382 NOTATION OF MIXTURES OF ISOMORPHOUS COMPOUNDS.

the metallic base of the protoxide, N for the metallic base of the sesquioxide; the general formula for mica may then be expressed thus::

Mica = [ Ꮇ Ꮎ,3 ᎦᎥᎾ, 3 (N Ꮎ ᎦᎥᎾ ) ] .

2

Now the components of potash-mica are principally silicate of potash and silicate of alumina, the potash being the metallic protoxide and the alumina being the metallic sesquioxide; but sesquioxide of iron, sesquioxide of manganese, and sesquioxide of chromium are also isomorphous with alumina: these compounds frequently displace a portion of the alumina from its combinations, and this is especially the case with the sesquioxide of iron. The peculiarity of isomorphous metals, when they displace each other, is this that the displacement is liable to occur in any possible proportion; for example, in different specimens of mica the relative proportions of iron and aluminum are liable to great variations; this arises from the fact that the ferric silicate and silicate of aluminum which are isomorphous, may be mixed in any conceivable proportion, and will crystallize together without altering the form of the mineral. The same fact may also be represented by stating that they may vary indefinitely in amount, provided only that the quantity of the two metals taken together in any one specimen furnishes such a proportion of a metallic sesquioxide as is equivalent to the silica in that portion of the mineral; that is to say, that the two proportionals of metal required for combination with the 3 proportionals of oxygen in the sesquioxide, may either consist wholly of aluminum, or a small but indefinite proportion of the aluminum may have its place supplied by a small but equivalent quantity of iron, or a large proportion of the aluminum may have its place supplied by a corresponding and equivalent proportion of iron.

Now the method of notation adopted in the preceding formulæ is employed to indicate precisely this-that the proportions of the two or more metals, the symbols of which are bracketed together, thus, (AlFeMn)"''‚Ð ̧, are liable to vary within any conceivable limits, provided that the united amount of all the metals so bracketed be exactly sufficient to form a true sesquioxide with the three proportionals of oxygen.

2

In like manner, in the case of the potash in felspar, the place of part of the potassium may be supplied by sodium; but the proportions of the two taken together require exactly the same amount of oxygen, and consequently saturate the same proportion of silica, that I atom of potash alone would have required.

POTASSIUM-ITS PROPERTIES.

383

This frequent partial displacement of one isomorphous metal by another in native crystallized minerals, renders much caution necessary in interpreting the results of an analysis. The difficulty of fixing the formula of a mineral of course increases with the complexity of its composition, and it is with the silicates especially that these difficulties are experienced. It is usual, when the analytical operations are completed, to ascertain the proportion of oxygen in the silica, then the proportion of oxygen contained in the sesquioxides, and lastly the quantity of oxygen in the protoxides; because, however much the proportions of the different metals may vary in different specimens of the same mineral, the ratio of the oxygen in both sets of bases to the oxygen in the silica remains uniform. In felspar, for instance, if the proportion of oxygen in the silica be taken as 12, that in the sesquioxide of aluminum is 3, and that in the protoxide of potassium or sodium is 1.

(559) POTASSIUM.-This remarkable metal was discovered by Davy, in the year 1807, and its isolation marks an important era in the progress of philosophical chemistry. The alkalies and the earths had long been suspected to be compound bodies, but up to that period they had resisted all attempts to decompose them. When once potassium, however, had been separated from its compounds, and potash had been proved to be an oxide of this metal, the decomposition of the other alkalies and earths followed as a necessary consequence: more correct ideas upon fundamental points of chemical theory were introduced; new methods of research were placed within reach of the analytical chemist, and potassium itself, from its powerful attraction for oxygen, became an important addition to the reagents of the laboratory.

Properties.-Potassium is a bluish-white metal, which is brittle, and has a crystalline fracture at 32°; at temperatures a little above this it is malleable; at 60° it is soft; as the temperature rises it becomes pasty, and at 144°5 it is completely liquid. Whilst in the soft condition, two clean surfaces of the metal admit of being welded together like iron; at a red heat it may be distilled, and it yields a beautiful green vapour. Potassium is light enough to float in water, having a specific gravity of only o'865. If exposed to the air, even for a few minutes only, it becomes covered with a film of oxide: when heated to its point of volatilization it bursts into flame, and burns with great violence. The powerful attraction of potassium for oxygen is seen on throwing the metal into water, in which case part of the water is immediately decomposed; half the hydrogen of the water is displaced by the potas

384

PREPARATION OF POTASSIUM.

sium and hydrate of potash is formed, 2 H2O+K2=2 KHO+H2, while the escaping hydrogen carries with it a small portion of the volatilized metal, and taking fire from the heat evolved, burns with a beautiful rose-red flame; the metal melts and swims about rapidly upon the water, and finally disappears with an explosive burst of steam, as the globule of melted hydrate of potash which is formed during its oxidation becomes sufficiently cool to come into contact with the water. Potassium decomposes nearly all gases which contain oxygen, if it be heated in contact with them ; and at a high temperature it will remove oxygen from almost all bodies into the constitution of which that element enters. It becomes necessary therefore to preserve the metal either in exhausted hermetically sealed glass tubes, or under the surface of some liquid, like naphtha, which does not contain oxygen. At a heat short of redness potassium absorbs hydrogen and becomes converted into a greyish mass (HK,?); but if more strongly heated, the hydrogen is again expelled. Potassium enters directly into combination with the halogens and with sulphur, selenium, and tellurium, burning vividly when heated with them. It likewise absorbs carbonic oxide with facility when heated moderately in it, or when the vapour of potassium is allowed to condense slowly in an atmosphere of the gas; a black mass is thus formed from which the metal cannot be recovered. It furnishes rhodizonate of potassium when treated with water, and occasions considerable waste in the ordinary method of preparing potassium.

