In a snow storm the force of the wind and the intensity of the cold usually change all this. The progress of the crystallization, which to be perfect must take place slowly, and under the condition of perfect repose, is at once hastened by the low temperature, and disturbed by the commotion in the air. Across the broad expanse of open plains, along mountain-sides, through groves of trees, and over the smooth surface of frozen lakes and rivers, millions of misshapen and broken crystals are driven by the wind, piled up in heaps, or accumulated in confused masses under the lee of every obstruction, having been subjected on the way to such violence of agitation and collision that the characteristic beauty and symmetry of the material is entirely destroyed.
If we examine attentively the falling flakes, whether of snow showers or of snow storms, at different times, under the varying circumstances in which snow forms and descends, we shall be surprised at the number and variety of the forms which they assume. They may be received and examined upon any black surface—the crown of a hat, or a piece of black cloth, for example—previously cooled below the freezing-point. At any one time the crystallizations are usually alike, but different snow-falls seem to have each its own special conformation. Sometimes, however, a change takes place from one style of flake to another in the course of the same storm or shower, and during the period of transition both varieties fall together from the air. Persons interested in such observations may easily make drawings with a pen of the different forms that present themselves from time to time, and thus in the course of a winter make a very curious and interesting collection.
The number and variety of the forms which the snowy crystallizations assume seem greatest in the polar regions, and the celebrated scientific navigator Scoresby studied them there with great attention during his various arctic voyages. He made drawings of ninety-six different forms, and the number has been increased since, by more recent observers, to several hundred.
It will be observed that all the forms have a hexagonal character. They consist of a star of six rays, or a plate of six angles. There is a reason for this, or rather there is a well-known property of ice in respect to the law of its crystallization which throws some light upon the subject. The law is this: that whereas every crystallizable substance has its own primitive crystalline form, that of ice is a rhomboid with angles of 60° and 120°, and consequently all the secondary forms which this substance assumes are controlled by these angles, and derive from them their hexagonal character.
The most striking of the methods adopted for the inspection of ice crystals is one discovered by Professor Tyndall, and consists of melting the ice from within. This is done by means of a lens, by which the sun's rays are brought to a focus within the mass of ice, so as to liquefy a portion of it in the interior without disturbing that at the surface.
