HYDROFLUO-SILICIC ACID. 181 manner, by the action of minute quantities of such agents of transformation. The frequent occurrence of minute quantities of fluorides in various minerals may thus have great significance. The specific gravity of fluoride of silicon is 3.60. Assuming 0.93 to represent the sp. gr. (weight of 1 vol.) of imaginary silicon vapour (see p. 165), and 131 to represent the sp. gr. of fluorine, the number 3.55 would be the sum of the weights of 1 vol. of silicon vapour and 2 vols. of fluorine; so that 2 vols. (1 eq.) of silicon vapour are combined with 4 vols. (2 eqs.) of fluorine, to form 2 vols. of fluoride of silicon. 134. Hydrofluo-silicic acid or silico-fluoric acid.-This acid is only known in the form of a solution, which is obtained by passing fluoride of silicon into water 3SiF2 + 2HO = 2(HF. SiF2) Hydrofluo-silicic acid. + SiO2. The gas must not be passed directly into the water, lest the separated silica should stop the orifice of the tube, to prevent which, the latter should dip into a little mercury at the bottom of the water, when each bubble, as it rises through the mercury into the water, will become surrounded with an envelope of gelatinous silica, and if the bubbles be very regular, they may even form tubes of silica extending through the whole height of the water. For preparing hydrofluo-silicic acid it will be found convenient to employ a gallon stoneware bottle (fig. 176), furnished with a wide tube dipping into a cup of mercury placed at the bottom of the water. 1 lb. of finely powdered fluor spar, 1 lb. of fine sand, and 64 measured ounces of oil of vitriol, are introduced into the bottle, which is gently heated upon a sandbath, the gas being passed into about 5 pints of water. After 6 or 7 hours the water will have become pasty, from the separation of gelatinous silica. It is poured upon a filter, and when the liquid has drained through as far as possible, the filter is wrung in a cloth to extract the remainder of the acid solution, which will have a sp. gr. of about 1.078. Fig. 176.-Preparation of hydrofluo-silicic acid. A dilute solution of hydrofluosilicic acid may be concentrated by evaporation up to a certain point, when it begins to decompose, evolving fumes of fluoride of silicon, hydrofluoric acid remaining in solution and volatilising in its turn if the heat be continued. Of course the solution corrodes glass and porcelain when evaporated in them. If the solution of hydrofluo-silicic acid be neutralised with potash, and stirred, a very characteristic crystalline precipitate of silico-fluoride of potassium is formedHF. SiF, + ко KF. SiF, + HO. Silico-fluoride of potassium. 2 = But if an excess of potash be employed, a precipitate of gelatinous silica will be separated, fluoride of potassium remaining in the solution HF. SiF, + 3KO 182 GENERAL REVIEW OF THE HALOGENS. One of the chief uses of hydrofluo-silicic acid is to separate the potash from its combination with certain acids, in order to obtain these in the separate state. 135. Fluoride of boron may be prepared by a process similar to that employed for fluoride of silicon, but it is also obtained by strongly heating a mixture of powdered anhydrous boracic acid with twice its weight of fluor spar in an iron tube The fluoride of boron is a gas which fumes strongly in moist air like the fluoride of silicon. It is absorbed eagerly by water, with evolution of heat. One volume of water is capable of dissolving 700 volumes of fluoride of boron, producing a corrosive heavy liquid (sp. gr. 1·77) which fumes in air, and chars organic substances on account of its attraction for water. This solution is known as fluoboric or borofluoric acid, and its formation is explained by the equation— BF3 When the solution is heated, it evolves fluoride of boron until its specific gravity is reduced to 1.58, when it distils unchanged. Hydrofluoboric acid is obtained in solution by adding a large quantity of water to fluoboric acid 3(BO,. 3HF) = BO, + 6HO + 3HF. 2BF, . Hydrofluoboric This acid resembles the hydrofluo-silicic; its hydrogen may be exchanged for metals to form borofluorides. 136. General review of chlorine, bromine, iodine, and fluorine.—These four elements compose a natural group, the members of which are connected by the similarity of their chemical properties, far more closely than those of any other group of elements. They are usually styled the halogens, from their tendency to produce salts resembling sea-salt in their composition (as, the sea), and such salts are called haloid salts. These elements are also called salt-radicals, from their property of forming salts by direct union with the metals. The equivalent weights of chlorine, bromine, iodine, and probably of fluorine, in the state of vapour, occupy the same volume as an equivalent of hydrogen, and each of these elements combines with an equal volume of hydrogen to form an acid which occupies the joint volumes of its constituents. If one volume of hydrogen represents one atom, then the equivalent weights of these elements (occupying the same volume as hydrogen) will also represent their atomic weights, and they are decidedly mon-atomic elements. The halogens also supply the most prominent example of the gradation in properties sometimes observed among the members of the same natural group of elements. In the order of their chemical energy, that is, of the force with which they hold other elements in chemical combination with them, fluorine should stand first, its combining energy being so great as to cause a serious difficulty in isolating it at all; chlorine would rank next, then bromine, and iodine last. ORES AND MINERALS CONTAINING SULPHUR. 