BROMIDE AND IODIDE OF SODIUM-SULPHATE OF SODIUM. 417

ration a large quantity of the nutritive juice of the meat is extracted, and this liquid when saturated with the salt forms the brine. Meat thus prepared is much less digestible and nutritious than fresh meat.

Chloride of sodium, when fused with rather more than onethird of its weight of sodium in a current of dry hydrogen furnishes a blue compound, supposed to be a subchloride, Na,Cl (Bunsen).

(584) BROMIDE OF SODIUM (NaBr=103) is analogous to bromide of potassium; it is soluble both in water and in alcohol, and crystallizes at temperatures above 86° in anhydrous cubes. At lower temperatures it forms hexagonal tables with 2 H2O.

(585) IODIDE OF SODIUM (NaI=150; Sp. Gr. 3'45) crystallizes at temperatures above 100° F. in cubes, which are anhydrous; but if crystallized at ordinary temperatures it yields large transparent, striated, oblique rhombic prisms, with 2 H2O. Iodide of sodium occurs native in sea-water in minute proportion, but small as this proportion is, it furnishes the commercial supply of iodine: many marine plants appropriate it to their nutrition, and when these plants are burned, the iodide remains in the residue : the ash thus obtained goes by the name of kelp. A ton of good Irish kelp from drift-weed furnishes about 8 lb. of iodine.

3

(586) SULPHATE OF SODIUM (Na,SO,, 10 H,O=142+180, or NaO,SO, 10 HO=71+90). Sp. Gr. anhydrous, 2597; cryst. 1:469: Composition in 100 parts of dry salt, Na,0, 43'67; SÐ ̧ 56′33; of crystallized salt, Na,, 19:24; SO„, 24·84; H2O, 55'91.—This salt has long been known under the name of Glauber's salt. It crystallizes usually in long four-sided prisms, terminated by dihedral summits. It is remarkably efflorescent, and loses the whole of its 10 atoms of water by mere exposure, at common temperatures, to the atmosphere. It has a saline, bitter taste, and is occasionally used medicinally as a purgative.

The solubility of sulphate of sodium in water offers some remarkable anomalies (55), which have been the subject of many inquiries, the most complete of which are those of Löwel.* It has already been mentioned (74) that a boiling saturated solution of this salt, if closed hermetically, may be kept for months without crystallizing, but the moment that air is admitted, the whole becomes semi-solid, from the sudden formation of crystals through

Three forms of sulphate of sodium may be obtained in crystals, viz. 1. the anhydrous sulphate; 2. the ordinary crystallized sulphate with 10 H.; and 3. the hydrate with 7 H., which crystallizes in rhombic prisms. Each of these varieties has a specific solubility. The 10-atom hydrate is the least

418

SULPHATE OF SODIUM.

the mass. It is most probable that the salt exists in the supersaturated solution in the form of the anhydrous salt, and that crystallization occurs when any circumstance occasions the formation of the less soluble 10-atom hydrate. The crystallization of such a solution may, for example, be instantly determined by dropping in a fragment of the sulphate, or by contact with a rod of glass or of metal. If, however, the glass rod or the metallic wire be boiled with water, and allowed to cool under water or in a closed vessel, it may be introduced into the supersaturated solution without causing the crystallization of the salt.

Crystallized sulphate of sodium is soluble in hydrochloric acid, with great depression of temperature. A convenient freezing mixture is obtained by pouring 5 parts of the commercial acid upon 8 of the crystallized sulphate.

Sulphate of sodium, to which the name of Thénardite has been given, has been met with nearly pure not far from Madrid, depo

soluble, and the 7-atom hydrate the most so of the three forms. The following table (Löwel, Ann. de Chimie, III. xlix. 50) exhibits the varying solubility of each form of the sulphate of sodium, as the temperature rises :

100 Parts of Water when saturated contain, of

From this table it appears that the solubility of the anhydrous salt decreases from 64°4 to the boiling-point (2177) of the solution. Below 64° the molecular constitution of the salt is changed, a saturated solution depositing, in vessels from which air is excluded, crystals of the 7-atom hydrate. 100 parts of water at 64°4 retain as much as 53 25 of the anhydrous salt, whilst at the boiling-point only 42'65 parts are held in solution. Hence

SULPHATE OF SODIUM.

419

sited at the bottom of some saline lakes, in anhydrous octohedra. It has likewise been found, not far from the same place, combined with sulphate of calcium, as Glauberite, in anhydrous crystals (Na€a 2 ᎦᎾ ).

Crystallized sulphate of sodium also frequently occurs in needles as an efflorescence upon plaster, and upon brickwork in damp situations.

Preparation. Sulphate of sodium is made from oil of vitriol and common salt in enormous quantities, under the name of saltcake, as a preliminary step in the manufacture of carbonate of sodium. The operation is carried on in a reverberatory furnace, connected with an apparatus for condensing the hydrochloric acid, which, till within the last few years, was allowed to escape into the atmosphere, to the serious injury of vegetation in the surround

if a solution saturated at 64° be simply heated to boiling, without allowing any loss of liquid by evaporation, it will deposit in hard, gritty, anhydrous crystals more than one-fifth of the salt which it previously held in solution.

In the case of the least soluble form of the sulphate, the 10-atom hydrate, the solubility increases until the temperature reaches 93°, at which point the salt begins to liquefy in its water of crystallization: its molecular constitution then undergoes a change, and it becomes gradually converted into the anhydrous variety, which at that particular temperature has a lower solubility than the hydrated salt, and consequently is partially separated in crystalline grains.

