If you interrogate geometry, it will reply to you that, of all the polygons inscribed in a circle (Fig. 6, _c_), there is but one whose sides are equal to the radius of that circle; and this polygon is exactly that of the bee and wasp's cell. Here, then, is a very singular coincidence. If you afterwards examine very minutely the work of the bee, you will find in each cell of the honeycomb a pyramidal base, composed of three equal rhombs, whose angles solve a grand geometrical problem, that of giving the maximum of space with the minimum of matter. The papier-maché combs of the wasp are formed of a single row of cells, each of which has a nearly level bottom. This is all that is required; for these cells are destined, not for the reception of honey, but only of the larvæ, the offspring of their architects. Do not think that you have but to pick up a thumbful of snow to procure your crystals! These change their form very quickly, and it is almost impossible to detect it in snow which has remained for any length of time upon the ground. The great flakes which fall in relatively mild weather, when the temperature borders upon freezing-point, are often nothing better than masses of small amorphous atoms of ice; to get at the crystals, you must remove the kind of icy varnish which encases them. * * * * * For the accurate observation of the crystallisation of water which precipitates itself in the air, we have at our disposal a means as simple as convenient--a pane of glass. All we have to do is to arrange everything in such a manner that the congelation shall be both slow and certain; on this condition alone can we obtain well-defined crystals. A cold room is best adapted for this kind of experimentation; and thus you will frequently see deposited upon the window-glass, in an uninhabited chamber, some exceedingly graceful designs, as follow. [Illustration: FIG. 7.] These are asteriæ,--arborescent, and leaf-like crystals,--imitating the beautiful foliage of ferns and mosses. The severer the cold, the more regular, be it understood, is the formation of these crystals. Owing to its dazzling whiteness, snow is a great reflector of light, and singularly illuminates the darkness of the winter nights. The long dreary nights of the polar world are lit up by the glories of the magnetic auroras, joined to the radiancy of the snow. This induces us to repeat a question which we have often addressed to ourselves, namely,--under what aspect must the very varied changes which the solar light experiences on the surface of our planet be presented to the inhabitants of Mars and Venus? A more attentive observation of the ashen-gray light of the moon, which appears to be principally produced by the reflection of the more or less luminous face of the earth, may perhaps one day provide us with an answer to our question.