:

which is procured from the burnt ashes of vegetables, hence the termination ash; the prefix pot was given on account of its being prepared in iron pots. It obtain ed the name of vegetable alkali, because it was supposed to exist only in vegetable substances and being prepared from nitre and tartar, it was called the "alkali of nitre," and likewise "salt of tartar," a name by which it is still known in the shops. By some it is distinguished by the name of "alkali," the plant from which it was originally procured. This substance, in its rough state, is prepared by burning wood, or other vegetable matter, and thus reducing them to ashes. The ashes are washed repeatedly with fresh waters, till the liquid comes off perfectly tasteless. The liquid thus obtained is evaporated, and the salt obtained is potash. If this substance is exposed to a red heat, many of the substances which are mixed with it are driven off, and what remains is much whiter, and on account of its colour it is called "pearl ash." In this state it is deemed sufficiently pure for the ordinary purposes of life, though by no means adapted to the purposes of the experimental chemist. Even when apparently freed from all extraneous substances, it is found to possess very different properties after having been subjected to certain processes. In one state it is mild and inactive; in another extremely acrid and corrosive. In the former cases it is united with carbonic acid gas, and is a carbonate of potash, and not pure potash. When deprived of this acid gas, it is powerful, corrosive, and highly caustic. Different methods have been proposed by different chemists to obtain this substance quite pure we shall transcribe that given by Professor Lowitz, of Petersburgh. He boils in an iron pot for two or three hours any quantity of potash, with double its weight of quicklime, and eight times the weight of the whole mixture of distilled or rain water. The liquor is to be set by to cool, and then filtered and evaporated, till a thick pellicle is formed on the surface. It is then set by till crystals are formed on it, which are crystals of extraneous salts, that are to be removed. The evaporation is to be continued, and the several pellicles removed as fast as they are formed. When the fluid ceases to boil, and no more pellicles arise, it is removed from the fire, and kept stirring till it is cold. It is then dissolved in double its weight of water; the solution is filtered and evaporated in a glass retort, till regular crystals begin to be deposited.

When a sufficient quantity has been formed, the liquid is decanted, and the salt is re-dissolved, after it is suffered to drain, in the same quantity of water. The decanted liquor is preserved in a well-closed bottle for several days, till it subside and become clear. It is then decanted, evaporated and crystallized again, and the process repeated, as long as the crystals afford with the least quantity of water solutions that are perfectly limpid.

Potash thus obtained is a white solid substance which is susceptible of crystallization, in long compressed, quadrangular prisms, terminating in sharp-pointed pyramids. These crystals, which are only obtained from very concentrated solutions, are soft and deliquescent. The taste is extremely acrid; and it is so corrosive, that it destroys the texture of the skin the moment it touches it: hence it has derived the name of caustic, and is employed in surgery for the purpose of opening abscesses, or for destroying excrescences. Its specific gravity is about 1.7. By a similar mode to that above described, pure soda may be prepared, substituting the carbonate of soda for the pearl-ash. They both possess the following properties:-1. They convert vegetable blues into a green colour. 2. They powerfully attract moisture. 3. They readily dissolve in water, and produce heat during the solution. They are not volatilized by a moderate heat: hence they are called fixed alkalies. Fixed alkalies have till very lately been numbered among the simple substances, not, however, without exciting in the minds of chemists a suspicion that they were compounds. Professor Davy has, in the course of the present and preceding years, put the matter beyond all doubt, and has proved to the satisfaction of every chemist, that they are a compound of oxygen and certain metallic bases, to which he has given the names of

