obtained: these masses, whilst still damp, are broken into small fragments, or granulated, by submitting them to the action of toothed rollers in a machine constructed for the purpose. The grains are next sorted by means of sieves into different sizes, after which they are thoroughly dried in closets heated by steam, and, finally, are glazed, or polished by placing them in barrels caused to revolve about 39 times in a minute. Mining powder is often glazed by adding powdered graphite in the polishing barrels; this operation retards the rate of ignition, and diminishes the hygroscopic character of the powder. A cubic foot of good English cannon powder weighs about 58 lb. ; if below 55 lb., it is considered unfit for use. The heavier the powder the greater is its explosive power. Two ounces of the best English powder, when introduced into a mortar of 8 inches diameter, set at an angle of 45°, should throw a 68-lb. shot from 260 to 280 feet, on level ground.

Good gunpowder burns rapidly in the open air, leaving little residue, not blackening or kindling paper upon which it is fired. It has been found that a powerful concussion of powder between two pieces of iron will frequently kindle it, and if the powder be placed upon lead, or even upon a board, it may be exploded by the blow of a leaden bullet fired at it. The temperature at which it takes fire is about 480°.

The object of granulating the powder, independently of its diminishing its tendency to absorb moisture, is to favour the rapidity of inflammation, by leaving interstices through which the flame is enabled to penetrate and envelope each grain. The ignition of the whole charge does not take place simultaneously throughout, nor is it desirable that it should do so, otherwise sufficient time would not be given to allow the ball to receive the full advantage of the expansive force of the air generated; too rapid an action would be expended upon the barrel of the gun itself, and effects would be produced like those due to fulminating mercury; where a prolonged heaving force is required, as in blasting for mining operations, the action of the powder is still further retarded by mixing it with sawdust; the powder employed for this purpose usually contains 65 parts of nitre, 20 of sulphur, and 15 of charcoal. In the formation of the fusee, the quick and slow match, and certain kinds of fireworks, gunpowder is mingled with combustibles in various proportions.

The analysis of gunpowder is easily effected: 100 grains of the powder for examination are dried over sulphuric acid in vacuo ; the loss indicates the amount of moisture. The residue is

NITRITE AND CHLORATE OF POTASSIUM.

403

digested in water, and washed: the solution, when evaporated in a counterpoised capsule, and weighed, furnishes the amount of nitre and other salts. Nitrate of barium, when added to a solution of these salts, acidulated with nitric acid, will yield the sulphuric acid in the form of sulphate of barium; and nitrate of silver, when added to the liquid filtered from the sulphate of barium, will give the data for ascertaining the amount of chlorine from the precipitated chloride of silver. The charcoal and sulphur are contained in the portion which did not dissolve in water; they may be separated by means of bisulphide of carbon, or by the use of benzol at a boiling temperature, which dissolves out the sulphur, and leaves it in the crystalline form by spontaneous evaporation, whilst the charcoal is left undissolved and may be weighed.

A mixture of 3 parts of nitre, 2 of carbonate of potassium, and I part of sulphur, produces a substance known as pulvis fulminans, which when heated on an iron shovel until fusion takes place, explodes suddenly with a very loud report.

(573) NITRITE OF POTASSIUM (KNØ, or KO,NO3) is a white anhydrous, deliquescent salt, which may be obtained in crystals. It may be procured by decomposing nitre by fusion at a red heat, dissolving the residue in water, and allowing the nitre to crystallize out of the deliquescent nitrite, which may be obtained by evaporating the solution to dryness.

(574) CHLORATE OF POTASSIUM (KClO, or KO,CIO,=122°5) ; Sp. Gr. 1989: Composition in 100 parts, K., 38.36, and Cl, 6164.-One mode of preparing this salt has already been explained (382). It may be more economically obtained by converting milk of lime into a mixture of chlorate and chloride of calcium by transmitting chlorine gas in excess; to a concentrated solution of the mixed salts, chloride of potassium is then added in the proportion of 74'5 parts of the chloride to 168 parts of the caustic lime originally employed. The chlorate of calcium and chloride of potassium decompose each other, and chlorate of potassium and chloride of calcium are formed: boiling water dissolves both the chlorate of potassium and the chloride of calcium. The two salts are easily separated by crystallization, as the chlorate requires 16 parts of cold water for solution, and the chloride is soluble to almost any extent. From this mixture the crude chlorate is deposited in 6-sided prisms, which, on being redissolved in water, are crystallized in anhydrous rhomboidal pearly tables; it has a cooling taste, somewhat analogous to that of nitre: 100 parts of boiling water dissolve 615 of the salt. When heated to

404

PERCHLORATE AND CARBONATE OF POTASSIUM.

between 700 and 800° the salt melts, and at a higher temperature is decomposed, furnishing oxygen gas of great purity, and leaving chloride of potassium as a fixed residue behind. When heated to redness, 100 parts of the salt leave 60'79 parts of chloride of potassium, and 39:21 of oxygen are evolved. The chlorate is a more powerful oxidizing agent than nitrate of potassium; and if combustible substances, such as sulphur or phos. phorus, be rubbed with it forcibly, the combination of the combustible with oxygen, accompanied by detonation, ensues. Chlorate of potassium is principally consumed in the manufacture of lucifer matches, and as an oxidizing agent in certain operations of the calico-printer. When added to a solution acidulated with hydrochloric acid, it is often used in the laboratory as an oxidizing agent.

The friction tubes employed for firing cannon are charged with a mixture of 2 parts of chlorate of potassium, 2 of sulphide of antimony, and 1 part of powdered glass. A mixture known under the name of white gunpowder, consisting of chlorate of potassium, dried ferrocyanide of potassium, and sugar, has sometimes been manufactured for blasting purposes; but its preparation is attended with very great danger, owing to the facility with which it explodes by friction, a circumstance which has caused several fatal accidents.

