HYPOSULPHITE OF SODIUM.

413

one another; after the baking follows a process of slow cooling, during which the heated material contracts uniformly in all directions as it assumes the dimensions proper to it when cold.

Unlike the silicates of the alkali-metals, most metallic silicates have a tendency to assume crystalline form on cooling, and it is not difficult to bring about crystallization of silicate of calcium, or silicate of aluminum, from ordinary glass, particularly from the coarser kinds, such as bottle-glass. Glass, in which some of the constituents have thus crystallized, has the appearance of porcelain; it is said to be devitrified. The devitrification may readily be shown by imbedding bottle-glass in sand, heating the glass almost to the melting-point, and then allowing it to cool slowly. In annealing some kinds of glass care must be taken not to heat the ware too strongly, lest it be devitrified during the process.

494. Melted glass, like melted borax (Exp. 241), is capable of dissolving small quantities of many of the metallic oxides, a transparent and often colored silicate of the oxide being formed, which imparts its hue to the entire mass of glass. In this way, glass may be obtained of almost any desired color. The green color of bottle-glass is due to silicate of protoxide of iron; but richer shades of green may be obtained by using protoxide of copper or oxide of chromium. Dioxide of copper gives a ruby-red color, and oxide of gold various shades of red, inclining to purple. The oxides of uranium, of antimony, and of silver yield yellow glasses; oxide of cobalt affords a beautiful blue, and the binoxide of manganese a violet glass; while mixtures of the oxides of cobalt and of manganese impart to the glass a black color.

495. Hyposulphite of Sodium (Na,S,O,, 5H,O).—This easily crystallized and tolerably permanent salt is of great use to the photographer, because its aqueous solution is capable of rendering soluble the chloride, bromide, and iodide of silver, compounds much employed by the photographer, and very insoluble in water. The photographic paper or glass, uniformly coated with some silver compound, is exposed to light in the camera or press, and then immersed in a strong solution of the hyposulphite, which forms with the silver compound, in those parts of the picture which have not been acted upon by the light, a double salt which

is soluble in water. This double salt and the superfluous hyposulphite must be washed away by soaking the picture several hours in water which is constantly renewed. Hyposulphite of sodium is also used as an "antichlore," or agent for removing the last traces of chlorine, or hypochlorous acid, from substances which have been bleached therewith. The salt may be best prepared by digesting sulphur with a solution of sulphite of sodium.

Exp. 242.-Dissolve 20 grms. of crystallized carbonate of sodium in 30-40 c. c. of water. Place the solution in a small Woulfe-bottle, and pass sulphurous acid gas (Exp. 96) through it until all the carbonic acid is expelled from the carbonate and effervescence ceases. The liquid then holds in solution sulphite of sodium (Na2SO,). Pour this solution into a bottle which can be tightly closed, and add to it 3 or 4 grms. of finely powdered sulphur; let this mixture stand corked up for several days in a warm place; the sulphur will gradually dissolve, and form a colorless solution, which on evaporation will yield crystals of hyposulphite of sodium. Time may be saved by keeping the solution of sulphite of sodium hot, but not boiling, during the digestion of the sulphur. The reaction has been already formulated (§ 243).

CHAPTER XXIV

POTASSIUM.

496. The proximate sources of sodium-compounds are the sea, and salt springs and deposits. Potassium-compounds, on the other hand, are derived indirectly from the soil. Arable soils are produced by the weathering and gradual decomposition of the common granitic rocks. Into the composition of these rocks there enter two minerals, called feldspar and mica, which are mixed silicates of potassium or sodium and aluminum or iron. The element potassium thus becomes a normal constituent of the earthy food of plants. The soil itself is not directly available as a source of potassium-salts, because no cheap and easy method has yet been devised for separating the potassium-compounds

CARBONATE OF POTASSIUM.

415

from the other ingredients of the soil. Plants, however, are able to pick out and assimilate the potassium-salts from these rocks and soils; so that by burning the plants and extracting the ashes with water a soluble potassium-salt is obtained. Plants thus concentrate the potassium from out great masses of earth, and make it accessible to us. The salt which is obtained from the ashes of plants by washing and evaporation, is called potash, or, if refined, pearlash, and it is from this substance that the bulk of potassiumcompounds are obtained.

Exp. 243.-Place a handful of wood-ashes on a filter, and pour hot water over them, collecting the filtrate in a bottle and returning it upon the ashes two or three times, in order to obtain a stronger solution. To exhaust the ashes of their potash, they must, of course, be treated with successive portions of hot water. This solution has a strong alkaline reaction upon test-paper. A few drops of it, poured into a testtube containing a little dilute acid, occasion a brisk effervescence, a reaction from which we readily surmise the truth, that the potassiumsalt contained in the solution is the carbonate of potassium. Proof that the gas evolved is carbonic acid can readily be obtained by conducting the gas into lime-water, as in Exp. 170. By evaporating the rest of the solution to dryness in a porcelain dish, we obtain a small sample of crude potash. By treating this potash with a quantity of cold water, insufficient to dissolve any but the most soluble portions of the mass, letting the mixture stand some time, and evaporating the partial solution to dryness, a whiter, purer carbonate is obtained, the pearlash.