POTASIUM, and SODIUM, or SODAIUM. Of these, and of the experiments which led to the discoveries, we shall proceed to give some account, having attended the repetition of his experiments at the lectures delivered last spring at the Royal Institution. Mr. Davy, in his first attempts to decompose the alkalies, made use of the aqueous solutions, and failed. He next made use of the potash in a state of igneous fusion, which he brought within the sphere of the galvanic battery: with this also he was unsuccessful in the main point; but some brilliant phenomena were produced. The potash appeared a

conductor in a high degree: a most intense light was exhibited at the negative wire, and a column of flame, which seemed to be owing to the developement of combustible matter, arose from the point of contact. Mr. Davy next tried several experiments on the electrization of potash rendered fluid by heat, with the hope of being able to collect the combustible matter, but he was still unsuccessful, "and I only," says he, "attained my ob. ject by employing electricity as the common agent for fusion and decomposition." Potash perfectly dried by ignition is a non-conductor, by a very slight addition of moisture, which does not perceptibly destroy its aggregation, it is rendered a conductor, and in this state it readily fuses and decomposes by strong electrical powers. A small piece of pure potash was placed upon an insulated disk of platina, connected with the negative side of the battery, in a state of intense activity; and a platina wire, communicating with the positive side, was brought in contact with the upper surface of the alkali, a vivid action took place, and the potash began to fuse at both points of electrization. There was a violent effervescence at the upper surface; at the lower or negative surface, there was no liberation of elastic fluid; but small globules, having a high metallic lustre, appeared; these were similar in visible character to quicksilver: some of them burnt with explosion and bright flame as soon as they were formed, and others remained, and were merely tarnished, and finely covered with a white film, which formed on them. "These globules," said the professor, "numerous experiments soon shewed to be the substance I was in search of, and a peculiar inflammable principle the basis of potash. I found that the platina was in no way connected with the result, except as the medium for exhibiting the electrical pow. ers of decomposition; and a substance of the same kind was produced, when pieces of copper, silver, gold, plumbago, or even charcoal, were employed for completing the circuit."

Soda, when acted upon in the same manner, exhibited an analogous result, and these effects equally took place in the atmosphere, and when the alkali was acted upon in the vacuum of an exhausted receiver; but these globules could not in either case be produced from crystallized alkalies. When a globule of the base of potash was exposed to the atmosphere, it immediately attracted oxygen,

and a white crust formed upon it, which proved to be pure potash. When the globules were strongly heated, and then suspended in oxygen gas, a rapid combustion with a brilliant white flame was produced, and these metallic globules were converted to an alkali, whose weight greatly exceeded that of the combustible matter consumed. When Mr Davy had thus detected the basis of the fixed alkalies, he had considerable difficulty to preserve and confine them, so as to examine their properties and submit them to experiments. He found, however, at length, that in recently distilled naph. tha they may be preserved many days, and that their physical properties may be easily examined in the atmosphere, when they are covered by a thin film of it. The basis of potash, at 60° Fahrenheit, is only imperfectly fluid; at 70° it becomes more fluid; and at 100° its fluidity is perfect, so that different globules may be easily made to run into one. At 50° it becomes a soft and malleable solid, which has the lustre of polished silver; and about the freezing point of water it becomes harder and brittle, and when broken in fragments exhibits a crystallized texture, of perfect whiteness and high metallic splendour. To be converted into vapour, it requires a temperature approaching that of the red heat. It is an excellent conductor of heat, and a perfect conductor of electricity.

Resembling the metals in all these properties, it is, however, remarkably different from any of them in specific gravity; for it will not sink in double distilled naphtha, whose specific gravity is only 7.70, that of water being considered as 1.000. Mr. Davy has determined by experiment, that its specific gravity is to that of mercury as 10 to 223, which gives a proportion to that of water nearly as 6 to 10; so that it is the lightest fluid body known. When this substance is introduced into oxymuriatic gas, it burns spontaneously with a bright red light, and muriate of potash is formed. When thrown upon water, it decomposes it with great violence, and an instantaneous explosion is produced with brilliant flame, and a solution of pure potash is the result.

