POTATO BUG

be placed in competition, an equal amount of ground yielding, according to Humboldt, thirty times greater weight of potatoes than of wheat. Potatoes consist almost wholly of starch, and are accordingly deficient in nitrogen and ill adapted for an exclusive diet. They are

POTENTILLA

W. of a line drawn between Chicago and St. Louis; 1874 it touched the Atlantic seaboard at numerous places, and, 1875, was common from Virginia to Maine. It thus traveled over 1,500 m. in a direct line within sixteen years and spread over an area of something like 1,500,000 sq. m. Spraying with solutions of Paris green is the cheapest effective protection to the plants attacked.

Potato, Sweet, a trailing vine of the morning-glory family, producing large thickened edible roots. The plant is widely cultivated for food in the tropics and the warmer portions of temperate countries. Its native country is unknown, but the evidence points strongly to an American origin. Its aboriginal American name was batatas, whence comes the word po

POTATO PLANT.

hardy, and grow well throughout a vast extent of the earth's surface. In 1907 the production in the U. S. of the species known as the Irish potato was 297,942,000 bu., valued at $183,880,000, from 3,124,000 acres; most productive states, New York, 34,850,000 bu.; Maine, 26,100,000; Michigan, 23,400,000; Wisconsin, 20,160,000; all others being under 20,000,000.

Potato Bug, name applied indiscriminately by farmers to a great many different insects that attack the potato, but commonly applied to the Colorado potato beetle (Doryphora ten

SWEET POTATO.

tato, later applied to the Irish, or round potato. The sweet potato is a staple crop in the S. of the U. S., where it thrives in sandy or loose soils. It is propagated by suckers or sprouts which spring from the tubers. It very rarely blossoms. There are many diseases of the sweet potato.

Potem'kin, Grigori, Prince, 1739-91; Russian statesman; b. Smolensk; originally an ensign in the army, superseded Orloff as lover of Catharine II; became practically the ruler of the empire; after crippling Turkey and founding Kherson in S. Russia, conquered the Crimea and the Kuban territory, became governor of the new possessions, dazzled Catharine with the vision of a new Byzantine empire, and on her visit, 1787, resorted to many stratagems to impress her with the splendor of her new dominions. In the following war with Turkey he conquered Otchakov, while Suvaroff won victory after victory. Catharine loaded him with honors and riches.

Potentil'la, genus of herbs and shrubs of the family Rosacea. There are many species, mostly herbs, about forty of which are natives of the U. S. The plants known as cinquefoil, five-finger, and tormentil belong to this genus.

POTHIER

Pothier (pō-tya'), Robert Joseph, 1699-1772; French jurist; b. Orleans; judge and Prof. of French Law at Orleans; principal work, "Pandects of Justinian Digested into New Order." His "Maritime Contracts" was translated by Caleb Cushing; "Contracts of Sale," by L. S. Cushing, and The Law of Obligations or Contracts," by W. D. Evans. Fully three quarters of the Civil Code is taken almost verbatim from his works.

Pot'hole, approximately vertical and cylindrie cavity in rocks, produced by a whirling current of water; occurs on the beds of streams whose steep grades give them high velocity, and they are often many feet in depth. It is believed that potholes are formed also beneath glaciers, where crevasses permit streams of water to plunge from the surface to the base. There is a series of holes referred to this action at Cohoes Falls, N. Y., one of which is over 60 ft. deep.

Poto'mac, river of the U. S., constituting the boundary between Maryland and Virginia and W. Virginia; formed by the junction of two branches, of which the N. rises in the Alleghanies of W. Virginia and the S. in the Shenandoah range, Virginia; resembles a bow in form; is nearly 400 m. long; receives as tributaries from Virginia the Shenandoah, Savage, and Monocacy; is an estuary from 6 to 8 m. wide for 100 m. of its lower course; enters Chesapeake Bay 75 m. from the Atlantic. Washington, D. C., is 125 m. above its mouth, to which the tide ascends, and it is navigable for large vessels. Above Washington are several falls.

