COMPOUNDS OF POTASSIUM.

261 characteristic of potassium, and allows it to be recognised in its compounds.

If a solution of nitrate of potash (saltpetre) in water be mixed with enough spirit of wine to allow of its being inflamed, the flame will have a peculiar lilac colour. This colour may also be developed by exposing a very minute particle of saltpetre, taken on the end of a heated platinum wire, to the reducing (inner) blowpipe flame (fig. 218), when the potassium, being reduced to the metallic state, and passing

Fig. 218.-Coloured flame test.

into the oxidising (outer) flame in the state of vapour, imparts to that flame a lilac tinge.

The difficulty and expense attending the preparation of potassium have prevented its receiving any application except in purely chemical operations, where its attraction for oxygen, chlorine, and other electronegative elements, is often turned to account.

The chloride of potassium (KCl) is an important natural source of this metal, being extracted from sea-water, from kelp (the ash of sea-weed), and from the refuse of the manufacture of sugar from beet-root. It also occurs

in combination with chloride of magnesium, forming the mineral known as carnallite (KC1. 2MgCl. 12HO), an immense saline deposit overlying the rock-salt in the salt-mines of Stassfurth in Saxony. Carnallite resembles rock-salt in appearance, but is very deliquescent; it promises to become the most important source of potassium hitherto discovered.

Bicarbonate of potash (KO. HO. 2CO2), which is much used in medicine, is obtained by passing carbonic acid through a strong solution of carbonate of potash, when it is deposited in crystals, being much less soluble in water than the normal carbonate.

Nitrate of potash (KO. NO,), or saltpetre, will be specially considered in the section on gunpowder.

Equivalent and atomic weights of potassium.-The chloride of potassium has been found to contain 35.5 parts by weight (1 eq.) of chlorine and 39 parts of potassium; 39 is therefore regarded as the equivalent weight of this metal. Since this represents the quantity required to displace one atom of hydrogen from its compounds, it is also taken as the atomic weight of potassium. Since potash contains 39 potassium combined with 8 oxygen, its atomic formula would be K, ( = 16); and hydrate of potash would be KHO. Chloride of potassium, however, has the atomic formula KCl corresponding with its equivalent formula, because the atomic weights of both its elements are the same as their equivalent weights.

The following less important compounds of potassium have not been noticed else

262

EXTRACTION OF COMMON SALT.

where, and are not of sufficient practical importance to require particular description in this work :—

184. Sodium is often found, in place of potassium, in the feldspars and other minerals, but we are far more abundantly supplied with it in the form of common salt (chloride of sodium, NaCl), occurring not only in the solid state, but dissolved in sea-water, and in smaller quantity in the waters derived from most lakes, rivers, and springs.

Rock-salt forms very considerable deposits in many regions; in this country the most important is situated at Northwich in Cheshire, where very large quantities are extracted by mining. Wielitzka, in Poland, is celebrated for an extensive salt mine, in which there are a chapel and dwelling-rooms, the furniture of which is made of this rock. Extensive beds of rock-salt also occur in France, Germany, Hungary, Spain, Abyssinia, and Mexico. Perfectly pure specimens form beautiful colourless cubes, and are styled sal gem; but ordinary rock-salt is only partially transparent, and exhibits a rusty colour, due to the presence of iron. In some places the salt is extracted by boring a hole into the rock and filling it with water, which is pumped up when saturated with salt, and evaporated in boilers, the minute crystals of salt being removed as they are deposited.