When a globule is placed upon ice, not even the solid form of the two substances can prevent their union; for it instantly burns with a bright flame, and a deep hole is made in the ice, which is found to contain a solution of potash. When a globule is dropped upon mois

tened tumeric paper, it immediately burns, and moves rapidly upon the paper, as if in search of moisture, leaving behind it a deep reddish brown trace. So strong is the attraction of the basis of potash for oxygen, that it discovers and decomposes the small quantities of water contained in alcohol and ether, even when they are carefully purified. When thrown into the mineral acids, it inflames and burns on the surface. In sulphuric acid, sulphate of potash is formed; in nitrous acid, nitrous gas is disengaged, and nitrate of potash formed. When brought in contact with a piece of phosphorus, and pressed upon, there is a considerable action: they become fluid together, burn, and produce phosphate of potash. When a globule is made to touch a globule of mercury about twice as large, they combine with a considerable heat; the compound is fluid at the temperature of its formation: but when cool it appears as a solid metal, similar in colour to silver. If this compound be exposed to air, it rapidly absorbs oxygen; potash which deliquesces is formed; and in a few minutes the mercury is found pure and unaltered.

When a globule of the amalgam is thrown into water, it rapidly decomposes it with a hissing noise, potash is formed, hydrogen disengaged, and the mercury remains free. The basis of potash readily reduces metallic oxides when heated in contact with them. It decomposes common glass by a gentle heat, and at a red heat effects a change even in the purest glass. Mr. Davy has discovered that its base, like that of potash, is white, opaque, and has the lustre of silver. The property of welding, which belongs to iron and platina, at a white heat only, is possessed by this substance at common temperatures. It is very similar, in its more obvious properties, to the base of potash; but it has greater specific gravity, being to that of water nearly as nine to ten, or as 9348 to 10,000.

In

oxygen gas it produces a white flame, and sends forth bright sparks, occasioning a very beautiful effect. In oxymuriatic acid gas it burns vividly, with numerous scintillations of a bright red colour. In the quantity of it renders mercury a fixed solid, of the colour of silver, and forms an alloy with tin. When amalgamated with mercury, the amalgam will combine with other metals.

Mr. Davy tried this with iron and pla

し

tina, and had reason to believe that these latter metals remain in combination with the mercury, even when deprived of the new substance by exposure to the air. From several curious and ingenious experiments to ascertain the proportions of the bases and oxygen in the two fixed alkalies, he concludes that 100 parts of potash consist of about 84 basis, and 16 oxygen; and 100 parts of soda consist of about 76 or 77 basis, and 24 or 23 oxygen; or that potash may be considered as consisting of about 6 parts basis, and 1 of oxygen; and soda of 7 basis, and 2 oxygen. In reply to the question, whether the bases of potash and soda should be called metals, it may be said that they agree with metals in opacity, lustre, malleability, conducting powers as to heat and electricity, and in their qualities of chemical combination. Even their low specific gravity does not appear a suffi

cient

reason for making them a new class; for amongst the metals themselves there are remarkable differences in this respect, platina being nearly four times as heavy as tellurium; and tellurium is not much more than six times as heavy as the basis of soda. Conceiving the basis of the two fixed alkalies to be metals, Mr. Davy has named one Potasium, and the other Sodium; adopting that termination, which by common consent, has been applied to other newly discovered metals.

On an examination of the volatile alkali, and after a great number of complex and tedious experiments, Mr. Davy saw reason to conclude that ammonia contains oxygen as an essential ingredient, and that this cannot well be estimated at less body may, therefore, as he says, be collthan 7 or 8 parts in the hundred: this sidered as the principle of alkalescence, with as much reason as the French have made it the principle of acidity. After making some general remarks on the preceding facts, he suggests the proba bility, that the muriatic, fluoric, and boracic acid all contain oxygen as one of their constituent principles. The earths of barytes and strontian, as being most analogous to the alkalies, were likewise examined, and both yielded oxygen. In concluding this very important communication, Mr. Davy remarks, that an immense variety of objects of research is presented in the powers and affinities of the new metals produced from the alkalies. In themselves they will undoubtedly prove powerful agents for analysis; and

