by boiling with a solution of hydrofluosilicic acid. The process is tedious, and requires care. Perchloric acid, CIO,, is obtained from the perchlorate of potash, KO, CIO,. The chlorates in solution give no precipitate with nitrate of silver. When heated they evolve oxygen; and if treated with sulphuric acid they evolve the characteristic yellow gas as mentioned under hypochloric acid. BROMINE. Symbol, Br; Equivalent, 78.26. This element is found in sea-water. At ordinary temperatures it is a brownish-red liquid, having a disagreeable odour, closely resembling chlorine and iodine in character. It is found in combination with magnesium in sea-water, as the bromide of magnesium, MgBr. It is obtained by agitating any solution of bromine with ether. After the less soluble salts have been separated, a stream of chlorine is sent through the solution, which decomposes the bromide of magnesium, and ether being added, dissolves the liberated bromine. The liquid is transferred to a retort with a receiver, and heat applied, when the bromine condenses in the cold part of the retort and receiver. Bromine, when associated with water has bleaching properties. Hydrobromic acid, HBr, is analogous to hydriodic acid, has nearly the same properties, and may be prepared by substituting bromine for iodine, in the preparation of hydriodic acid. Till lately there was only one known oxide of bromine, bromic acid, BrO,, which is prepared by adding bromine to a solution of caustic potash, until all alkaline properties are destroyed. The change is the same as that described under chloric acid. A few months ago a compound resembling CIO was discovered, and on analysis found to have the formula BIO-hypobromous acid. IODINE. Symbol, I; Combining Equivalent, 127. At ordinary temperatures, iodine is a bluish-black metallic-looking substance, having the appearance of scales of black lead. It occurs chiefly in combination with sodium and potassium in sea-water. It is a constituent of marine plants and animals. It is chiefly prepared from the ashes of sea-weeds, called kelp. The ashes are mixed with water, filtered and evaporated, until the solution is reduced to a very small quantity: the salts of sodium, potassium, and magnesium being removed as they crystallize. The mother-liquor left after the removal of these salts, contains the iodine in combination with magnesium and sodium. This is placed in a leaden retort with sulphuric acid and the binoxide of manganese. On the application of heat the iodine distils over as a dark purple vapour, which is condensed. This process may be imitated by placing in a retort connected with a receiver, a solution of the iodide of potassium, a little powdered binoxide of manganese, and sulphuric acid: on the application of heat, a beautiful violet vapour passes over, which condenses in the cold part of the retort and receiver in small shining scales. Decomposition : : KI+MnO, + 2(HO,SO,) = KO,SO, + MnO,SO, + 2HO+ I; or, (KI + MnO, + 2(H,SO) = K,SO4 + Mn,SO. + 2HO+I). Iodine stains the skin; it acts violently on the animal system, and is used in medicine. When iodine is free, it produces a splendid blue colour in contact with starch. If any soluble iodide be first liberated with a little chlorine water, it is then capable of producing the characteristic colour with starch. F Iodine forms with oxygen two acids, iodic acid, IO,, periodic acid, IO,. and In combination with hydrogen it forms an acid known as hydriodic acid, HI. This acid is a gas, but it may be obtained in solution by passing sulphuretted hydrogen through water in which powdered iodine is suspended. In this case, the sulphur of the sulphuretted hydrogen is liberated, and the hydrogen combines with the iodine, forming hydriodic acid. HS + I = HI + S. The liquid is warmed, filtered, and a colourless solution of the acid is obtained, which soon becomes of a brown colour on exposure to air, by its decomposition into free iodine and water, HI O HO+ I. = Iodine dissolves freely in alcohol. If a polished silver plate be held over the vapour of iodine, it first becomes yellow, then violet, and then a deep blue, owing to a combination of the iodine with the silver. The iodide of silver thus formed is decomposed by light. FLUORINE. Symbol, F; Equivalent, 18'9. Owing to the powerful affinities of this element for the metals, also for silicon and hydrogen, it has never been isolated. Its most frequent combination is with calcium, as the fluoride of calcium, CaF, or fluor-spar. It exists, also, in a state of combination, in the bones and teeth of animals. So far as it has been examined, it is said to be like chlorine. a gas, There is no known oxygen compound of fluorine; but in combination with hydrogen, it forms an acid known as hydrofluoric acid, HF. To obtain this compound, the powdered fluoride of calcium is heated with concentrated sulphuric acid in a platinum or lead retort connected with a receiver of the same metal, kept cool by being surrounded with a freezing mixture. A volatile, colourless liquid is obtained, which emits suffocating white fumes in the air. It unites with water, and very dilute solutions attack glass. It has, therefore, to be kept in leaden, or gutta-percha bottles. The decomposition is easily explained: CaF + HO,SO, = HF + CaO,SO; or, (CaF+H,SO, = HF+Ca,SO.). If the concentrated acid be dropped upon the skin, it occasions ulcers of a very malignant and dangerous kind. Hydrofluoric acid is used to etch or engrave letters on glass. The glass to be operated upon is first covered with bees'-wax, and the lines to be etched scratched through with a needle. The glass is then exposed to the action of the vapour of hydrofluoric acid, or the acid is poured upon it in a liquid state. Where the glass is unprotected with wax, it is corroded. It is in this manner that thermometers, and other glass vessels, are graduated. The power of the acid to etch glass may be observed by having a leaden saucer, into which some powdered fluor-spar and sulphuric acid are placed. The glass to be etched is first covered with a coating of wax, and the device drawn upon it with a penknife so as to remove the wax from those parts to be etched. The glass which should cover the leaden saucer is now placed on the top, with the waxed side downwards. The saucer should now be gently heated, care being taken that the temperature does not rise sufficiently to melt the wax. In a few minutes the glass may be removed and cleaned with a little hot turpentine, when it will be found to be etched. This THIRD DIVISION. NITROGEN. Symbol N; Combining Equivalent, 14. gas is without taste, odour, or colour, rather lighter than common air, and distinguished from all the other gases by its negative properties. It forms by volume of the atmosphere, and enters into a variety of combinations. Nitrogen exists in the vegetable kingdom. It is probably derived from the ammonia of the atmosphere. It is a constituent of muscular tissue to the extent of about 17 per cent. It is not found in the mineral crust of the earth, except in some kinds of coal. Fig. 11. Nitrogen may be prepared, 1. By burning a small piece of phosphorus in a confined portion of common air under a gas jar standing over water, by which phosphoric acid is produced, and the nitrogen liberated. The white solid flakes of phosphoric acid are dissolved in the water over which the jar is placed. (See fig. 11). It must stand till the jar has become clear. Theory of change:— Air Oxygen Nitrogen. 2. By passing chlorine through a solution of ammonia: 3Cl + NH2 = 3HCl + N free. 3. By mixing iron filings and sulphur with water, and allowing the mixture to remain under a gas jar for twentyfour hours. The oxygen of the atmosphere is absorbed and the nitrogen set free. 4. By treating nitric acid with lean butcher's meat in a retort. This change is not well understood. The atmosphere, which is the great storehouse of nitrogen, is composed of a mixture of nitrogen and oxygen, with variable proportions of carbonic acid, aqueous vapour, and traces of ammonia. The analysis of common air may be effected in the following manner :-Introduce, by means of a wire, into a |