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patches upon the haulms and leaves. These spots appear about the time the plants attain their full growth, and when carefully examined are found to be surrounded by a ring of a paler colour. The whole of this outer ring is infested with a fungus called the Botrytis or Peronospora infestans, which is a constant accompaniment of the disease, if not its cause. If the weather be dry the progress of the disease is slow, but if a moist warm day supervene it will be found that the mould spreads with great rapidity, and sometimes the whole plant becoines putrid in a few days. The disease first shows itself in a tuber by appearing as a brownish spot, and the part affected may be cut out, leaving the remainder quite wholesome. None of the plans adopted for mitigating the potato disease have been very effective. The potato is also attacked by various insects, the most destructive being the Colorado beetle. The tubers consist almost entirely of starch, and being thus deficient in nitrogen, should not be too much relied on as a staple article of diet. Potatoes are extensively used as a cattle-food, and starch is also manufactured from them. In Maine, Vermont, and Northern New York this is an important industry. Enormous crops of this valuable esculent are grown in the United States, and much attention has been given to their improvement.
Potato-bug, a name given in America to many insects injurious to the potato, such as the Colorado beetle (which see).
Potchefstroom (pot'shef-strōm), a town in the Transvaal, South Africa, on the Mooi river, about 25 miles N. of the Vaal river. Pop. 3900.
Potemkin (pot-yom'kin), GREGORY ALEXANDROVITCH, Russian general, a favourite of the Empress Catharine II., born in 1736, died in 1791. Descended from an ancient Polish family, and early trained to the military profession, he soon after her accession attracted the attention of Catharine, who appointed him colonel and gentleman of the chamber. Soon after he gained the entire confidence of Catharine, and became her avowed favourite. From 1776 till his death, a period of more than fifteen years, he exercised a boundless sway over the destinies of the empire. In 1783 he suppressed the khanate of the Crimea, and annexed it to Russia. In 1787, being desirous of expelling the Turks from Europe, he stirred up a new war, in the course of which he took Oczakoff by storm (1788). In the following year
(1789) he took Bender, but as the finances of Russia were now exhausted Catharine was desirous of peace. Potemkin, however, resolved on conquering Constantinople, resisted the proposal to treat with the enemy, and went to St. Petersburg to win over the empress to his side (March, 1791); but during his absence Catharine sent plenary powers to Prince Repnin, who signed a treaty of peace. When Potemkin learned what had been done he set out for the army, resolved to undo the work of his substitute; but he died on the way, at Nicolaieff.
Potential, a term in physics. If a body attract, according to the law of universal gravitation, a point whether external or of its own mass, the sum of the quotients of its elementary masses, each divided by its distance from the attracted point, is called the potential. The potential at any point near or within an electrified body is the quantity of work necessary to bring a unit of positive electricity from an infinite dis tance to that point, the given distribution of electricity remaining unaltered.
Potential Energy, that part of the energy of a system of bodies which is due to their relative position, and which is equal to the work which would be done by the various forces acting on the system if the bodies were to yield to them. If a stone is at a certain height above the earth's surface the potential energy of the system consisting of the earth and stone, in virtue of the force of gravity, is the work which might be done by the falling of the stone to the surface of the earth.
Potential Mood, that mood of a verb which expresses an action, event, or circumstance as merely possible, formed in English by means of the auxiliaries may or can.
Potentilla, a genus of herbaceous perennials, nat. order Rosacea, found chiefly in the temperate and cold regions of the northern hemisphere, containing about 120 species. They are tall or procumbent herbs, rarely undershrubs, with digitate or unequally pinnate leaves, and yellow, red, purple, or white flowers. Some are favourite garden flowers. P. anserina is also called silver-weed, goose-grass, or wild tansy, the leaves of which are greedily devoured by geese; and P. fragariastrum, barren strawberry. P. reptans is a well-known creeping plant with conspicuous yellow flowers. The roots of P. anserina are eaten in the Hebrides, either raw or boiled. P. Tormentilla is used in Lapland and the Orkney Islands
both to tan and to dye leather, and also to dye worsted yarn. It is also employed in medicine as a gargle in the case of enlarged tonsils and other diseases of the throat, and for alleviating gripes in cases of diarrhoea.
Poten'za, a town of Southern Italy and a bishop's see, capital of the province of the same name, on a hill of the Apennines near the Basento, 85 miles E.S.E. of Naples. I is walled, and is indifferently built. It зuffered severely by earthquake in 1857, most of the buildings having fallen and many lives being lost. Pop. 20,353. The province is partly bounded by the Gulf of Taranto and the Mediterranean. Its chief productions are maize, hemp, wine, silk, cotton. Area, 3845 square miles; pop. 540,287.
