KO.HO with an equivalent weight of potassium, in an atmosphere free from oxygen; decomposition of the water then takes place, and two equivalents of potassa are produced; thus:

KO.HO+K=2K0+H.

Properties.-Anhydrous potassa is a hard gray solid, fusible at a little above a red heat, and convertible into vapor at a very high temperature. When this substance is added to an equivalent quantity of water, very energetic combination takes place, and so much heat is developed that the resulting hydrate of potassa fuses and becomes red hot. Since the anhydrous potassa has received no application, it need not further occupy our attention.

HYDRATE OF POTASSA, CAUSTIC POTASSA.

KO.HO. Eq. 56.

§ 143. This is a compound of very great importance, since it is constantly employed in chemical operations.

Preparation.-Hydrate of potassa is generally prepared from the carbonate. One part of carbonate of potassa is dissolved in ten parts of water, in a clean iron pan, and the solution raised to the boiling point; to this solution is now added, by small portions at a time, a quantity of milk of lime, prepared by slaking one part, at least, of good quicklime with warm water, in a covered pan; the mixture should be allowed to boil for a minute or two after each addition of lime, and the water which evaporates should be replaced; when all the lime has been added, a small portion of the liquid is removed from the pan, and allowed to stand till the lime has subsided; the clear liquid is then decanted, and mixed with excess of dilute hydrochloric acid; should any considerable effervescence be produced, fresh portions of milk of lime are added to the boiling liquid till this is no longer the case; the pan is then covered, and the mixture boiled for a quarter of an hour, after which it is removed from the fire, and allowed to stand until all the solid particles have subsided; the clear liquid may be drawn off with a siphon (those portions which are still turbid may be set aside in stoppered bottles of green glass), and rapidly evaporated, in a silver basin, either to the requisite state of concentration, or, if the solid hydrate be required, until the hydrate itself begins to pass off in white fumes, when it may be poured upon a clean iron plate, and allowed to cool. If a scum of carbonate of potassa be formed upon the surface of the fused hydrate, it should be removed before pouring the latter from the pan.

The decomposition is thus represented :

KO.CO2+CaO.HO=KO.HO+CaO.CO,.

In this process, simple though it appears, considerable care is required to insure a good result; thus, if too small a quantity of water be present, the carbonate of potassa will not be decomposed by the lime; in fact, a strong solution of potassa is capable of withdrawing the carbonic acid from carbonate of lime. Again, if the mixture be not boiled after each addition of milk of lime, the carbonate of lime will not subside readily, and the resulting solution of potassa will be turbid. It is important that the pan in which the potassa is prepared be made of untinned iron, or of silver, since tin and copper would be acted upon by the alkali; the vessel in which the solution is allowed to subside should be covered, for the potassa is very prone to be reconverted into carbonate by absorption of carbonic acid from the air. The solution of potassa should be preserved in stoppered bottles, the glass of which is free from oxide of lead, since this latter is soluble in potassa; bottles of German glass, or of common green glass, are the best.

The hydrate of potassa thus prepared is liable to contain various impurities,

derived from the lime and carbonate of potassa; these are particularly noticed, and the method of discovering them described, in the section upon reagents. A purer variety of hydrate of potassa, known as alcohol-potash, is prepared by agitating the syrupy hydrate with alcohol, decanting the alcoholic solution, and evaporating. Pure potassa is also prepared from carbonate of potassa obtained by incinerating well-washed bitartrate of potassa; the lime for this purpose is generally obtained by the calcination of oyster-shells in an open fire.1

Properties.-Hydrate of potassa, when perfectly pure, is a white, hard solid (as met with in commerce, it is often in the form of thin sticks (potassa fusa), which have been cast in moulds, and have a bluish-green color, due to the presence of manganate of potassa). It fuses below a red heat, and at a higher temperature volatilizes in white vapors. The water cannot be expelled by heat. Placed in water, it dissolves rapidly, with disengagement of heat and a slight hissing sound. It also dissolves very readily in alcohol, which, after a short time, it decomposes, yielding a brown solution. When exposed to the air, it deliquesces to a syrupy liquid, which gradually absorbs carbonic acid, and passes into carbonate of potassa.