497. Carbonate of Potassium (K,CO,) is a hygroscopic and very soluble salt. When exposed to damp air it becomes moist, and finally deliquesces. In this respect it does not resemble soda-ash, which is not hygroscopic, and is, for this reason among others, better adapted than potash for transportation, storing, and most commercial uses. Carbonate of potassium fuses at a red heat, but cannot be decomposed by heat alone. At a red heat it is decomposed by silica, as is also the carbonate of sodium, carbonic acid being expelled with effervescence, whilst the silica unites with the alkali. Advantage is taken of this property in the analysis of minerals which contain a large quantity of silica, and are not easily decomposed by acids. The finely powdered mineral is fused with about three times its weight of carbonate of sodium or of potassium, or, better, with thrice its weight of a

416

HYDRATE OF POTASSIUM.

mixture of 5 parts of carbonate of sodium with 7 parts of carbonate of potassium. The mixed carbonates produce a more fusible mixture than either alone (§ 492). The fused mass is then treated with dilute chlorhydric acid, which decomposes the alkaline silicates, and dissolves all the bases of the mineral which were before combined with the silica.

Carbonate of potassium was the most important source of alkali, until Leblanc's process made soda cheaper than potash. It is still largely consumed in the manufacture of soap, glass, caustic potash, and other compounds of potassium; but sodium-salts have, to a great extent, displaced potassium-salts in commerce and the arts.

498. Carbonate of Potassium and Hydrogen (KHCO1).—This salt, commonly called the "bicarbonate" of potassium (KO, H,O, 2CO,), is prepared by passing a current of carbonic acid through a strong solution of carbonate of potassium; crystals of the bicarbonate will be deposited, which are permanent in the air, and require about 4 parts of cold water for solution. When the solution of this salt is long exposed to the air, or boiled, it loses one-fourth of its carbonic acid; when the dry salt is fused, it loses half its carbonic acid, and is converted into the carbonate. It is a valuable salt to the chemist and the apothecary, because it can be readily obtained in a state of purity; when itself made from a refined carbonate of potassium, it may be advantageously used as the material from which to prepare other pure compounds of this important element.

499. Hydrate of Potassium (KHO).-The manufacture of hydrate of potassium, from carbonate of potassium, resembles, in every detail, the preparation of caustic soda from carbonate of sodium (Exp. 233). The carbonate of potassium is dissolved in 10 or 12 times its weight of water, and decomposed by a cream of lime; carbonate of calcium is precipitated, and hydrate of potassium remains in solution. All that has been said of the concentration of the solution of hydrate of sodium (§ 488) is true, also, of hydrate of potassium.

Hydrate of potassium is a hard, whitish substance, possessing a peculiar odor and a very acrid taste. Like the hydrate of sodium, it rapidly absorbs moisture and carbonic acid from the air;

and since the carbonate of potassium thus formed is a deliquescent salt, this change will go on until the entire mass of hydrate is converted into a syrup of the carbonate; whereas, in the case of hydrate of sodium, the absorption of water and carbonic acid is soon arrested by the formation of a coating of non-deliquescent carbonate of sodium upon the surface of the lump of hydrate. In chemical industries and speculations, the question of success or failure often turns on such points as this; the advantage of a new material, for example, often depends upon just such differences as this between caustic soda and caustic potash.

500. The hydrate of potassium, cast into small sticks, is employed by physicians as a cautery,-a use which illustrates forcibly its destructive effect upon animal and vegetable matters. Like hydrate of sodium, its solution destroys ordinary paper, and cannot be filtered except through asbestos, or gun-cotton. A clear solution is best obtained by decantation from the subsided impurities. All vessels made of materials which contain silica are attacked by this caustic solution; and even platinum is slowly oxidized in its presence; vessels of gold and silver resist it best. This hydrate, like that of sodium, forms soaps with oils or fats; the sodium-soaps are hard, the potassium- soft. At a high temperature hydrate of potassium volatilizes without change; heat alone cannot decompose the caustic alkalies. In the chemical laboratory, solutions of caustic potash and caustic soda are in frequent use for absorbing acid gases, such as carbonic acid, and especially for separating the hydrates of other metals from solutions of their salts.

Exp. 244.-Dissolve a crystal of blue vitriol (sulphate of copper) in a few centimetres of cold water, and add to the solution several drops of a solution of caustic soda (or potash). The hydrate of copper is precipitated as a delicate, blue, insoluble powder, while colorless sulphate of sodium (or potassium) remains in solution.

CuSO, +2NaHO = CuH2O, + Na.SO. Sulphate of Copper.

Hydrate of Copper.

Exp. 245.-Place in a small flask 4 or 5 grms. of chalk or marble (carbonate of calcium), and 7 or 8 c. c. of water; then cautiously add chlorhydric acid, little by little, until effervescence ceases and the chalk is dissolved. When the effervescence is not violent, the flask