We have spoken of attracting and repelling poles; but a few words of explanation seem desirable. Every magnet possesses two such poles; and if iron filings be scattered over a magnet, each particle becomes also endowed with two poles. Now suppose that similar particles, devoid of weight, and floating in the atmosphere, come together, what will happen? Obviously, the repellent poles will retreat from each other, while the attractive will approach, and ultimately interlock. Further: if the particles, instead of a single pair, possess several pairs of poles arranged at definite points over their surfaces, you can then picture them, in obedience to their mutual attractions and repulsions, building themselves together in masses of definite shape and structure. [Illustration: VARIOUS FORMS OF SNOW-CRYSTALS.] You have, then, only to imagine the aqueous particles in cold calm air to be gifted with poles of this description, compelling the said particles to assume certain definite aggregates, and you have before your mind’s eye the invisible architecture which creates the visible and beautiful crystals of the snow. The important part played by this crystallizing force in ice as well as snow, will be understood from the following remarks by Professor Tyndall, who may justly be described as the most eminent living authority on the subject:-- At any temperature below 32° F.,--that is, freezing-point,--the movement of heat is sufficient to loosen the molecules of water from their rigid bonds of cohesion. But at 32° the movement is so diminished that the atoms lock themselves together, and unite in a solid. This act of union, however, is controlled by well-known laws. To the unintelligent eye a block of ice seems neither more interesting nor more beautiful than a sheet of glass; but to the instructed mind the ice is to the glass what an oratorio of Handel is to the scream of a ballad-singer. Ice is music, glass is noise; ice represents order, glass confusion. In the latter, the molecular forces have brought about an inextricable intertangled network; in the former, they have woven a rich and regular embroidery, the designs of which are infinitely beautiful. Let us suppose ourselves examining a block of ice. In what way shall we get at its structure? A sunbeam, or if that be wanting, a ray of electric light is the anatomist to which we must confide the work of dissection. We direct this ray straight from our lamp across the plate of transparent ice. It shivers into pieces the icy edifice, exactly reversing the order of its architecture. The crystallizing force, for example, had silently and systematically built up atom after atom; the electric ray dislocates them (so to speak) just as silently and systematically.