Pote'rium, a genus of plants, nat. order Rosacea and sub-order Sanguisorbeæ. P. Sanguisorba, or salad-burnet, which grows on dry and most frequently chalky pastures, is said to be native about Lake Huron. is valuable for fodder, and is used in salad. It has pinnate leaves and tall stems surmounted by dense heads of small flowers.
Poti, a Russian town in Transcaucasia, on the eastern coast of the Black Sea It has extensive harbour works, and is connected by railway with Tiflis, but the trade is being drawn away by Batoum. Pop. 3112.
Pot Metal, an inferior kind of brass (copper, 10 parts; lead, 6 to 8), used for making various large vessels employed in the arts. Also a kind of stained glass in which the colours are incorporated with the substance by being added while the glass is in a state of fusion.
Potocki (po-tots'ki), an ancient Polish family, taking its name from the castle of Potok, and still holding possessions in Galicia and the Ukraine. Among its most distinguished members was Count Ignatius, grand marshal of Lithuania before the downfall of Poland, and a fellow-patriot of Kosciusko, born 1751. In 1791 he took refuge in Saxony, and published a political tract upon the establishment and fall of the constitution, returning, however, to share in the last struggle for independence. He then passed some time in the prisons of St. Petersburg and Warsaw, and died at Vienna 1809.
Poto'mac, a river of the United States, which forms the boundary between Maryland and Virginia, passes Washington, and after a course of nearly 400 miles flows into Chesapeake Bay, being about 8 miles wide
at its mouth. The termination of the tidewater is at Washington, about 125 miles from the sea, and the river is navigable for large ships all that distance. Above Washington are several falls which obstruct navigation.
Pot'oroo. See Kangaroo Rat.
Potosi (pot-o-se'; common pronunciation, po-to'se), a city of Southern Bolivia, in the department of same name, on the slope of the mountain mass of Cerro de Pasco, more than 13,000 feet above the sea-level, in bare and barren surroundings. It is regularly built, and has a cathedral, a mint, &c. It has long been celebrated for its silver-mines, which were at one time exceedingly productive, and have again begun to show an improved return. The city was founded in 1547, and the population increased so rapidly that in 1611 it amounted to 150,000, but at present it is only about 12,000.-The department has an area of 51,000 sq. miles, and is celebrated for its mineral wealth, especially silver. Pop. 237,755.
Pot-pourri (pō-pö-re; French) signifies the same as olla podrida (which see); also, and more generally, a musical medley, or a literary composition made up of parts put together without unity or oond of connection.
Potsdam, a town in Prussia, a bishop's see, capital of the province of Brandenburg, and the second royal residence of the kingdom, is charmingly situated in the midst of wooded hills, 17 miles south-west of Berlin, on the Havel, which here has several la es connected with it. It is, on the whole, one of the handsomest and most regularly built towns in Germany, and with its suburbs now covers a large space. The principal edifices are the royal palace (remodelled 1750), with interesting memorials of Frederick the Great; Garrison Church, containing the tombs of William I. and Frederick the Great; the Nikolai Church, the French Protestant Church, built after the model of the Pantheon at Rome; the town-house; and the Barberini Palace, erected by Frederick the Great in imitation of that at Rome, but rebuilt in 1850-52. Immediately to the west, outside the Brandenburg Gate (resembling a Roman triumphal arch), are the palace and park of Sans Souci. The palace, a building of one story, was erected under the direction of Frederick the Great; the grounds are finely laid out, and contain various fountains, &c., and an orangery 330 yards long. In the same neighbourhood is the New Palace, a vast brick building ex
hibiting much gaudy magnificence. A third palace in the environs of the town is called the Marble Palace. Potsdam was an unimportant place till the Great Elector selected it as a place of residence and built the royal palace (1660-71). Pop. 54,125.
Potstone (Lapis ollāris), a species of talc containing an admixture of chlorite. Its colour is green of various shades; it is greasy and soft, but becomes hard on being exposed to the air. It derives its name from its capability of being made into vases, &c., by turning. It was obtained by the ancients from quarries in the island of Siphnos and in Upper Egypt. It is now quarried in the Valais in Switzerland, in Norway, Sweden, Greenland, and the neighbourhood of Hudson's Bay.