The hydrate with 7 H, is more soluble than either of the foregoing forms; but under ordinary circumstances it cannot exist in contact with the atmosphere, and is only deposited from supersaturated solutions in closed vessels, or in flasks which have been allowed to cool covered with small capsules, so as to prevent the entrance of particles of dust or of foreign matter. Crystals of the 7-atom hydrate may also be obtained by pouring a boiling solution of the sulphate into a capsule and allowing it to cool under a bell-glass, over a vessel of chloride of calcium. In whatever mode the crystals of the 7-atom hydrate have been produced, they undergo change from very slight causes, and become white and opaque with evolution of heat, either when exposed to the air, or when the solution is allowed to crystallize around them, or when touched with a glass rod. The solubility of the 7-atom hydrate rises with the temperature, as is shown in the table; but this form of the salt cannot exist at temperatures above 84°; for when heated to this point its crystals begin to liquefy in their water of crystallization; and, in consequence of a molecular change, crystals of the anhydrous variety are deposited.

From the foregoing details it will be easy to perceive why it is that a hot solution of the sulphate deposits crystals so slowly :-When a solution of sulphate of sodium, saturated at its boiling-point, is poured into an open capsule, a film of crystals of the anhydrous sulphate is formed at first upon its surface, owing to the rapid evaporation of a portion of the solvent. No crystals, however, are deposited in the body of the liquid until the temperature has fallen to about 91°. The film of crystals first formed is gradually redissolved, and crystals of the 10-atom hydrate are formed as the temperature continues to fall. If the solution be evaporated at temperatures above 93°, acute rhombic octohedra of the anhydrous salt are produced: but if a boiling saturated solution be allowed to cool in closed vessels, no crystals are deposited until the temperature falls to 64°, when oblique rhombic prisms of the 7-atom hydrate are formed.

420

ing district.

SULPHATE OF SODIUM.

One of the best forms of furnace is shown in section in fig. 333 the course of the flues, however, is not exactly such

as is there represented: A, the smaller of the two compartments which compose the furnace, is of cast iron; into this (the decomposer) from 5 to 6 cwt. of common salt are introduced, and an equal weight of sulphuric acid, of specific gravity 16, is gradually mixed with it, a gentle heat being applied to the outside; enormous volumes of hydrochloric acid gas are disengaged, and pass off by the flue, d, to the condensing towers, E and F; these towers are filled with fragments of broken coke or stone, over which a continuous stream of water is caused to trickle slowly from h, h. A steady current of air is drawn through the furnace and condensing towers, by connecting the first tower with the second, as represented at g, and the second tower with the main chimney, K, of the works. In the first bed of the furnace, about half the hydrochloric acid is expelled from the salt: the pasty mass thus produced is then pushed through a door for the purpose into the roaster, or second division, в, of the furnace. In this state it consists of a mixture of acid sulphate of sodium and undecomposed salt. The reaction in the first bed of the furnace may be represented as follows:

2 NaCl + H2SO, NaCl + NaHSO4 + HCl.

=

In the second stage of the operation a higher temperature is required; the acid sulphate of sodium then reacts upon the unchanged chloride, and the conversion into normal sulphate of sodium is complete; thus NaCl + NaHSO, HCl + Na,SO. The hydrochloric acid gas, as it is liberated from в, passes off through the flue, d, and is carried on to the condensing towers. Heat is

4

=

SULPHATE OF POTASSIUM AND SODIUM-SULPHITE OF SODIUM. 421

applied to the outside of the roaster, B; the smoke and products of combustion circulate in separate flues around the chamber, in the direction indicated by the arrows, but never come into contact with the salt-cake in B.

Sulphate of Potassium and Sodium (NaK ̧2 SO4, Penny; Sp. Gr. 2.668).—This double salt is anhydrous; it may be formed by dissolving the two salts in water and evaporating. Gladstone has shown that the employment of a large excess of sulphate of sodium does not alter the composition of the salt, the sulphate of sodium in excess crystallizing in its usual form.

It is obtained upon a large scale from kelp liquors during the manufacture of iodine, and is known under the name of plate sulphate, from the manner in which it is deposited in hard crystalline layers or plates, upon the sides of the crystallizing vats. During the act of crystallizing it emits vivid scintillations of phosphorescent light: this phosphorescence is most striking when the temperature is near 100° F. A very brilliant effect is produced by dashing a pailful of the warm mother-liquor upon a crop of crystals in a vat from which the mother-liquor has been drained off a few hours previously.

An Acid Sulphate of Sodium, often called bisulphate of soda, (NaHSO, or NaO,HO,2 SO=120; Sp. Gr. 2.742) corresponding to the acid sulphate of potassium, may be formed. It is more easily deprived of basic hydrogen by heat than the acid sulphate of potassium; 2 NaHSO, yielding Na2SOSO2+H2O. The anhydrosalt, by a stronger heat loses its sulphuric anhydride, and may hence be employed as a convenient source of this anhydride (416).

3

(587) SULPHITE OF SODIUM (NaSO3,7H2O=126+126 or NaO,SO, 7 Aq=63+63; Sp. Gr. 1'736) is now prepared largely under the name of antichlore, for the purpose of removing the last traces of chlorine from the bleached pulp obtained from rags in the manufacture of paper. It is procured by passing sulphurous anhydride, obtained by the combustion of sulphur in air, over moistened crystals of carbonate of sodium, so long as the acid gas is absorbed; the mass is dissolved in water and crystallized. Sulphite of sodium forms efflorescent, oblique prisms, which fuse at 113°; they are soluble in about 4 parts of cold water: the solution has a slightly alkaline reaction and a sulphu

rous taste.

An Acid Sulphite of Sodium (NaHSO) may be obtained in crystals.

(588) NITRATE OF SODIUM, or Cubic Nitre (NaNO, or NaO,NO=85; Sp. Gr. 2:26), occurs abundantly 2 or 3 feet