having an affinity for oxygen, stronger than any other known substances, they may possibly supersede the application of electricity to some of the undecompounded bodies. Further experiments, it is said, have enabled Mr. Davy, since his communication to the Royal Society, from which the above has been partly abridged, to decompose, in the most satisfactory manner, the barytes and strontites, and to show that the other alkaline earths are oxydes of highly combustible metals. It cannot now be doubted, that, in the hands of the great chemist, other bodies, hitherto deemed simple, or at least never yet analyzed, will speedily yield to the powers, either of the highly inflammable metals already discovered, or of a still further increase of the galvanic battery. Mr. Davy has decomposed carbonic acid by means of those metals, and has oxydated them by muriatic acid; and an excellent writer says, "it is now by no means improbable that charcoal itself, hitherto regarded as the most refractory of all substances, may be decomposed by the new instruments; and that the means of obtaining it pure, and even crystallized, shall at last be found; a discovery which would enable art to vie with nature in the fabrication of her most valuable produce." At any rate, to use the words of the Professor himself: "In sciences kindred to chemistry, the knowledge of the nature of the alkalies, and the analogies arising in consequence, will open many new views; they may lead to the solution of many problems in geology, and show that agents may have operated in the formation of rocks and earths, which have not hitherto been suspected to exist." See Philosophical Transactions of the Royal Society for 1808. Part I.

POT stone, in mineralogy, a species of the Clay genus. The colour of this mineral is a greenish gray, of different degrees of intensity. It occurs massive. The internal lustre is glistening and pearly. Fracture, sometimes curved, foliated, sometimes imperfectly slaty. It is soft, feels greasy, and difficultly frangible. It is found in beds with serpentine, at Como in the Grisons, in some parts of Saxony, and in Hudson's Bay. It is very nearly allied to indurated talc. It is refractory in the fire, and may be used for lining furnaces. It may be turned in a lathe, and made into a variety of vessels fit for culinary and other purposes. POTATOE, in botany, the English

name for a species of the tuberose-rooted Solanum. See SOLANUM.

POTENT, or POTENCE, in heraldry, a term for a kind of cross, whose ends all terminate like the head of a crutch. It is otherwise called the Jerusalem cross.

POTENTILLA, in botany, cinquefoil, a genus of the Icosandria Polygynia class and order. Natural order of Senticosa. Rosacea, Jussieu. Essential character: calyx ten-cleft; petals five; seeds roundish, naked, fastened to a small juiceless receptacle. There are thirty-two species, chiefly natives of the South of Europe.

POTERIUM, in botany, burnet, a genus of the Monoecia Polyandria class and order. Natural order of Miscellaneæ. Rosaceæ, Jussieu. Essential character: male, calyx four-leaved: corolla fourparted: stamina thirty to forty; female, calyx four-leaved; corolla wheel-shaped, five parted; pistils two; berry formed of the hardened tube of the corolla. There are five species.

POTHOS, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of Piperitæ. Aroideæ, Jussieu. Essential character: spathe; spadix simple, covered; calyx none; petals four; stamina four; berries two-seeded. There are thirteen species.

POTTERY, the manufacture of earthen ware, or the art of making earthen vessels. The inferior kinds of porcelain, or pottery, are prepared by the same process as that which has been described under the word PORCELAIN; less pure, but more fusible materials being employed, and of course, a less degree of heat being applied.

The better kinds of English stone-ware are composed of pipe-clay and pounded flints. The yellow stone-ware is made of the same materials, in other proportions. The first is glazed by throwing sea-salt into the furnace in which it is baked, when the heat is strong; the salt is converted into vapour, and this, being applied to the surface of the stone-ware, vitrifies it, and forms an excellent glazing. The yellow stone-ware is glazed by dipping the baked ware in water, in which is sus pended a mixture of pounded flint, glass, and oxide of lead. In the glazing of some kinds of stone-ware, oxide of tin enters into the composition with the oxide of lead, and gives a whiter glaze. All the coarser kinds of pottery are glazed with oxide of lead, this promoting so much the fusion and vitrification, that the low

heat at which they are baked is sufficient.