Hydrate of potassa is the most powerful alkali which we possess; its solution in water has a very soapy feeling on the skin, and immediately restores the blue color to reddened litmus, or imparts a brown color to turmeric. When brought in contact with acids, the water is displaced, and the potassa combines with the acid to form a potassa-salt. The salts formed by potassa with strong acids, when neutral in constitution, are also neutral in reaction. These salts are, with few exceptions, soluble in water.

The aqueous solution of potassa (liquor potassa), which is so very useful both as a chemical and medicinal agent, is prepared by arresting the evaporation of the liquid obtained by decomposing the carbonate of potassa with lime, as soon as it has attained a certain strength; this is indicated by the specific gravity of the solution, the amount of hydrate of potassa in which increases with its density. By reference to the tables given in larger chemical works, we may ascertain the amount of potassa contained in solutions of various densities. The liquor potassæ used in medicine has a specific gravity of 1.06, and appears to contain about 5 per cent. of real alkali. The ordinary solution employed in the laboratory contains about 25 per cent., and has a specific gravity 1.27.

Solution of potassa boils at a higher temperature than water, and the stronger the solution the higher will be its boiling-point; this solution, and all other strong alkaline solutions, should never be heated in vessels of glass or porcelain, since they readily attack these materials. Solution of potassa attacks cork, and must therefore be preserved in stoppered bottles.

A solution of hydrate of potassa in a very small quantity of hot water, if allowed to cool in a stoppered bottle, deposits small rhombohedral crystals of the formula KO+5HO, which dissolve in water with production of cold. Potassa, as has been already mentioned, may be decomposed by a powerful galvanic current; certain metals, iron and zinc, for example, at a high temperature, are also capable of abstracting its oxygen. Hydrate of potassa, when fused with sawdust and many other organic matters, oxidizes them at the expense of the water which it contains, hydrogen being evolved, and the potassa remaining as carbonate, the carbonic acid being formed by the oxidation of the carbon of the organic matter. We have seen that potassa is reduced by carbon at a high temperature. If chlorine be passed over potassa at a red heat, it displaces the oxygen, chloride of potassium being formed; whilst, if chlorine be allowed to act upon a solution of potassa, we obtain chloride of potassium, and a salt of potassa with an oxygen

Pure potass is sometimes prepared by decomposing sulphate of potassa with hydrate of baryta.

acid of chlorine; the same is the case with bromine and iodine; an analogous reaction takes place between potassa and sulphur or phosphorus, a sulphide or phosphide of potassium being formed, together with a salt of potassa with one of the oxygen-acids of these elements.

Uses of Potassa.-The solid hydrate of potassa is extensively used by the chemist for drying gases, for decomposing mineral silicates and various organic substances. It is also applied in surgery as a caustic. Solution of hydrate of potassa is extensively used by the soap-maker for preparing soft soap, and is constantly employed in the laboratory, where its very powerful alkaline properties render it useful for displacing weaker bases, and for absorbing acid gases, for example, carbonic acid, in organic analysis. In medicine, the solution of sp. gr. 1.06 is administered as an antacid, and as a solvent of uric acid in cases of gravel, &c.

NITRATE OF POTASSA, NITRE, SALTPETRE. KO.NO,

§ 144. This salt occurs in nature as an incrustation upon the surface of the earth in hot climates, such as India, Arabia, and South America. It is also found in certain caverns in Ceylon and other parts; these natural excavations occur in a limestone, which contains magnesia and feldspar. Some of these caves are the resort of innumerable bats, whose excrement collects in them, and doubtless is a great source for the production of nitre in these localities.

Saltpetre, as it is found in these crusts or deposits, is always more or less contaminated with the nitrates of lime, magnesia, and soda, besides their chlorides and sulphates.

Nitrate of potassa is also found in small quantities in the juices of plants, and in some waters.