Pott, AUGUST FRIEDRICH, German philologist, born in 1802. He studied at Göttingen, became a teacher in the gymnasium at C'elle, and subsequently privat-docent in the University of Berlin. Died 1887.
Potter, HENRY CODMAN, D.D., LL.D., D. C. L., author and divine, born at Schenectady, N. Y., May 25, 1835. A graduate of P. E. Theological Seminary, Va.; rector Grace Church, N. Y., 1868; Bishop of N. Y., 1887. Has published numerous works and is an energetic social reformer. In 1900 he visited the Philippines and published his views thereon.
Potter, JOHN, D.D., English classical scholar and divine, primate of all England, born in 1674, was the son of a linen-draper of Wakefield. In 1706 he became chaplain to Queen Anne. In 1708 he was appointed regius professor of divinity at Oxford, in 1715 was raised to the see of Oxford, and in 1737 appointed Archbishop of Canterbury. He died in 1747. His works include Archæologia Græca, a work on Greek antiquities, A Discourse on Church Government (1707), an edition of Clemens Alexandrinus (1714), and theological works (Oxford, 1753).
Potter, PAUL, a celebrated Dutch painter of animals, born at Enkhuisen in 1625. He received his first instruction in art from his father, Pieter Potter (1587-1655), a painter of some note. He devoted himself specially to the study of animals, producing his firstsigned picture, The Herdsman, in 1643. His works, specimens of which are in the more important European galleries, are highly esteemed. His colouring is brilliant, and the separate parts are delicately executed, yet without stiffness or mannerism.
His pictures are generally of small size; but there is a celebrated one of large size in the museum of the Hague. It represents a man and cattle, with a bull in the foreground, and is known as Paul Potter's bull. He died at Amsterdam in 1654, at the early age of twenty-nine. His engravings are much esteemed, and his paintings command high price.
Potteries, THE, a district of Staffordshire, head-quarters of the English earthenware and porcelain manufacture, compris ing the towns of Burslem, Hanley, Stoke, Longton, Newcastle-under-Lyme, Tunstall, &c.
Potter's Clay. See Clay.
Pottery, the art of forming vessels or utensils of any sort in clay. This art is of high antiquity, being practised among various races in prehistoric times. We find mention of earthenware in the Mosaic writings. The Greeks had important potteries at Samos, Athens, and Corinth, and attained great perfection as regards form and ornamentation. Demaratus, a Greek, the father of Tarquinius Priscus, king of Rome, is said to have instructed the Etruscans and Romans in this art. Glazed earthenware was long supposed to be of no older date than the 9th century of our era, and to have originated with the Arabs in Spain; but the discovery of glazed ware in Egypt, of glazed bricks in the ruins of Babylon, of enamelled tiles and glazed coffins of earthenware in other ancient cities, proves that this is not the case. The Arabs, however, seem to be entitled to the credit of having introduced the manufacture of glazed ware into modern Europe. The Italians are said first to have become acquainted with this kind of ware as it was manufactured in the Island of Majorca, and hence they gave it the name of majolica. They set up their first manufactory at Faenza in the 15th century. In Italy the art was improved, and a new kind of glaze was invented, probably by Luca della Robbia. The French derived their first knowledge of glazed ware from the Italian manufactory at Faenza, and on that account gave it the name of faïence. About the middle of the 16th century the manufactory of Bernard Palissy at Saintes in France became famous on account of the beautiful glaze and rich ornaments by which its products were distinguished. A little later the Dutch began to manufacture at Delft the more solid but less beautiful ware which thence takes ite
name. The principal improver of the pot-
cess of fermentation or disintegration renders it finer in grain and not so apt to crack in the baking. But even after this process the ingredients composing the paste are not intimately enough incorporated together nor sufficiently fine in texture until another operation has been undergone, called slapping or wedging, which consists in repeatedly breaking the lumps across and striking them together again in another direction, dashing them on a board, &c. This final process of incorporation is now most frequently performed by machinery.
