Of these classes the first is called " lamellar," and is divided into many different species: one of the latter ia a thin transparent hexagonal plate, or a hexagonal plate with white lines parallel to the sides of the polygon, and sometimes there is a starlike figure in the centre; the magnitudes vary, and the greatest is about inch diameter. Another species, and this is the most ordinary appearance of snow, is the atelhifonn ; the figures 1, 2, and 3 represent some of the most remark varieties of this kind ; its magnitude varies, but the diameter of the greatest is about inch, and it occurs most abundantly when the tem perature of the air is near the freezing point of water. Sometimes the stars appear to have twelve points, but Dr. Scoresby thinks that there are formed merely of two stellar plates applied one on the other The six following figures represent assemblages of hexagonal crystals the diameters of the first two kinds are respectively and inch and those of the rest are inch diameter ; they are usually formed at temperatures between and 20' (Fehr.).
The second class is also lamellar, but it differs from the former in having a spherical nucleus, either transparent or white, about inch diameter ; and sometimes spienlar radii proceed from thence in different directions at angles of 60° with each other. The temperature at which this class is formed varies also from the freezing-point to 20° (Fans).
The third class consists of spiculre or six-sided prisms ; of these, the finer sort, which are formed at the temperature of 28° resemble white hairs very delicate and clear, and about 1 inch long ; the coarser kinds are formed in the lower region of the atmosphere, at about the freezing temperature.
The fourth class is of a pyramidal form and about g, inch high, but Dr. Scoresby could not determine whether the base was triangular or hexagonal The fifth class consists of hexagonal crystals united together by a slender spieular crystal, so as to resemble two wheels with an axle. Both of these kinds are very rare. Dr. Scoresby saw the latter only twice and the former only once.
31. Huber Burnand, speaking of the character of the snow which fell at Yverdun in 1829 and 1S30, states that it was crystallised in stellar plates with six rays, along each of which were disposed filaments arranged like feathers, and these again supported finer filaments similarly arranged ; the plates, which were extremely thin, were per fectly plane and regular. e Rib!. Univ.,' 1830.) It is also related in the same work, that in 1829 the frost at Yverdun assumed every (lay a different form, being sometimes disposed in parallel groups or fillets ; sometimes it resembled leaves, and occasionally spines about an inch long, which were terminated by a flat rosette with six divisions.
The severe weather experienced in the vicinity of London, and over the south-west and eastern parts of England at the beginning of the year 1855, of which no parallel had taken place since that of 1814, which it greatly resembled, as well for depression of temperature as for the duration of the frost, was remarkable also for the peculiar character and continuous fall of snow, which first made its appearance on January 16th, and lay on the ground till after the end of February.
We are indebted to Mr. Glaisher, the Secretary of the British Meteoro logical Society, for a particular account of this snow and its crystals, which is annexed to the Report of the Society read at the fifth annual meeting, May 22, 1855. Much of this snow, 31r. Glaisher observes, was " of that peculiar character which former writers designated Polar Snow, it having been chiefly composed of crystalline particles, which they supposed to be confined, with rare exceptions, to the Arctic regions. This supposition, however, is not supported by the recent prevalence of innumerable crystals, that have exhibited a degree of crystalline formation equal to any that has been recorded in colder latitudes The primary figure or base of each crystal I mined to be a star of six radii, or a hexagon of 'amines, and the com pound varieties to include combinations of spicule, prisms, cubes, and rhomboids, aggregated upon and around the central figure, according to the degree of its complexity." The paper is illustrated by 88 small and 63 greatly enlarged figures of the snow crystals, the joint pro ductions of the pencils of Mr. and Mrs. Glaisher, and probably the most valuable series of such representations extant. The figures above, copied from Dr. Scoresby, 6, 7, and 8, it will be observed, have an inner tracing of the hexagon. Similar crystals to these were observed by Mr. Glaisher, who had not seen them previously, nor any figures of them, except those of Dr. Scoresby of those seen by him in the Arctic Seas. But the forms of snow-crystals are doubtless dependent on t he temperature of the air and the amount and distributiou of the aqueous vapour it contains, the differences iu those respects of the different strata of air, the interchange of currents, and other physical cir cumstances, and not otherwise upon difference of locality than as that may involve a difference in those circumstances. The forms ordinarily characteristic of the snow of one latitude or region may be produced in any other, wherever snow can occur. It does appear, however, that the most regular and complex stellate aggregations accompany the lowest temperatures. The extraordinary complexity of many of these crystals of snow; when considered in connection with the principle of regelation which governs the union of separate portions of ice, leads to the suspicion that infinitesimally small crystals result from the solidification of vapour, which undergo accretion and are subse quently incorporated with each other by regelation, in directions accordant with those of the crystalline forces to which the elementary forms are due. The crystallography, however, of solid water, or ice, which of course is that of snow, is at present in a condition of singular contradiction. We appear not to know with certainty to what system of crystallisation it should be referred, or even whether it does not belong to more than one system; though there is strong evidence that it must belong, in part at least, to the rhombohedral system, to which it has usually been referred. The facts known respecting it will he stated with the other properties of ice, in the article WATER.