In some of those localities where nitre-incrustations are found, this salt appears to exist in small quantities in the soil, being collected upon its surface by the heat of the sun, which causes the superficial moisture to evaporate, and deposit the salt dissolved in it, while the crust of earth, thus becoming very dry and porous, draws up fresh quantities of moisture, containing nitrates, from beneath, which is in turn evaporated; in this manner the crust of salt deposited gradually increases in thickness.

In other localities, however (e. g. in some of the caverns above alluded to), the saltpetre is evidently formed gradually, by the decomposition of animal and vegetable matter, in contact with certain bases.

Many examples might be quoted of the production of nitrates in this manner; we may mention, as one, the production of the so-called saltpetre-rot, a plumose incrustation of nitrates which is frequently observed upon the base of the external walls of buildings in crowded cities, imperfectly drained, when nitrogenized organic matter (manure) mixes with earthy salts in the street, or attaches itself to the mortar of the buildings.1

§ 145. It has already been stated that the production of nitric acid, from organized substances, appears to depend upon the oxidation of the ammonia produced in the putrefaction of nitrogenized bodies, at the expense of the atmosphere, in the presence of powerful bases, with which the nitric acid thus formed may combine; thus:

NH,+0,=3H0+NO,.

The existence of the bases in a porous condition is believed to assist the formation of nitric acid, probably by the great condensing power which porous

This species of incrustation must not be confounded with a similar efflorescence frequently observed on the walls of new buildings (particularly if these are plastered over), and which consists of sulphates and carbonates of the alkalies contained in the building materials, and gradually brought to the surface as the structure dries.

substances are known to exert over gases, thus bringing them within the sphere of action of chemical affinity. This argument receives great support from the fact that ammonia may be converted into peroxide of nitrogen, by passing it over spongy platinum heated to 572° F. (300° C.) It is obvious that, if that oxide can be thus formed, there is no obstacle to its final conversion into nitric acid.

Artificial production of Nitrates.-In countries where nitre does not occur, or where it is not easily imported, large quantities are prepared artificially, by what is termed the process of nitrification, the conditions necessary for the forma tion of nitrates being carefully attended to.

Vegetable and animal refuse containing nitrogen, such as the sweepings of slaughter-houses, dung, weeds, &c., is made into heaps with earth, limestone, old mortar, and ashes; these heaps are sheltered from rain, and are moistened from time to time with urine; after several months, an incrustation of nitrates forms on the surface; when sufficiently rich (or ripe), the nitrified earth, as it is termed, is extracted in the manner to be presently described.

In the Prussian saltpetre plantations, the nitre-beds are constructed in such a manner that they are never completely removed. That side of the mound which is exposed to the prevailing wind is perpendicular, while the back portion is built up in steps. The heap is watered from behind, while, as the front wall is exposed to the desiccating action of the wind and sun, the nitrates are there collected, and the rich outer coating is removed from time to time, fresh portions of material being added to the heap from behind.

In Sweden, where nitre is a revenue-tax, most of the peasants possess a nitreplantation on a small scale. Heaps are constructed in sheds, of the materials enumerated above, watered from time to time with urine, and maintained in a porous condition by the insertion of twigs. The mass is turned occasionally, and allowed to remain generally about two years.

In other countries (e. g. in Switzerland, where the stables are erected against the sides of the mountains), the liquid manure that penetrates through the roughly boarded floors of the stables, is collected beneath, in pits filled with a mixture of the above-mentioned substances.

The time required for the production of nitrates, in any quantity, varies considerably with the temperature of the atmosphere. It has been observed that from 59° to 68° F. is the temperature most favorable to the production of nitrates, while their formation is completely checked at 32° F.

PREPARATION AND PURIFICATION OF NITRE.-The separation of the nitrates from the earth is effected by lixiviation. The nitrified earth is broken up into small lumps, and placed in large tubs or troughs with false perforated bottoms, and lateral openings below these, stopped with plugs until required. The perforated bottom is covered with a layer of straw or small twigs, to prevent the holes from becoming stopped up by portions of the earth. Sufficient water is added to cover the earth, and allowed to remain together with it for about twelve hours, in order that the salts may be thoroughly extracted. The solution or lye

Some chemists imagine this power capable even of inducing the nitrogen and oxygen of the air to unite directly to form nitric acid, provided some impulse be imparted to their particles by the presence of organic matter undergoing decomposition, or of ready-formed ammonia. In support of their argument, they call attention to the large amount of animal matter required to produce any quantity of nitric acid, and to the circumstance that ammonia continually escapes into the atmosphere, whence it may be rapidly absorbed by porous earth or rocks, being one of those gases over which such bodies exert their influence in the most powerful manner.