183 Their equivalent weights follow the inverse order of their chemical energies: fluorine, 19; chlorine, 35·5; bromine, 80; iodine, 127;-numbers which, of course, also represent their relative specific gravities in the state of vapour. A similar gradation is observed in the physical state and colour of those three which are well known; chlorine being a yellow gas, bromine a red liquid, boiling at 145° F., and iodine a black solid, boiling at 347° F. Even in the exceptions which occur to the order of chemical energy above alluded to, the same progression is noticed; thus fluorine has so little attraction for oxygen that no oxide is known, chlorine has less attraction for oxygen than bromine (chloric acid being less stable than bromic), whilst bromine has less than iodine, which is said to be capable even of uniting directly with ozonised oxygen to form iodic acid. The compounds of these elements with hydrogen are all gases distinguished by a powerful attraction for moisture and great similarity of odour. Their potassium-salts all crystallise in the same (cubical) form. The fluoride of silver is soluble in water; the chloride is insoluble in water, but dissolves very easily in ammonia; the bromide dissolves with some difficulty in ammonia; and the iodide is insoluble. SULPHUR. 137. Sulphur is remarkable for its abundant occurrence in nature in the uncombined state, in many volcanic districts. It is also found, as sulphuretted hydrogen, in many mineral waters, and very abundantly in combination with metals, forming the numerous ores known as sulphurets or sulphides, of which the following are the most abundant : Sulphur is plentifully distributed also, in combination with oxygen and a metal, in the form of sulphates, of which the most conspicuous are: In plants, sulphur is also found in the form of sulphates, and as a constituent of the vegetable albumen (of which it forms about 1.5 per cent.) present in the sap. It is also contained in certain of the essential oils remarkable for their peculiar pungent odour, such as— Essence of garlic, Essence of mustard, Sulphide of allyle,* CH,S Sulphocyanide of allyle, CH,. C2NS. In animals, sulphur occurs as sulphates, as a constituent of albumen, fibrine, and caseine (in neither of which does it exceed 2 per cent.); and * Allium, garlic. 184 EXTRACTION OF SULPHUR. in bile, one of the products from which (taurine, C,H,NO,S,) contains 25 per cent. of sulphur. For our supplies of sulphur we are chiefly indebted to Sicily, where large quantities of it are found in an uncombined state in beds of blue clay. Magnificent crystalline masses of sulphate of strontia are often found associated with it; the sulphur itself sometimes occurs in the form of transparent yellow octahedra, but more frequently in opaque amorphous masses. The districts in which sulphur is found are usually volcanic, and those which border the Mediterranean are particularly rich in it. Sulphur has also been found in Iceland and California. The native sulphur being commonly distributed in veins through masses of gypsum and celestine, has to be separated from these by the action of heat. When the ores contain more than 12 per cent. of sulphur, the bulk of it is melted out, the ore being thrown into rough furnaces or cauldrons with a little fuel, and smothered up with earth, so as to prevent the com bustion of the sulphur, which runs down in the liquid state to the bottom of the cauldron, and is drawn out into wooden moulds. But when the proportion of sulphur is small, the ore is heated so as to convert the sulphur into vapour, which is condensed in another vessel. The operation is conducted in Sicily in rows of earthen jars (A, fig. 177), heated in a long furnace, and provided with short lateral pipes, which convey the sulphur into similar jars (B) standing outside the furnace, in which the vapour of sulphur condenses in the liquid state, and flows out into pails of water. The SULPHUR DISTILLED FROM PYRITES. 185 sulphur obtained by this process is imported as rough sulphur, and contains 3 or 4 per cent. of earthy impurities. In order to separate these it is redistilled, in this country, in an iron retort (A, fig. 178), from which the vapour is conducted into a large brick chamber (B), upon the sides of which it is deposited in the form of a pale yellow powder (flowers of sulphur, or sublimed sulphur). When the operation has been continued for some time the walls of the chamber become sufficiently hot to melt the sulphur, which is allowed to collect, and afterwards cast in wooden moulds, forming roll sulphur or brimstone. Distilled sulphur is obtained by allowing the vapour to pass from the retort into a small receivingvessel (C) cooled by water, where it condenses in the liquid state; this variety of sulphur is preferred for the manufacture of gunpowder, for reasons which will be stated hereafter. Sulphur is readily distilled on a small scale in a Florence flask (fig. 179), another flask cut off at the neck (see p. 166) being employed as a receiver. The flask containing the sulphur should be supported upon a thin iron wire triangle, and heated by a gauze-burner, at first gently, and afterwards to the full heat. Flowers of sulphur will at first condense in the receiver, and will be followed by distilled sulphur when the temperature increases. A slight explosion of the mixture of sulphur vapour and air may take place at the commencement of the distillation. An ounce of sulphur may be distilled in a few minutes. Fig. 179.-Distillation of sulphur. We are by no means entirely dependent upon Sicily for sulphur, for this element can be easily extracted from iron and copper pyrites, both which are found abundantly in this country. Iron pyrites forms the yellow metallic-looking substance which is often met with in masses of coal, sometimes in distinct cubical crystals, and which is to be picked up in large quantities on some sea-beaches, where it occurs in rounded nodules, rusty outside, but having a fine radiated metallic fracture. When this mineral is strongly heated it gives up part of its sulphur; at a very high temperature one half of the sulphur may be separated but by an ordinary fur- Fig. 180.-Furnace for distillation of sulphur Each vessel contains 100 lbs. of pyrites, and yields 14 lbs. |