The wheel and lathe are the chief, and almost the only, instruments used in pot. tery; the first for large works, and the last for small. The potter's wheel consists principally in the nut, which is a beam, or axis, whose foot or pivot plays perpendicularly on a free-stone sole or bottom. From the four corners of the beam, which does not exceed two feet in height, arise four iron bars, called the spokes of the wheel; which, forming diagonal lines with the beam, descend, and are fastened at bottom to the edges of a strong wooden circle, four feet in diameter, perfectly like the felloes of a coachwheel, except that it has neither axis nor radii, and is only joined to the beam, which serves as an axis, by the iron bars. The top of the nut is flat, of a circular figure, and a foot in diameter; and on this is laid the clay which is to be turned and fashioned. The wheel, thus disposed, is encompassed with four sides of four different pieces of wood, fastened on a wooden frame; the hind piece, which is that on which the workman sits, is made a little inclining towards the wheel; on the fore piece are placed the prepared earth; on the side pieces he rests his feet, and these are made inclining, to give him more or less room. Having prepared the earth, the potter lays a round piece of it on the circular head of the nut, and, sitting down, turns the wheel with his feet till it has got the proper velocity: then, wetting his hands with water, he presses his fist or his fingers-ends into the middle of the lump, and thus forms the cavity of the vessel, continuing to widen it from the middle; and thus turning the inside into form with one hand, while he proportions the outside with the other, the wheel constantly turning all the while, and he wetting his hands from time to time. When the vessel is too thick, he uses a flat piece of iron, somewhat sharp on the edge, to pare off what is redundant; and when it is finished, it is taken off from the circular head, by a wire passed underneath the vessel.

The potter's lathe is also a kind of wheel, but more simple and slight than the former; its three chief members are, an iron beam or axis, three feet and a half high, and two feet and a half diameter, placed horizontally at the top of the beam, and serving to form the vessel upon; and another large wooden wheel, all of a piece, three inches thick, and two or three feet broad, fastened to the same beam at

the bottom, and parallel to the horizon. The beam or axis turns by a pivot at the bottom, in an iron stand. The workman gives the motion to the lathe with his feet, by pushing the great wheel alternately with each foot, still giving it a greater or lesser degree of motion, as his work requires They work with the lathe, with the same instruments, and after the same manner as with the wheel. The mouldings are formed by holding a piece of wood or iron, cut in the form of the moulding to the vessel, while the wheel is turning round, but the feet and handles are made by themselves, and set on with the hand; and if there be any sculpture in the work, it is usually done in wooden moulds, and stuck on, piece by piece, on the outside of the vessel.

POUCH, in military affairs, a case of black stout leather, with a flap over it, which is generally ornamented by a brass crown, &c. for the battalion men; a fuse for the grenadiers; and a bugle-horn for the light infantry. The pouch hangs from a buff cross-belt over the left shoulder, and is worn in that manner, by the infantry, for the purpose of carrying the ammunition,

POULTICE. See PHARMACY.

POUNCE, gum sandaric pounded and sifted very fine, to rub on paper, in order to preserve it from sinking, and to make it more fit to write upon.

POUNCE is also a little heap of charcoal dust, inclosed in a piece of muslin, or some other open stuff, to be passed over holes pricked in a work, in order to mark the lines or designs thereof on paper, silk, &c. placed underneath; which are to be afterwards finished with a pen and ink, a needle, or the like. This kind of pounce is much used by embroiderers, to transfer their patterns upon stuffs; by lace-makers; and sometimes also by engravers.

POUND, a certain weight, which is of two kinds, viz, the pound troy, and the pound avoirdupois; the one is divided into 12 ounces, the other into 16. The pound troy is to the pound avoirdupois as

576 to 700.

POUND also denotes a money of account; so called because the ancient pound of silver weighed a pound troy

See MONEY.

POUND, in law, any place inclosed to keep beasts in ; a common pound belongs to a lordship, or village, and there ought to be such a pound in every township. Some persons have of late very reasona ably complained of the ancient practice of keeping beasts for many days, in a com