4. Hexagonal kind of snow-crystal I have but once seen. A variety, consisting apparent ly of a triangular pyramid, was observed; but whether its base was a triangular or six-sided figure, similar to No. 96, is doubtful. These pyramids were about the thirtieth part of an inch in height, and fell along with some other curious figures, during a fresh gale of wind From the northward, in very large quantity. Figures 41, and 47, represent this kind or crystal.
5. Spiculx or prisms haring one or both extremities inserted in the centre of a lamellar crystal.—This is the most singular genus 1 have ever seen, and has been observed but twice. It resembles a pair of wheels, united by au axletree; the wheels consisting of hexa gonal or other lamellar crystals, and the axle of a slen der prism. Fig. 43, '15, 46, and 43, represent this modification of snow-crystal. Fig. 46, consists of but one tabular crystal and a prism: and Fig. 45, of three lamina and two prisms. The length of this was one-sixth of an inch: of the other kind, from one thirtieth to one-tenth. Some of this extraordinary figure occurred along with the last-described genus: of which kinds, principally, a quantity of snow three or four inches in depth, once fell on the deck of the ship in which I sailed, in the course of a few hours. The temperature, when this kind of fell, was in one instance 22°, and in the other 20'.
Plate CCCCI.XXXVIII. contains representations of ninety-six different snow-crystals, magnified From thirty to about four hundred times. The Italic letter following the number of the figure, refers to the se cond column of the annexed Table, by which, the state of the atmosphere and weather, when each crys tal was observed, may be seen. The fractional num her which succeeds the Italic letter, shows the diame ter of the crystal in parts of an inch. The largest crystal represented was one-third of an inch diameter; the smallest one-thirty-fifth. They were all perfect figures. Many instances, it may be observed, occur of mutilated and irregular specimens; sonic wanting two or three radii, and others having radii of differ ent sizes and shapes. But in low temperatures, the
greatest proportion of crystals that fall are probably perfect geometrical figures. This constant regard to equality in the form and size of the six radii of the stellates; the geometrical accuracy of the different parts of the hexagons; the beauty and precision of the internal lines or the compound figures, with the proper arrangement of any attendant ramifications, and the general completion of the regular figure,— compose one of the most interesting features in the Science of Crystallography.
The crystals thus described by Mr. Scoresby, and those delineated by Dr. Nettis, are obvious ice com pound crystals: but they afford us no clue to deter mine the primitive form or the simple crystals of which they consist. This form has generally been supposed lobe the rhombohedron; but Mr. Mobs and Mr. Haidinger maintain, that there is not, in the whole compass of rhombohedral forms. an example of the same formation as the stars with six radii of snow, while it is common enough in those species which belong to the prismatic system." From Dr. Brewster's experiments on ice,(See our article on Ice,) it necessarily follows that the form of crystallizell water must, as he has inferred, belong either to the rhombohedail or to the pyramidal system. The pris matic system being thus excluded, so far at least as ice is concerned, Mr. Haidinger considers the pyra midal form as the most probable one.%from the cir cumstance of tin ore and rutile, which belong to the pyramidal class, producing, by regular composition, crystallizations similar to the star-like figures of snow. That this is the case, has been put beyond a doubt by more recent observations by Dr. Brewster, made dur ing the frost of the 18th and 19th November 1827, in which he found numerous and regular quadrangular plates, in the hoar frost, crystallized upon leaves and stones.