(575) PERCHLORATE OF POTASSIUM (KCIO, or KO,CIO,=138·5); Comp. in Ico parts, K,→, 33'93; Cl2, 66.07.-This salt crystallizes in anhydrous prismatic needles, which are very sparingly soluble in cold water; its principal properties and the mode of preparing it have been already described (383). When heated to redness it gives off 46.11 per cent. of oxygen, leaving 53.89 of chloride of potassium.

(576) CARBONATE OF POTASSIUM (K ̧€Ð ̧=138, or KO,CO,=69); Sp. Gr. 2.207; Comp. in 100 parts, K2Ð, 6811; €, 31.89. -This important salt is obtained in large quantities for commercial purposes by lixiviating wood ashes, and evaporating the solution until it crystallizes; the mother-liquor, when it cools, is poured off from the crystallized salts, as it retains the more soluble carbonate of potassium, and when evaporated to dryness, affords the potashes of commerce, and these, when calcined, yield the impure carbonate known as pearlash. Different plants, when burned, furnish varying quantities of the alkali, which they extract from the soil: the leaves and young shoots, where the vital action is the most vigorous, are the parts which furnish the greatest quantity of alkali. Herbaceous plants, therefore, generally

CARBONATE OF POTASSIUM.

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furnish more than shrubs, and shrubs more than an equal weight of timber. It appears from the experiments of Violette that the variation in the quantity of ash obtained from different parts of the same tree is extremely great. Thus, taking the quantity of ash found in the heart-wood as the unit of comparison, the proportion in other parts was as follows:

and it is stated by Chevandier (Ann. de Chimie, III. x. 129) that the quantity of ash varied as follows for 100 parts of different portions of the wood of the undermentioned trees after drying at 284° ;

In the wine-producing countries, a considerable quantity of carbonate of potassium of good quality is furnished by burning the refuse yeast after the fermentation is complete. The yeast, for this purpose, is pressed, dried in the sun, and burned in shallow enclosures: this dry yeast furnishes nearly 10 per cent. of its weight of the carbonate. Potassium does not exist in plants in the form of carbonate; it occurs in them in union with the radicles of different organic acids: these organic acids are destroyed by the action of the heat during incineration. Such acids always contain more carbon than is sufficient, when oxidized by the air, to form the amount of carbonic acid radicle requisite to neutralize the potassium; and the carbonate of potassium thus produced, as it is not decomposed by a red heat, remains behind. In the ashes of plants various other saline substances are likewise present, but those which are soluble consist, in addition to the carbonate, principally of the sulphate and chloride of potassium; these alkaline salts usually amounting to from 10 to 20 per cent. of the entire quantity of ash.

A purer carbonate is obtained for chemical purposes by deflagrating a mixture of purified cream of tartar with an equal quantity of pure nitre. The mass is thrown, in small portions at a time, into a red-hot crucible: in this operation the nitre yields oxygen to the vegetable acid, converting the carbon which it contains into carbonic acid, which enters into combination with the alkali both of the tartar and of the nitre, since the two acids undergo mutual decomposition: the carbonate of potassium is extracted from the dry mass by lixiviation.

406

CARBONATE OF POTASSIUM-ALKALIMETRY.

Carbonate of potassium is a deliquescent salt, which is with difficulty obtained in oblique rhombic octohedral crystals (K ̧¤Ð ̧,2 H ̧Ð). Its reaction upon test-paper is strongly alkaline; it has an acrid, alkaline taste. Its solutions have a peculiar lixivial smell: 100 parts of water at 60° dissolve 90 of carbonate of potassium; and at the boiling-point take up 205 parts, or rather more than twice their weight, of the salt. Alcohol does not dissolve it. Carbonate of potassium fuses by exposure to a red heat, and at a very high temperature is partially volatilized; at a red heat it is decomposed by silica, carbonic anhydride being expelled with effervescence, whilst the silica, uniting with the alkali, forms with it a true silicate, the basis of one of the varieties of glass. Advantage is taken of this property, in the analysis of mineral substances which contain a large quantity of silica, and which are not easily decomposed by the action of acids. For this purpose the mineral to be analysed is reduced to an extremely fine powder by careful levigation; a portion of this powder is accurately weighed, and then intimately mixed with about 3 times its weight of carbonate of potassium (or, still better, with thrice its weight of a mixture of 5 parts of dried carbonate of sodium and 7 parts of carbonate of potassium); the whole is introduced into a platinum crucible, and exposed to a bright red heat for an hour. The mass enters into fusion, carbonic anhydride escapes with effervescence, and a silicate of potassium is formed; by which means all the bases of the mineral, which before were combined with the silica, are set at liberty. Upon now treating the mass with diluted hydrochloric acid, the silicate of potassium is decomposed, the earths and metallic oxides are dissolved, and the silica is partially dissolved and partially separated in the hydrated form. In order to decompose the hydrate of silica, the solution is evaporated to dryness, moistened with hydrochloric acid, and again treated with water; the whole is now placed upon a filter, and the silica, after being well washed, remains behind in a state of purity. The analysis of the filtered liquid is then finished according to the ordinary method adopted for substances directly soluble in acids.

Carbonate of potassium is largely consumed in the arts, as for example, in the manufacture of soap and of glass, and for preparing caustic potash and other compounds of potash. It also furnishes the chemist with one of his most indispensable reagents.

(577) Alkalimetry.—Since the quantity of alkaline carbonate, technically known as alkali, is liable to great variations in different samples of the ash,—and since the commercial value of pearlash