In making earthenware vessels, if they are of a circular form, the first operation after the paste has been made is turning, or what is technically called throwing them on the wheel. This is an apparatus resembling an ordinary turninglathe, except that the surface of the chuck, or support for the clay, is horizontal instead of vertical. The chuck is in fact a revolving circular table, in the centre of which a piece of clay is placed, which the potter begins to shape with his hands. The rotary motion of the table gives the clay a cylindrical form in the hands of the potter, who gradually works it up to the intended shape. It is then detached from the revolving table and dried, after which, if intended for finely-finished ware, it is taken to a lathe and polished. It is at this stage that the handles and other prominent parts are fitted on, which is done by means of a thin paste of clay called slip. The articles are now removed to a room in which they are dried more thoroughly at a high temperature. When they have reached what is called the green state they are again taken to a lathe and more truly shaped, as well as smoothed and burnished. When the articles are not of a circular form, and accordingly cannot be produced by means of the wheel, they are either pressed or cast in moulds of plaster of Paris. In the
Though the various kinds of pottery and
Y Y Y Y
Successive Stages of Earthenware Vessel on the Potters Wheel.
former case the paste used is of the same consistence as that employed on the wheel; in the latter moulds of the same sort are used, but the clay mixture is poured into them in the condition of slip. By the absorption of the water in the parts next the dry mould a crust is formed of greater or less thickness, according to the time that the liquid is allowed to remain. The moulds are in two or more pieces, so as to be easily detached from the moulded article.
When shaped and dried the articles are ready for the kiln, in which they are exposed to a high temperature until they acquire a sufficient degree of hardness for use. The paste of which the earthenware is composed is thus converted into what is called bisque or biscuit. While undergoing this process of baking the articles are inclosed in larger vessels of baked fire-clay, called saggers, to protect them from the fire and smoke, and to distribute the heat more uniformly. The whole firing lasts from forty to forty-two hours. After the kilns have been allowed to cool very slowly, the articles are taken out, and if they are not to be decorated in colour, and sometimes also when they are to be so decorated, they are immersed in a vitrifiable composition called glaze, which, after the vessels have been a second time subjected to heat in glazed saggers, is converted into a coating of glass, rendering the vessels impermeable to water.
These processes are all that are necessary to complete a plain earthenware vessel, but very frequently the vessels are adorned with printed or painted decorations executed in colours, such as may be burned into the substance of the article. There are two methods of printing on earthenware: pressprinting, which is done on the bisque, and bat-printing, done on the glaze. In both cases an engraving is first executed in copper, and thence transferred, by means of a sheet of paper containing an impression, to the article requiring to be printed; but the processes are slightly different in detail. When the vessel has received its impression it is ready to be fired in the enamel kiln. Painting on earthenware is effected with a brush over the glaze.
All the numerous varieties of earthen ware are made in the manner just described, with only slight modifications in the nature of the ingredients of their composition or the processes of manufacture. Stoneware may be formed of the clays which are used for other vessels, with the addition of dif
ferent sorts of sand, and sometimes of cement. A greater degree of heat is applied than in the case of ordinary earthenware, and when some fluxing substance is added it has the effect of producing that state of semi-fusion which is the distinguishing quality of stoneware. A kind of semi-vitrified ware, first made by Wedgwood, takes its name from him. It is made of two different kinds of pastes, both very plastic. This ware is incapable of taking on a superficial glaze; but by a process called smearing, which is simply baking at a high heat in saggers coated internally with a glaze, acquires a remarkable lustre.
Porcelain or chinaware is formed only from argillaceous minerals of extreme delicacy, united with siliceous earths capable of communicating to them a certain degree of translucency by means of their vitrification. Porcelain is of two kinds, hard and tender. Both consist, like other earthenwares, of two parts - a paste which forms the biscuit, and a glaze. The biscuit of hard porcelain is composed of kaolin or china clay, and of decomposed felspar. The glaze consists of a felspar rock reduced to a fine powder, and mixed with water, so as to form a milky liquid into which the articles are dipped after a preliminary baking. Tender porcelain biscuit is made of a vitreous frit, composed of siliceous sand or und flints, with other ingredients added, all baked together in a furnace till half-fused, and then reduced to a condition of powder. The glaze of tender porcelain is a specially prepared glass ground fine, and made into a liquid by mixing with water. The processes employed in manufacturing porcelain wares are very much the same as those used for other kinds of earthenware, but requiring more delicacy and care. The biscuit paste even of hard porcelain has so little tenacity compared with that of earthenware that it cannot easily be shaped on the wheel, and is consequently more frequently moulded. The paste of tender porcelain is still less tenacious, so that the wheel cannot be used for it at all, and a little mucilage of gum or black soap must be added before it can be worked even in moulds. During the baking, too, it becomes so soft that every part of an article must be supported. Tender porcelain receives two coats of glaze.
Metallic oxides incorporated with some fusible flux, such as borax, flint, &c., are used for painting on porcelain. The colours are mixed with essential oils and turpen