590 a polished iron tray, or into an iron mould to produce the customary form of "sticks," and allowed to cool. The solidified preparation must be at once bottled up, because it attracts the moisture and carbonic acid of the air with great avidity and deliquesces. According to the present writer s experience (Journ. Soc. Chem. Ind., May 1884). nickel basins are far better adapted than iron basins for the concentration of potash ley. The latter begin to oxidize before the ley has come up to the traditional strength, while nickel is not attacked so long as the percentage of real KHO is short of 60. For the fusion of the dry hydrate nickel vessels cannot be used ; in fact, even silver is perceptibly attacked as soon as all the excess of water is away ; absolutely pure KHO can be produced only in gold vessels. Regarding the action of potash on platinum, see PLATINUM (stipra, p. 191). Glass and (to a less extent) porcelain are attacked by caustic potash ley, slowly in the cold, more readily on boiling. Frozen caustic potash forms an opaque, white, stone-like mass of dense granular fracture; specific gravity = 2- 1. It fuses considerably below and is perceptibly volatile at a red heat. It is extremely soluble in even cold water, and in any proportion of water on boiling. The solution is intensely " alkaline " to test-papers. It readily dissolves the epidermis of the skin and many other kinds of animal tissue, hence the well-known application of the " sticks " in surgery. A dilute potash ley readily emulsionizes fats, and on boiling " saponifies " them with formation of a soap and of glycerin. Caustic potash is the very type of an energetic (mono-acid) basic hydrate (see CHEMISTRY, vol. v. pp. 486, 488). According to Tiinnermann s and Schiff s determinations, as calculated by Gerlach, the relation in pure potash ley between specific gravity at 15 C. and percentage strength is as follows : Percentages of KHO or KoO. Specific Gravity, if percentage refers to Percentages of KHO or K 2 O. Specific Gravity, if percentage refers to K,,0. KHO. K 2 O. KHO.
1 10 15 20 i-ooo 1-010 1-099 1-154 1-215 i-ooo 1-009 1-083 1-128 1-177 25 30 40 50 60 1-285 1-355 1-504 1-660 1-810 1-230 1-288 1-411 1-539 1-667 All commercial caustic potash is contaminated with ex cess of water (over and above that in the KHO) and with carbonate and chloride of potassium ; sulphate, as a rule, is absent. Absolutely pure potash has perhaps never been seen ; a preparation sufficing for most purposes of the analyst is obtained by digesting the commercial article in strong (85 per cent, by weight) pure alcohol. The hydrate KHO dissolves in the alcohol of the solvent ; the chloride and the carbonate unite with the water and form a lower layer or magma, from which the alcoholic solution of the KHO is decanted off, to be evaporated to dryness and fused in silver vessels ("potasse a 1 alcool"). The metal (potassium) has been known to exist since Lavoisier, but was first obtained as a substance by Hum phry Davy in 1807. He prepared it from the hydrate by electrolysis. Gay-Lussac and Thenard subsequently found that this substance can be reduced to the metallic state more easily by passing its vapour over white hot metallic iron ; but even their method as a mode of preparation was soon superseded by Brunner s, who, to the surprise of his contemporaries, produced the metal by simply distilling its carbonate with charcoal applying an old-established principle of ordinary metallurgy. Brunner s process is used to the present day for the production of the metal. One of those cylindrical, neckless, wrought -iron bottles which serve for the storing of quicksilver is made into a retort by taking out the screw-plug at the centre of one of the round ends and .substituting for it a short, ground-in, iron outlet pipe. This retort is charged with a black flux made from a mixture of pure and crude bitartrate so adjusted that the flux contains as nearly as possible the proportion of free carbon demanded by the equation K.jC0 3 + 2C = 2K + 3CO. It is then suspended horizontally within a powerful wind-furnace, constructed for coke as fuel. At first a mixture of coke and charcoal is applied, to produce the right tem perature for chasing away the moisture and enabling one to, so to say, varnish over the retort with borax and thus protect it against the subsequent intense heat. After these preliminaries coke alone is used and the fire urged on to, and maintained at, its maximum pitch, when potassium vapour soon begins to make its appearance. The condensation of this vapour, however, demands special methods, because even the cold metal would quickly oxidize in the air and act most violently on liquid water. Bmmier used to condense the vapour by passing it into a small copper vessel charged with rock- oil (see PARAFFIN, vol. xviii. p. 237), in which liquid the condensed metal sinks to the bottom and thus escapes the air. Donne and Maresca dispense with rock-oil altogether ; they receive the vapour in a dry condenser made of two flat rectangular trays of wrought iron which fit closely upon each other, enclosing a space such as might be used as a mould for casting a thin cake of any ordinary metal. This condenser has a short neck into which the outlet pipe of the retort fits ; and the pipe must be as short as possible, be cause it is essential (Donne and Maresca) that the hot vapour pass abruptly from its original high to a low temperature, to evade a certain range of medium temperatures at which the metal com bines with carbonic oxide into a black solid, which may obstruct the outlet pipe. The formation of this bye -product cannot be altogether avoided ; hence a long borer is inserted into the con denser from the first to enable one to clear the throat of the retort at a moment s notice. The condenser is kept as far as possible cold by the constant application to it of damp cloths. As soon as the distillation is finished the (still hot) condenser is plunged into a bucketful of rock-oil, to cool it down, the mould opened (under the oil), and the now solid metal taken out. The crude metal is always contaminated with some of the black solid and other mechanical impurities. To remove these the best method is to redistil it from out of a small iron retort and condense the vaponr in rock- oil according to Brunner s original plan. The purified metal is soft enough to be moulded (under rock-oil) into globular pieces, which are preserved in bottles filled to the top with the protecting liquid. But even this does not prevent gradual oxidation ; bright metallic potassium can be maintained in this condition only by preserving it in a sealed-up glass tube within a vacuum or in an atmosphere of hydrogen or some other inert gas. The black solid above referred to is a most dangerous substance. When exposed to the air it turns red and then explodes either spontaneously or on the slightest provocation by friction or pressure. Even if kept under rock-oil it gradually becomes explosive. The distillation of potassium, in fact, is a dangerous operation, which had better be left in the hands of specialists. Pure potassium is a bluish- white metal ; but on exposure to ordinary air it at once draws a film of oxide, and on prolonged exposure deliquesces into a solution of hydrate and carbonate. At temperatures below C. it is pretty hard and brittle ; at the ordinary temperature it is so soft that it can be kneaded between the fingers and cut with a blunt knife; specific gravity = 865. It fuses at 62"5 C. (Bunsen), and at 720 to 730 C. (Carnelley and Williams), i.e., considerably below its boiling point, begins to distil with formation of an intensely green vapour. When heated in air it fuses and then takes fire and burns into a mixture of oxides. Most remarkable, and charac teristic for the group it represents, is its action on water. A pellet of potassium when thrown on water at once bursts out into a violet flame and the burning metal fizzes about on the surface, its extremely high temperature precluding absolute contact with the liquid, except at the very end, when the last remnant, through loss of temperature, is wetted by the water and bursts with explosive violence. What really goes on chemically is that the metal decom poses the water thus, K -f H 2 O = KHO + H, and that the hydrogen catches fire, the violet colour of the flame being due to the potassium vapour diffused throughout it. Similar to that on water is its action on alcohol : the alcohol is converted into ethylate, while hydrogen escapes, K + C 2 H 5 . OH = C 2 H 5 . OK + H, this time without inflam mation. So strong is the basilous character of the element
that, in opposition to it, even ammonia behaves like an