Potomac Forma'tion, lowest of the geological formations representing the Cretaceous period on the Atlantic coast of N. America; rocks are variegated clays, friable sandstones, gravel, and sand, with many alternations, forming a series from 5 to 500 ft. thick; rest unconformably on crystalline rocks and on upturned and eroded strata of the Newark system; are overlaid by fossiliferous Eocene and Neocene strata.

Potosí (pō-to-se'), capital of department of same name, Bolivia; on side of Potosí Mount, a peak of the E. Cordilleras; 47 m. SW. of Sucre; is one of the highest inhabited places in the world (13,324 ft.). The climate is cold and changeable. Silver lodes were discovered here, 1546, and were long the richest known deposits in the world. The deposits are far from being exhausted, but, owing to the great depth to which shafts have been sunk and the difficulty of drainage, most of them have become unprofitable. During the eighteenth century Potosí, with its immediate vicinity, is said to have had a pop. of 170,000. The fine mint and the cathedral attest its former grandeur. Pop. (1906) est. at 23,450.

Potosí (Mexico). See SAN LUIS POTOSí. Pots'dam, city of Brandenburg, Prussia; at confluence of Ruthe and Havel, 17 m. SW. of Berlin; has many fine streets, public squares, promenades, public edifices, gates, bridges, and monuments; contains large barracks and other

POTTERY AND PORCELAIN

military establishments, educational and be nevolent institutions, and a great number of royal palaces and summer houses. Here is an astrophysical observatory, with two equatorial refractors, for spectroscopic, photographic, and other observations. Potsdam was founded by Elector Frederick William, but its modern splendor is principally due to Frederick the Great. Its industries comprise market gardening, manufactures of firearms, and the production of sugar, chocolate, tobacco, and cotton and woolen goods. Pop. (1905) 61,414.

Potsdam Sand'stone, in geology, a formation of the Cambrian period occurring in New York and Canada N. of the Adirondack Mountains. It is the basal member of the New York system, and was long supposed to be the oldest American Paleozoic formation. The name has been applied also to sandstones at the base of the Paleozoic series in various other parts of N. America, but it has now become known, from a comparison of fossils, that not all such sandstones are of the same age as the Potsdam. In the district N. of the Adirondacks the sandstone has a maximum thickness of 500 ft. It is extensively quarried for paving blocks and to less extent for flagging and building

stones.

Pot'stone, variety of talc, sometimes wrought, like soapstone, into pots, stoves, and kettles; abounds in Europe, and is coarser and more granular than the best soapstone.

Pottawat'tomies, tribe of N. American Indians belonging to the Algonquian family; at beginning of seventeenth century occupied the lower peninsular of Michigan; driven W. by Iroquois tribes, settled on Green Bay; then spread over what is now S. Michigan, N. Illinois, and N. Indiana; aided the French against the English; sided with the English in Revolutionary War till Wayne's victory, then signed Treaty of Greenville (1795); now composed of Pottawattomies of Huron, 75, in Michigan; Prairie Band, 560, in Kansas; and citizen Pottawattomies, 750, in Oklahoma.

Pot'ter, Paul, 1625-54; Dutch painter; b. Enkhuyzen; was unrivaled in painting domestic animals, which he invariably studied from life; best pictures are small, exhibiting exquisite finish, free handling, and brilliant effects of sunshine; but some are life size. The latter include the "Young Bull," in the Museum of The Hague. He also executed admirable etchings.

Pot'tery and Por'celain, words of several applications. Pottery denotes (1) objects made of material, generally clay, which is molded while soft and then baked till it becomes hard; (2) a place where such objects are produced; (3) the art and process of their manufacture. In a narrower and more customary sense, the word pottery is applied only to the coarser varieties of such objects; porcelain comprising the finer, translucent, or semitranslucent kinds. Both are generally made of clay, and are keramic ware (see KERAMICS), but the terms are stretched to cover some wares that are not clay wares.

