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solution of perchloride of platinum, to evaporate to dryness over the water-bath, and to wash the residue with alcohol so long as anything is dissolved. This salt when heated to redness is decomposed, the platinum loses its chlorine, and the chloride of potassium may be dissolved out of the grey residue with cold water, whilst metallic platinum is left behind: 100 parts of the chloride of platinum and potassium are equivalent to 15.98 of potassium, or to 19.26 of potash.

§ II. SODIUM: Na=23. Sp. Gr. 0972; Fusing-pt. 207°.7.

(581) SODIUM may be obtained from its carbonate by a process analogous to that used in procuring potassium. Deville recommends the employment of the following mixture in the extraction of sodium: dried carbonate of sodium, 717 parts; powdered charcoal, 175 parts; finely powdered chalk, 108. These materials are mixed intimately and kneaded into a stiff paste with oil, and calcined in a covered iron pot; the mass is then introduced into an iron retort and distilled, with the precautions described when speaking of potassium; it ought to yield nearly one-third of its weight of sodium: the object of adding the chalk is to prevent the charcoal from separating from the carbonate of sodium when this salt fuses. As a reducing agent sodium is but little inferior to potassium in energy, and since its combining number is lower, and the metal is much less expensive, it may generally be substituted for potassium with advantage in such operations.

Sodium has a bluish-white colour; in appearance and properties it much resembles potassium, but is somewhat more volatile ; it fuses at a temperature of 207°.7. It burns with a bright yellow flame. When dropped into cold water, it decomposes a portion of it, extricating hydrogen, but the gas does not take fire unless the water be heated previously.

The great storehouse of sodium is common salt, which is met with in nature in extensive deposits; it is also contained in vast quantities in the water of the ocean; the immense quantities of soda consumed in the arts are almost exclusively obtained from chloride of sodium, by a process presently to be described, though sodium occurs in several minerals, such as albite or sodium-felspar, and cryolite, the double fluoride of sodium and aluminum. Borax or the acid borate of sodium, and trona or the sesquicarbonate, as well as the nitrate, are also native compounds of sodium.

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The metal forms two well-known oxides, one of which contains twice as much oxygen as the other :

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A blue suboxide appears also to exist.

(582) OXIDE OF SODIUM, or SODA (Na,0=62, or NaO=31), forms the basis of the important series of salts of sodium. It can be procured in an anhydrous state by burning the metal in dry air or oxygen gas: it is of a yellowish-white colour, attracts moisture as greedily as the corresponding oxide of potassium, and this water cannot again be expelled from it by heat. In appearance and properties the hydrate closely resembles that of potash; it may be formed from the carbonate by a similar method; its action upon acids, however, is less energetic. According to Filhol the solid hydrate (NaHO) has a sp. gr. of 213. The following table shows the proportion of anhydrous soda which is contained in solutions of hydrate of soda of different densities :—

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Hydrate of soda is extensively used in the manufacture of hard soaps.

Caustic soda is now manufactured on a large scale in the alkali works, which supply it in the form of a solid hydrate containing 60 per cent. of anhydrous soda; for this purpose advantage is taken, as proposed by Mr. Gossage, of the caustic soda present in the solutions of crude soda.*

* In order to effect this the crude solution obtained from the black-ash vats is evaporated down till it acquires a sp. gr. of 15 or 16, during which operation almost all the carbonate, sulphate, and chloride of sodium crystallize out. The solution (technically known as red liquor) is of a red colour, owing to the presence of a peculiar soluble compound of sulphide of sodium and sulphide of iron, and is likewise contaminated with ferrocyanide, and occasionally with sulphocyanide of sodium. By forcing air under pressure

414

CHLORIDE OF SODIUM.

The sulphides of sodium correspond in number with those of potassium, which they closely resemble. They may be prepared by analogous methods.

(583) CHLORIDE OF SODIUM (NaCl=58·5); Sp. Gr. 2:078; Comp. in 100 parts, Na, 39°32; Cl, 60·68.—This important and well-known compound, formerly called muriate of soda, constitutes common culinary table-salt. It is found native in the solid form, and it exists in solution in sea water in a proportion of about 2.7 per cent., which amounts to nearly four ounces per gallon, or to a bushel in from 300 to 350 gallons.

The extraction of the chloride from sea water was formerly practised to some extent upon the southern coast of our own island: in Great Britain the manufacture is now unimportant, though in the southern countries of Europe the preparation of bay-salt is still a branch of industry of some magnitude. In conducting this process the sea water is allowed to run into shallow pools, in which the water evaporates and the liquor becomes concentrated by the heat of the sun crusts of the salt are formed, and are raked off from time to time: the rough crystals thus obtained furnish the bay-salt of commerce. The concentrated sea water, or bittern, is employed as a source of bromine. Balard, who has devoted much attention to the study of these mother-liquors, has devised a method of extracting from them not only sulphate of sodium, but also an important quantity of salts of potassium, in the form of a double sulphate of potassium and magnesium, as well as of a double chloride of potassium and magnesium. The process requires a careful attention to the temperature at which the crystallizations are effected. At temperatures below 27° the chloride of sodium still present in the brine decomposes the sulphate of magnesium, chloride of magnesium and sulphate of sodium being formed; whilst at temperatures above Ico° F., a sparingly soluble, double sulphate of sodium and magnesium is formed.*

for several hours through the hot liquid, the iron is precipitated as sesquioxide, and the sulphur compounds are converted into sulphates. The completion of the oxidation is effected by the addition of nitrate of sodium. The entire process of oxidation may indeed be more rapidly effected by means of the nitrate. After its addition the evaporation is carried further, until the mass by degrees becomes heated nearly to redness. When the temperature rises to 310°, large quantities of ammonia are evolved, and subsequently, as the heat becomes much greater, nitrogen escapes abundantly (Pauli). The ammonia is produced partly by the decomposition of the cyanogen compounds, but also in part from the removal of oxygen from water by the sulphides, whilst the hydrogen reduces the nitric acid to the form of ammonia. The fused soda is poured into sheet-iron vessels, in which it solidifies, and is preserved for the market. (Hofmann's Jury Report, Int. Exhib. 1862, p. 28).