At Droitwich, in Worcestershire, the salt is obtained by evaporation from the waters of certain salt springs. In some parts of France and Germany the water from the salt springs contains so little salt that it would not pay for the fuel necessary to evaporate the water, and a very ingenious plan is adopted, by which the proportion of water is greatly reduced without the application of artificial heat. For this purpose a lofty scaffolding is erected and filled with bundles of brushwood, over which the salt water is allowed to flow, having been raised to the top of the scaffolding by pumps. In trickling over the brushwood this water exposes a large surface to the action of the wind, and a considerable evaporation takes place, so that a much stronger brine is collected in the reservoir beneath the scaffolding; by several repetitions of the operation, the proportion of water is so far diminished that the rest may be economically evaporated by artificial heat. The brine is run into boilers and rapidly boiled for about thirty hours, fresh brine being allowed to flow in continually, so as to maintain the liquid at the same level in the boiler. During this ebullition a considerable deposit, composed of the sulphates of lime and soda, is formed, and raked out by the workmen. When a film of crystals of salt begins to form upon the surface, the fire is lowered and the temperature of the brine allowed to fall to about 180° F., at which temperature it is maintained for several days whilst the salt is crystallising. The crystals are afterwards drained and dried by exposure to air. The grain of the salt is regulated by the temperature at which it crystallises, the size of the crystals increasing as the temperature falls. It is not possible to extract the whole of the salt in this way, since the last portions which crystallise will always be contaminated with other salts present in the brine, but the

EXTRACTION OF COMMON SALT.

263

mother-liquor is not wasted, for after as much salt as possible has been obtained, it is made to yield sulphate of soda (Glauber's salt), sulphate of magnesia (Epsom salts), bromine and iodine.

The process adopted for extracting the salt from sea-water depends upon the climate. In Russia, shallow pits are dug upon the shore, in which the sea-water is allowed to freeze, when a great portion of the water separates in the form of pure ice, leaving a solution of salt sufficiently strong to pay for evaporation.

Where the climate is sufficiently warm, the sea-water is allowed to run very slowly through a series of shallow pits upon the shore, where it becomes concentrated by spontaneous evaporation, and is afterwards allowed to remain for some time in reservoirs in which the salt is deposited. The coarse crystals thus obtained are known in commerce as bay-salt. Before they are sent into the market they are allowed to drain for a long time, in a sheltered situation, when the chloride of magnesium with which they are contaminated deliquesces in the moisture of the air and drains off. The bittern or liquor remaining after the salt has been extracted is employed to furnish magnesia and bromine.

Great improvements have been made during the last few years in the economical extraction of the salts from sea-water. It will be remembered that 1000 parts of sea-water contain about

In a warm climate, that of Marseilles, for example, the water is allowed to evaporate spontaneously until it has a specific gravity of 1.24. During this evaporation it deposits about four-fifths of its chloride of sodium. It is then mixed with one tenth of its volume of water and artificially cooled to 0° F. (see p. 116), when it deposits a quantity of sulphate of soda, resulting from the decomposition of part of the remaining chloride of sodium by the sulphate of magnesia. The mother-liquor is evaporated down till its specific gravity is 1-33, a fresh quantity of chloride of sodium being deposited during the evaporation. When the liquid cools it deposits a double salt composed of chloride of potassium and chloride of magnesium, from which the latter may be extracted by washing with a very little water, leaving the chloride of potassium fit for the market.

This process is instructive as illustrating the influence exerted upon the arrangement of the various acids and bases in a saline solution by the temperature to which the solution is exposed, the general rule being that a salt is formed which is insoluble in the liquid at that particular temperature.

The great tendency observed in ordinary table salt to become damp when exposed to the air is due chiefly to the presence of small quantities of chloride of magnesium and chloride of calcium, for pure chloride of sodium has very much less disposition to attract atmospheric moisture, although it is very easily dissolved by water, 2 parts of this liquid being able to dissolve one part (by weight) of salt.

In the history of the useful applications of common salt is to be found one of the best illustrations of the influence of chemical research upon the development of the resources of a country, and a capital example of a manufacturing process not based, as such processes usually are, upon mere experience, independent of any knowledge of chemical principles, but upon a direct and intentional application of these to the attainment of a particular object.

264

MANUFACTURE OF ALKALI.

Until the last quarter of the eighteenth century the uses of common salt were limited to culinary and agricultural purposes, and to the glazing of the coarser kinds of earthenware, whilst a substance far more useful in the arts, carbonate of soda, was imported chiefly from Spain under the name of barilla, which was the ash obtained by burning a marine plant. known as the salsola soda. But this ash only contained about one-fourth of its weight of carbonate of soda, so that this latter substance was thus imported at a great expense, and the manufactures of soap and glass to which it is indispensable were proportionally fettered.