POTTERY AND PORCELAIN

Clay is very widely dispersed over all the continents and the larger islands of the world, and everywhere has been used for making vessels since the most

remote antiquity. It is easy to mold by hand or by tools, keeps its shape well if kept moist, and can be remolded as often as desired. When baked, or fired, it hardens without serious shrinkage or deformation, and will then keep its shape permanently. Different kinds of clay give different colored baked wares. The clay which forms the chief ingredient of porcelain is called kaolin. This is white and mealy, and consists of decomposed feldspar. Clay made very thin, to be applied to the surface of a piece of pottery and baked with it, is called slip.

FIG. 1.-PHOENICIAN VASE, FROM THE CESNOLA COLLECTION.

The glaze or enamel with which the clay body is covered in many kinds of pottery and porcelain is more diverse in composition. tremely thin glaze of some fine kinds of ancient

The ex

pottery has not been successfully analyzed. The equally thin glaze on the hard stoneware of which vinegar jugs, Selterswater bottles, English beer bottles, etc., are made, is produced by throwing common salt into the oven, where it is decomposed, the soda of the salt combining with the alumina of the clay to form a thin, hard silicate. Other kinds of stoneware, and some kinds of soft earthenware, are covered with a lead glaze, made from white lead, flint, fragments of glass, and other materials in smaller quantities. These are ground together to a very fine powder, and mixed with water to form a thin paste, which is applied to the surface by dipping, or with the brush. The glaze of porcelain is made of a mixture of quartz and kaolin, or of feldspar and kaolin, with small quantities of other ingredients. In all glazes the requirements are that they shall combine readily with the surface of the body, and cling to it, that they shall fuse and become vitrified at a lower

FIG. 2.-ITALIAN MAJOLICA

VASE.

POTTERY AND PORCELAIN

temperature than that needed for firing the body, and that the surface, when baked, shall be hard and insoluble in ordinary liquids. It is also generally a requisite that the glaze should not contract so much in the firing as to crack; but the crackle, which is so great an ornament to many Oriental wares, is produced by allowing the glaze to shrink and separate, and then, in some cases, filling the cracks with color before another firing.

When a glaze is opaque and put on rather thickly, it is called enamel. The most common enamel is stanniferous-that is, made in part of an oxide of tin. Such an enamel is perfectly opaque, pure white, lustrous, and with a soft surface which receives color well, so that good painting on the surface of such an enamel is often more beautiful than the painting on porcelain. The pigments used for

painting on pottery and porcelain are fusible or vitrifiable colors. These have often

a

very different appearance before firing to that which they are to give to the finished piece.

The chief implement used by the maker of pottery or porcelain vessels is the potter's wheel, a horizontally revolving disk, on which the lump of clay is held, and which the workman revolves at his wish. By the rotary mo

FIG. 3.-EARTHENWARE KILN.

tion a true roundness is given to the vessel, and the soft clay is shaped and reshaped very quickly by a skillful hand, the vase or jar being drawn out larger, or spread broader, or modified in its shape by a touch as it whirls. Handles and spouts are molded separately, either by hand or in plaster molds and stuck on to the body while still soft. Elaborately modeled ornaments, figures of men and animals, and the like, whether attached to vessels or forming ornamental objects by themselves, are made with the sculptors' modeling tools.

When the shaping is done, the piece is allowed to dry somewhat before firing, but care must be taken not to let it dry so much as to crack. Common earthenware is fired only once. Glazed and enameled wares are fired once for the body and once for the covering. Painted wares are fired a third time for all colors which are applied upon the glaze, and a fourth time, always with decreasing temperatures, for gilding. The firing (in the kiln) is sometimes repeated more frequently, because the degree of heat and the length of the ex