(560) Preparation.-1. Davy originally obtained potassium by decomposing a fragment of hydrate of potash (which had become slightly moistened upon its surface by exposure to the air for a few minutes) by the current of a voltaic battery of 200 or 250 pairs of six-inch plates, on Wollaston's construction. The dry hydrate is an insulator, but a trace of moisture confers upon it a sufficient degree of conducting power: under such circumstances globules of metallic potassium are separated at the negative wire, and may be preserved under naphtha. They burn vividly in air, leaving an intensely alkaline residue. This method of procuring the metal, however, furnishes it only in very small quantity, and is difficult and expensive.

2.-Gay-Lussac and Thénard, in 1808, invented a method by which potassium may be obtained by purely chemical means in greater abundance. Iron turnings were heated to whiteness in a curved gun-barrel, which was covered with a clay lute, to preserve it from the action of the air at a high temperature, and melted hydrate of potash was allowed to pass slowly over the ignited iron;

EXTRACTION OF POTASSIUM.

385

decomposition ensued, the iron combined with the oxygen, and potassium along with hydrogen passed forwards, the potassium condensing in a copper receiver which was kept cool.

3. The process by which potassium is now obtained consists in decomposing the carbonate of potassium by charcoal, a plan originally invented by Curaudau, and improved by Brunner. This operation has been carefully studied by Mareska and Donny (Ann. de Chimie, III. XXXV. 147). In order to ensure a successful result, attention to a number of minute precautions is requisite. The material which is best adapted to its preparation is the potassium salt of some vegetable acid, which, when decomposed by heat in a vessel from which air is excluded, leaves a large quantity of carbon. For this purpose the acid tartrate of potassium, or crude tartar, is preferred. About 6 lb. of this substance is placed in a capacious iron crucible furnished with a cover, and ignited till it ceases to emit combustible vapours. A porous mass of carbonate of potassium, intimately mixed with very finely divided carbon, is thus obtained: this is rapidly cooled by moistening the exterior of the crucible with cold water; the charred mass, when cold, is broken up into lumps about the size of a hazel-nut, and quickly introduced into a wrought-iron retort. This retort is usually made of one of the iron bottles in which mercury is imported; it is introduced into a furnace, a, as shown at b, fig. 329, and placed horizontally upon supports of fire-brick, ff; a wrought-iron tube, d, 4 inches long, serves to convey the

vapours of potassium produced during the distillation into a receiver, e, which it is found most advantageous to construct of the form shown on an enlarged scale in fig. 330. It consists of two pieces of wrought iron, a, b, which are fitted closely to each other, so as to form a shallow box only a quarter of an inch deep, and are confined in their places by clamp screws: the iron plate

II.

C C

386

PREPARATION AND PURIFICATION OF POTASSIUM.

should be one-sixth of an inch thick, 12 inches long, and 5 inches wide; the receiver is open at both ends, the socket fitting upon the neck of the iron retort. The object of preparing the receiver of this particular form is to ensure the rapid cooling of the potassium, and so to withdraw it from the action of the carbonic oxide which is disengaged during the whole process. Before this receiver is connected with the tube, d, the fire is slowly raised until the retort attains a dull red heat; powdered vitrified borax is then sprinkled over its exterior; the borax melts, and forms a coating which protects the metal from oxidation. The beat is then urged until it becomes very intense. A mixture of coke and charcoal forms a fuel well adapted to this purpose; care should be taken that the temperature of the furnace be raised as equally throughout every part as possible. When a full reddishwhite is attained, vapours of potassium begin to appear, and burn with a brilliant flame: the receiver is now adjusted to the iron neck of the retort, which is not allowed to project more than a quarter of an inch through the iron plate which forms part of the front wall, c, of the furnace, lest the tube should become obstructed by the accumulation of solid potassium. Should any obstruction occur, it must be removed by thrusting in an iron rod; if this fails, the fire must be immediately withdrawn; this is readily effected by removing the fire bars, g, from the furnace, with the exception of two which support the retort; the fuel thus falls into the ashpit. The receiver is kept cool by the application of a wet cloth upon its exterior. When the opera

tion is complete, the receiver with the potassium is removed, and instantly plunged into a vessel of rectified Persian naphtha, provided with a cover. The vessel is kept cool by immersion in water. When this apparatus is sufficiently cold, the potassium is detached, and preserved under naphtha.

In order to obtain the maximum produce of potassium, it is necessary that the mixture of carbonate of potassium and carbon should contain 1 atom of the carbonate to 2 of carbon, or 138 parts of the carbonate by weight to 24 of carbon. Upon the application of heat the mixture is wholly converted into carbonic oxide and potassium; K ̧¤Ð ̧ + ¤¿ K, +30. The charge usually yields about one-fourth of its weight of crude potassium, some loss during the process being inevitable. Donny and Mareska found this loss to amount to about one-third of the entire quantity of the metal contained in the charge.

=

The potassium so obtained is not pure; it is necessary to subject it to a second distillation in an iron retort.

This pre