During the wars of the French Revolution the price of barilla had risen so considerably, that it was deemed advisable by Napoleon to offer a premium for the discovery of a process by which the carbonate of soda. could be manufactured at home, and to this circumstance we are indebted for the discovery, by Leblanc, of the process at present in use for the manufacture of carbonate of soda from common salt, a discovery which placed this substance at once among the most important raw materials with which a country could be furnished.

185. Manufacture of carbonate of soda from common salt.-The salt is spread upon the hearth of a reverberatory furnace (fig. 219),* and mixed

with an equal weight of sulphuric acid, which converts it into the sulphate of soda (p. 147), expelling the hydrochloric acid in the form of gas, which would prove highly injurious to the vegetation in the neighbourhood, and is therefore usually condensed by being brought into contact with water (see p. 148). The flame of the fire is allowed to play over the surface of the mixture of salt and sulphuric acid until it has become perfectly dry; in this state it is technically known as salt-cake, and is next mixed with about an equal weight of limestone and rather more than half its weight of small coal; this mixture is again heated upon the hearth of a reverberatory furnace, when it evolves an abundance of carbonic oxide, and yields a mixture of carbonate of soda with

The hearth of this furnace is usually divided, as seen in the figure, into two compartments, in one of which (lined with lead), more remote from the grate, the decomposition is effected, the acid being poured in through the funnel, while in that nearest to the grate, lined with fire-brick, the whole of the hydrochloric acid is expelled, and the sulphate of soda fused.

SODA-ASH-SODA-CRYSTALS.

265

lime and sulphide of calcium; this mixture is technically known as black ash.

The change which has been effected in the sulphate of soda will be easily understood; for when this salt is heated in contact with carbon (from the small coal) it loses its oxygen, and becomes sulphide of sodium, whilst carbonic acid is evolved; thus

Again, when carbonate of lime is heated in contact with carbon, carbonic oxide is given off, and lime remains—

CaO. CO, + C = 2CO + CaO .

Finally, when sulphide of sodium and lime are heated together in the presence of carbonic acid, carbonate of soda and sulphide of calcium are produced

NaS

CaO + CO,

NaO.CO =

+ CaS.

When the black ash is treated with water, the carbonate of soda is dissolved, leaving the sulphide of calcium, and by evaporating the solution, ordinary soda ash is obtained. But this is by no means pure carbonate of soda, for it contains, in addition to a considerable quantity of common salt and sulphate of soda, a certain amount of caustic soda or hydrate of soda formed by the action of the lime upon the carbonate of soda. In order to purify it, the crude soda-ash is mixed with small coal or saw-dust and again heated, when the carbonic acid formed from the carbonaceous matter converts the hydrate of soda into carbonate, and on dissolving the mass in water and evaporating the solution, it deposits oblique rhombic prisms of common washing soda, having the composition, NaO. CO2 + 10Aq. (soda-crystals).

A little reflection will show the important influence which this process has exerted upon the progress of the useful arts in this country. The three raw materials, salt, coal, and limestone, we possess in abundance. The sulphuric acid, when the process was first introduced, bore a high price, but the resulting demand for this acid gave rise to so many improvements in its manufacture that its price has been very greatly diminished,a circumstance which has, of course, produced a most beneficial effect upon all branches of manufacture in which the acid is employed.

The large quantity of hydrochloric acid obtained as a secondary product has been employed for the preparation of bleaching powder, and the important arts of bleaching and calico-printing have thence received a considerable impulse. These arts have also derived a more direct benefit from the increased supply of carbonate of soda, which is so largely used for cleansing all kinds of textile fabrics. The manufactures of soap and glass, which probably create the greatest demand for carbonate of soda, have been increased and improved beyond all precedent by the production of this salt from native sources.

The crystals of carbonate of soda are easily distinguished by their property of efflorescing in dry air (p. 48), and by their alkaline taste, which is much milder than that of carbonate of potash, this being, moreover, a deliquescent salt. The crystals are very soluble in water, requiring only 2 parts of cold and less than their own weight of boiling water; the solution is strongly alkaline to test papers.

The substance commonly used in medicine under the name of carbonate