_Repulsion of Bodies._--A remarkable and playful experiment, by which the repulsion of bodies similarly electrified is illustrated, consists in placing a basket containing a heap of small pieces of paper on an insulated stand, and connecting it with the prime conductor of the electrical machine; when the pieces of paper rise rapidly after each other into the air, and descend on the lecture-table like a fall of snow. The effect is greatly increased when a metal disc is substituted for the basket. ORIGIN OF THE LEYDEN JAR. Muschenbroek and Linnæus had made various experiments of a strong kind with water and wire. The former, as appears from a letter of his to Réaumur, filled a small bottle with water, and having corked it up, passed a wire through the cork into the bottle. Having rubbed the vessel on the outside and suspended it to the electric machine, he was surprised to find that on trying to pull the wire out he was subjected to an awfully severe shock in his joints and his whole body, such as he declared he would not suffer again for any experiment. Hence the Leyden jar, which owes its name to the University of Leyden, with which, we believe, Muschenbroek was connected.--_Faraday._ DANGER TO GUNPOWDER MAGAZINES. By the illustration of a gas globule, which is ignited from a spark by induction, Mr. Faraday has proved in a most interesting manner that the corrugated-iron roofs of some gunpowder-magazines,--on the subject of which he had often been consulted by the builders, with a view to the greater safety of these manufactories,--are absolutely dangerous by the laws of induction; as, by the return of induction, while a storm was discharging itself a mile or two off, a secondary spark might ignite the building. ARTIFICIAL CRYSTALS AND MINERALS.--“THE CROSSE MITE.”