I.

NOW, in the ordinary acceptation of the word, is suggestive of a soft flocculent matter of considerable opacity, falling in flakes, and, as compared with water, of little density—a foot of fresh-fallen snow producing but from a tenth to a twelfth part of water. Snow, however, does not always fall in flakes; under certain conditions of atmosphere and temperature it occasionally falls in groups of slender needle-like particles or spiculæ, which under the microscope exhibit no structural detail worthy of remark, but are irregular and jagged in outline. This is one of the most imperfect forms of snow crystallization, and occurs generally at a temperature but little above freezing, and at the commencement of a severe and continued frost, or immediately preceding a general thaw.

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.

Fig. 1. Fig. 2. Fig. 3.

Fig. 4. Fig. 5. Fig. 6.

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.

Fig. 7. Fig. 8. Fig. 9.

Fig. 10. Fig. 11. Fig. 12.

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. 13. Fig. 14. Fig. 15.

Fig. 16. Fig. 17. Fig. 18.

[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.

It is not always that the primitive spiculæ are divergent in groups of six. At times they arrange themselves irregularly in clusters, and crystallization proceeds with results of a character

Fig. 19. Fig. 20. Fig. 21.

Fig. 22. Fig. 23. Fig. 24.

somewhat different, but scarcely less beautiful, of which [Fig. 27] (page 142) may be considered a type. This is analogous to the fanciful forms of frost seen on the interior of a pane of glass, and is frequently to be found where the water is very shallow, and where its mixture with some gritty substance, or blade of grass, or other obstruction, has in all probability interfered with a more geometric arrangement. By degrees the whole surface of the water becomes interlaced with needles and pinnæ, whether singly or in groups, and thin laminated surfaces of ice which cover all interstices. Then, according to external influences, the ice either thickens, obliterating all this beautiful tracery, or it melts away before the rising temperature of the day. It often happens, however, that these processes occur after dark, or that the water freezes so rapidly as to disappoint the wishes of the observer. At moderate temperatures these changes are best observed; but, in our opinion, they are somewhat dependent on other atmospheric conditions. The formation of the needles is common to the freezing of water under all circumstances, and they vary from a few inches to a few feet in length.

Fig. 25.

To return to the crystals of snow. [Fig. 3] ([page 136]) is another elementary figure, common to temperatures about the freezing-point; it is not often less than half an inch in diameter, and is a miniature copy of the water crystal.

Fig. 26.

Another simple order of figures, and containing within themselves the germ of the most symmetrical combinations, is that of which [Fig. 4] and 5 ([page 136]) are types; they exhibit secondary spiculæ diverging from the principal radii at an angle of 60°.

Around the simple it frequently happens that a secondary and smaller star is arranged, as in [Fig. 6] ([page 136]), the radii of which are intermediate between those of the former. An angle of 30° is, however, of unfrequent occurrence, and it seems probable that in this and similar cases it is the union of two crystals of distinct hexagonal formation.

Fig. 27.

Sometimes it happens that the secondary spiculæ, which we see in [Fig. 4] and 5, are continued down the main radii until they form a contact with each other, as in [Fig. 7] ([page 137]). The star thus enclosed about the centre generally becomes laminated and of great transparency. In other varieties, as in [Fig. 8] ([page 137]), it is intersected by the rays of the secondary or intermediate crystal.

Having traced the elementary principles of these figures to the first formation of a simple nucleus, we will proceed to the consideration of the more compound varieties, in which the nucleus is a conspicuous element of construction.

Fig. 28.

The figures we have been considering, although possessed of unity of design in a high degree, are found to exhibit no great perfection of structural detail when examined beneath a lens; those that we are about to inquire into belong to a more perfect order, much more minute and very compound.

Fig. 29.

[Fig. 28] is a figure of this class, much enlarged and drawn as seen beneath a microscope. It was highly crystallized, and the angles and planes of which it is composed were sharply and well defined. The prisms at the end of the radii were cut into facets, and glistened with brilliancy, as did the six prisms around the centre. The radial arms were sharply cut, six-sided shafts, very different from the snowy rounded spiculæ of the elementary figures. It was easily discernible to the naked eye, and principally those parts which are white in the engraving, and which communicate to the copy very much the effect of the original when under the full influence of direct light. The centre is laminated, hexagonal in form, and within it we perceive the secondary star of prisms; also that each addition to the radii diverges at an angle of 60°.

[Fig. 29] is another, highly crystallized, and composed of parallel prisms, divergent from the radial arms at an angle of 60°, and without nucleus. The irregular blade-like terminations arise from an ill-advised eagerness in the observation of their originally very complicated structure, by which they were in a moment dissolved, without injury, however, to the symmetry of the figure.

Fig. 30.

[Fig. 30] is a beautiful compound of the higher order of crystallized bodies with the more elementary, the nucleus belonging to the former, and the radii at their extremities to the

Fig. 31.

latter. This at first sight appears an anomaly; but we explain it on the supposition that the entire structure of the original crystal has been of a high order, the shafts six-sided, as they remain still at their base, and the leafy incrustrations to have been regularly distributed prisms, as in the preceding figure; that the crystal, in its descent, has passed through various temperatures of intense cold, probably exchanged for a warmer at one instant of time, in which it has partially thawed, and again passing into a cold stratum in approaching the ground, has been once more congealed, giving rise to the white opacity and irregular form of its terminations. And this explanation is the more reasonable, as will be gathered from a description of the dissolving or thawing of these bodies.

[Fig. 31] is a crystal seen just previous to its returning to the primitive drop of water. Originally composed of the ordinary radial arms, each supporting prisms of the form seen in [Fig. 29], and with a simple hexagonal nucleus, under the influence of a very slightly increased temperature the rigidity of each line has become relaxed, whilst the crystalline matter, all but fluid and no longer heaped up into prisms, is distributed over a wider area, according to the laws of attraction and corresponding area of surface.

Fig. 32.

Fig. 33.

A very different order of figures are those of which [Fig. 32], 33, 34, and 35 are types. The originals were exceedingly small—so minute, indeed, that the specks containing all these beauties of detail were almost inappreciable to the naked eye. It will readily be perceived that they differ greatly from the order arising out of the primitive star or its secondary radii. The base of these must be referred to the hexagon, as shown at [Fig. 2.]The most highly elaborate of our illustrations, shown at [Fig. 33], exhibited a succession of planes raised one above another, the centre of each radial arm intersected by a slender crystalline shaft laden with delicate prisms. [Fig. 35] preserves more the form of the ordinary hexagon, and was cut very regularly into facets. Of [Fig. 34] and [35] we were unable to observe the exact disposition of the raised surfaces, and have delineated the outline only: these figures fell, with several others far more complicated, during the continuance of a very unusual degree of cold for these latitudes.

Fig. 34.