice, like hail, the evils which would have arisen under such a pro- 

 vision in the economy of nature, must have been many and great. 



ON CRYSTALS FOUND IN PLANTS. 



It has been proved, by the microscopic examinations of distinguished natural- 

 ists, that saline substances are spontaneously crystallized within the cells of plants ; 

 the crystals having been found existing in infinite numbers throughout the bark, 

 wood, and leaves of a great variety of trees and shrubs. The facts stated in 

 this section are mostly taken from an interesting paper, read before the 

 Association of American Geologists, by Professor J. W. Bailey, of West Point, 



