At other times a light feathery snow may be seen to fall, composed almost entirely of stars of six spiculæ or radii, united in the centre by a white molecule. These are seldom less than from four to five tenths of an inch in diameter, and are generally collected in tufts of half-a-dozen or more together, which in calm weather waft uninjured to the ground. Sometimes these are mixed with other stars of more intricate figure, to be spoken of presently. Fig. 1 illustrates this variety, and is enlarged to double the proportions of the original.
Sometimes a heavy fall of ordinary snow may be accompanied by a number of minute specks, glistening among the flakes like fragments of talc or mica, as seen sparkling in a mass of granite. On careful investigation these prove to be thin laminated hexagons of the most perfect delicacy and symmetry of form, as shown in Fig. 2.
The hexagon and star being the base of all the crystals of snow yet observed, we will proceed to show how the more elaborate figures are compounded of these two primary elements.
To explain various peculiarities of structure which occur in several of the larger drawings, we will refer to the process of crystallization as carried on at low temperatures on the surface of still or gently-moving water.
Water freezes at an angle of 60°. On its first congelation, under favourable circumstances for observation, we perceive in parts, generally about the centre and around the margin, a corrugation of its surface. This corrugation presently discovers a series of distinct figures, needle-like in form, and analogous to the spiculæ of snow. As the process continues, to each of these needles, while yet forming, a serrated incrustation of leafy or arborescent character is attaching itself, so that in time the greater number of them become each the centre of a crystalline pinna, not unlike a frond of the lady fern. Fig. 25 (page 140) is a sketch of one, the size of the original, as observed by T. G. Rylands, Esq., of Warrington, and sent to us during the severe winter of 1855. The overlapping observable on one side of the pinna is a peculiarity generally to be found in three out of the six leaves forming the entire crystal.
Fig. 26 (page 141) represents the crystal when complete; the drawing was made by ourselves, and gives with great exactitude the figure of the needles, which, it will be observed, diverge from the main stem uniformly at an angle of 60°. The position maintained by them around the centre of the crystal is beautifully adaptive, and well worth examination.