* The following is the modification of Balard's plan, adopted by M. Merle :

CHLORIDE OF SODIUM.

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Immense beds of common salt are met with in Cheshire, at Wielitzka in Poland, and at Cardona in Spain. It has also recently been found in abundance in the north of Ireland, near Belfast, and on the southern borders of the Durham coal-field. Near Northwich, the principal deposit of rock salt in England, the mineral occurs in two beds, situated one above another, separated by about 30 feet of clay and marl intersected with small veins of salt the two beds together are not less than 60 feet in thickness, 300 yards broad, and a mile and a half long. These beds occur in magnesian limestone. The celebrated and beautiful mine of Wielitzka contains sufficient salt to supply the entire world for ages. It is calculated that the mass of rock salt here is 500 miles in length, 20 miles broad, and not less than 1200 feet in thickness. This salt deposit occurs in the chalk formation.* Chloride of sodium is sometimes found crystallized, and is then termed sal gem, or rock salt. Where coal is cheap, the solubility of the chloride is frequently taken advantage of in diminishing the labour of raising the salt to the surface, water being let down into the bed of salt and allowed to remain till it has become saturated: it is then pumped out and the brine is boiled down and crystallized. Some brine springs contain too small a proportion of salt to render it profitable to effect the evaporation by heat; the water in these cases is therefore concentrated by graduation, as at Salzburg: this process consists in exposing the brine, diffused over a large surface, to the air, by pumping it up to a height, and then allowing it to trickle slowly over large stacks of fagots, piled in suitable buildings screened from rain, but freely exposed to the prevailing wind: after this process has been repeated eight or ten times, the solution

Sea water is concentrated in the salt-pans by spontaneous evaporation till it acquires a density of 124, by which time it has deposited about ths of the quantity of common salt which it contains. The mother-liquor is then diluted with one-tenth of its bulk of water, and cooled down artificially by the use of Carré's refrigerator, (note, Part I. p. 298,) till the temperature falls to o° F. As the liquor passes through the refrigerator, sulphate of sodium is deposited in nearly a pure state, and is dried in a centrifugal hydro-extractor. The mother-liquor-retaining chlorides of magnesium, sodium, and potassium-is boiled down to a sp. gr. of 1331, and deposits nearly all the remaining chloride of sodium. The hot liquor is run into shallow coolers, where it deposits the whole of the potash in the form of the double chloride of magnesium and potassium. On treating this with half its weight of water, the salt is decomposed, the deliquescent chloride of magnesium is dissolved, together with one-fourth of the chloride of potassium, and the solution is returned to be recrystallized with the fresh mother-liquor-while the undissolved chloride of potassium is ready for sale after it has been dried.

*Some specimens of the salt from this mine decrepitate when thrown into water, owing to the escape of condensed gas (EH.; Rose), which is liberated during the solution of the crystals.

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CHLORIDE OF SODIUM.

acquires a density of about 1140, and is sufficiently concentrated to allow the evaporation to be finished as usual by the direct application of heat. In the first basin an insoluble double sulphate of calcium and sodium is deposited, partly in the form of mud, or schlot as the Germans term it, partly in the form of a hard scale, which adheres to the bottom of the pan: when the liquor reaches a density of 1236 it is decanted into another pan, and evaporated; the crusts of salt are removed as they are formed.

The appearance of the salt varies according to the rate at which the evaporation is conducted; when the brine is boiled down rapidly, it furnishes the mealy, fine-grained salt used upon our tables; if evaporated more slowly, the hard, crystallized salt preferred for fishery purposes is obtained. The salt of commerce always contains a certain proportion of chloride of magnesium, which gives it a slightly deliquescent character, and adds to the pungency of its flavour. It is stated, that when the proportion of chloride of magnesium in the brine is considerable, the crystals of chloride of sodium form a scum over the surface which much retards the evaporation. This inconvenience may be remedied by the addition of a quantity of sulphate of sodium, which decomposes the chloride of magnesium and converts it into sulphate.

Properties.-Chloride of sodium has an agreeable, saline taste. It crystallizes in colourless transparent cubes, which are anhydrous, soluble in about 3 parts of cold water, and scarcely more soluble at a temperature of 212°; the saturated solution has a sp. gr. of 1.205. Water at 32° dissolves 35'5 per cent. of the salt, and 412 per cent. at 229°5, the boiling-point of the solution. When heated suddenly, the crystals decrepitate with violence; at a bright red heat they fuse, and by a stronger heat are converted into vapour. Chloride of sodium is insoluble in pure alcohol, but is taken up in considerable quantity by dilute spirit. By exposing its aqueous solution to a temperature of about 14°, it crystallizes in hexagonal tables, which contain 2 H2O: as the temperature rises, the water is separated, the crystals fall to pieces, and become converted into a heap of minute cubes.

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Chloride of sodium is consumed in large quantities in the manufacture of the salts of sodium, it is extensively employed in glazing stoneware, and is an article of daily domestic use, being indeed an essential constituent of the food both of man and of animals, who languish if it be supplied in insufficient quantity. The process of salting meat is resorted to on account of the powerfully antiseptic qualities of the chloride of sodium. In this ope

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