2 One thousand cubic inches of good nitrified earth yield about five ounces of saltpetre. The surface of the mounds upon which the nitrates collect is removed from time to time, and set aside for lixiviation. It is generally about three years before a large nitre-mound is completely removed.

is afterwards allowed to run off from the openings at the bottom of the vessel.1 Generally, the liquor obtained from one quantity of earth is poured upon a second, and even a third, in order that a tolerably concentrated solution of nitrates may be obtained. This lye, which contains the nitric acid in combination with lime, magnesia, potassa, soda, and ammonia, besides considerable quantities of chlorides and sulphates of these bases, is now mixed with a strong solution of carbonate or sulphate of potassa, when the whole of the nitric acid is converted into nitrate of potassa.

The solution is allowed to stand until clear, when it is decanted, and transferred to a boiler, where it is rapidly boiled down until it has attained a certain strength. A large quantity of the impurities are separated in this operation; small quantities of earthy salts are first deposited, and as the solution becomes more concentrated, the chlorides of potassium and sodium (of which it contains. very considerable quantities) separate in small crystals, the solubility of these salts in water being only slightly increased by heat, when compared with that of nitre under similar circumstances. When the liquor has attained a certain specific gravity, it is drawn off from the boiler, and allowed to remain undisturbed, in large pans, at a temperature of about 122° F. (50° C.) until the chlorides have separated as far as possible; it is then decanted into other vessels, and allowed to crystallize.

The nitre thus obtained is still contaminated with small quantities of alkaline chlorides and with organic coloring matter. It is now digested with the smallest quantity of hot water necessary for the complete solution of the saltpetre (whereby a further quantity of chlorides is sometimes separated). The solution is then boiled with a small quantity of glue or gelatin, which possesses the property of rendering insoluble the whole of the organic matter by its coagulation, and collects as a scum upon the surface of the liquid, from which it is easily removed. When the solution is sufficiently concentrated, it is allowed to run through funnel-bags into crystallizing-pans, where it is continually agitated with wooden stirrers, until crystals are no longer deposited. The object of this may be explained in a few words: if a solution of nitre is allowed to crystallize undisturbed, it deposits very large striated crystals, containing considerable cavities, in which are inclosed portions of the mother liquor; if the latter contain any impurities, they will consequently be, to some extent, retained by the crystals. But if a solution of nitre, as it crystallizes, be continually stirred, the salt is deposited in very fine grains (called saltpetre-flour), which may be very easily freed from any trifling quantity of impurity that may adhere to their surfaces.

In order to avoid loss of product by washing this saltpetre-flour with water (which must, of course, dissolve a considerable quantity), recourse is had to a very ingenious method of purification, dependent upon the power possessed by water of exerting its solvent action upon several salts simultaneously, the amount of one salt present in a quantity of water not preventing the solubility of another, or of a third salt, in the same water.

The saltpetre-flour is placed in a trough, similar to that employed in the process of lixiviation; a saturated solution of pure nitre is then poured upon it, and allowed to remain in contact with it for a short time. Any chlorides that may

The lixiviated earth still retains a small quantity of nitrates, and is used, in the saltpetre plantations, for the construction of fresh mounds.

2 The solubility of nitre at 212° F. (100° C.) is about 14 times greater than it is at ordinary temperatures, while that of chloride of potassium is only about twice as great, and that of chloride of sodium is but slightly increased. If, therefore, a solution containing these three salts be concentrated, the greater quantities of the chlorides will be deposited as the water decreases, while the nitrate of potassa will not exhibit any symptom of crystallizing out.

The crystals of chlorides deposited in the above process are allowed to collect in a small basket suspended in the lye.