Silica has hitherto been considered to be insoluble in pure water; at least M. Bischoff states that only one part of silica dissolves in 769,230 parts of water; but by a method hereafter to be explained, Professor Graham has actually obtained a limpid solution of silica in pure water. Boron is a constituent of boracic acid, a natural production in Thibet and Monte Corbalo in Tuscany. It is a greenish-brown solid, insoluble in water, but when heated to about 600° it burns in open air with a vivid flame. Fluorine is a constituent of a very beautiful mineral, well known as fluor spar, which is found in cubic crystals of a green, yellow, or purple colour. Hydrofluoric acid obtained chemically from the mineral is highly volatile and extremely corrosive. Three of the non-metallic simple substances, chlorine, bromine, and iodine, are connected by the most remarkable analogies. They are marine productions, for chlorine is obtained from common sea-salt and in greater purity from rock-salt, both of which are compounds of chlorine and the metal sodium. When sea-water is evaporated, salt and a substance called bittern remain, which contains a salt whence bromine is separated. Again, when kelp, the ashes of burnt seaweeds, is purified from the carbonate of soda and the chloride of potassium, a salt is left which is the iodide of potassium, whence iodine is obtained. Iodine is also found in sponges, oysters, and other low sea animals, as well as in certain mineral springs, and sometimes in combination with silver. These three elemental bodies have little affinity for one another, but they combine powerfully with other substances. Chlorine is a yellowish-green gas, twice as heavy as atmospheric air, with a noxious suffocating smell and astringent taste. It has a powerful bleaching property, and when combined with water, which absorbs twice its volume of the gas, it is used for bleaching linen, in calico-printing, and other arts. The clear solution of chloride of lime is still more in use for the same purpose, as well as for an antidote against contagion and unwholesome smells. Carbon does not burn in chlorine gas, yet it is capable of supporting combustion, for oil of turpentine, phosphorus, thin leaves of tin and copper, and powdered antimony, take fire spontaneously in it. This gas shows its power by the development of intense heat, but not by brilliant light, because the results of its combustion are mostly vapours, or such gases as have a feeble illuminating power; so chlorine differs materially from oxygen in the phenomena of combustion. Mr. Faraday observes, however, that the bleaching powder is analogous to ozone in being an intermediate state, for chlorine is pernicious and violently destructive as a gas, perfectly innocuous and quiescent in common salt and in its other natural combinations, while in the bleaching substances its energy is subdued by art, so as to make it an important agent in various manufactures.