Perspiration is essential to health, and when deficient may be promoted by exercise, the warm bath, or friction. At ordinary times, it should never fail of encouragement from the washing of the hands and face. Under the article ABLUTION, it has been observed that the use of linen has in some measure superseded the necessity of bathing; but as this succedaneum has not extended to the parts of the body here mentioned, the uses of abJution to these, for opening the pores, and cleansing the skin of the salts, mingled with dust from the atmosphere, which are left upon it by perspiration, are evident. The vessels through which perspiration is performed, lie obliquely under the scarfskin ; and are so amazingly small that according to a calculation made by Lewenhoeck, it appears that the mouths of one hundred and twenty-five thousand may be covered with an ordinary grain of sand. The matter of perspiration is a fine subtile fluid, for the excretion of which there are no glands; and which exhales from the body to the quantity of half that of the food.

PERUVIAN bark. See CINCHONA.

PETARD, in military economy, a metalline engine, somewhat resembling, in shape, a high-crowned hat. The petard may be considered as a piece of ordnance. It is made of copper mixed with brass, or of lead with tin; and its charge is from five to six pounds of powder, which fills it to within three finger-breadths of the mouth. Being stopped with a wooden tampion, and the month bound over with a cloth very tightly tied with ropes, it is covered with a madrier, or wooden plank, pierced to receive its mouth, and fastened down with ropes.

Its use is in clandestine attacks, to break down gates, bridges, or barriers, to which it is hung; and in countermines, to break through the enemy's galleries, and give his mines vent. Its invention is ascribed to the Huguenots, who took Cahors by its means, in the year 1579.

PETRIFACTION, in natural history, the conversion of wood, bones, and other substances into stone. Petrified bodies are more or less altered from their original state, according to the different substances among which they have lain in the earth. Some are found but very slightly changed; and others so highly impregnated with crystaline, sparry, pyritical, or other extraneous matter, as to appear mere masses of stone, or lumps of the matter of the common pyrites: but generally with the external dimensions, and more or less of the internal figure of the bodies into the pores of which this matter has made its way.

PETROLEUM, (also called rock-oil, which name is a translation of the former), an extremely subtile and penetrating fluid, of the bituminous kind, found in rivers, in wells, and trickling down the sides of hills, along with little streams of water. The substances which mineralogists have distinguished by the names asphaltum, maltha, petroleum, and naptha, are thought by some modern philosophers, to be mere varieties of one species, and form a series which passes into coal. Asphaltum forms the connection with pitch-coal. This is found in veins, and in small masses, and also sometimes on the surface of lakes. Maltha is softer, has a degree of tenacity, and a strong bituminous smell. Petroleum is semi-liquid; semi-transparent; of a reddish brown colour, and fetid odour. Naptha is

of a lighter colour, more or less transparent, perfectly liquid, light, odoriferous, volatile and inflammable. In several parts of France, petroleum is found floating on the water, and is known by the name of oil of Gabian.

PETROMYZON, the lamprey, a genus of fishes of the order Cartilaginei. It is shaped like an eel. Their are nine species. The Petromyzon marinus or the great lamprey, is usually of a brown olive colour tinged with yellowish white. It frequently grows to the length of three feet, is an inhabitant of the seas, but ascends the rivers early in the spring, in which it resides a few months, and then returns to the ocean. It is viviparous, and supposed to subsist almost entirely on worms and fishes. Its heart is enclosed not in a soft, but in a cartilaginous pericardium, constituting thus a singular deviation from the general structure of animals. Its spine is a cartilage rather than a bone. Fishes of this genus fasten themselves with the jagged edges of the mouth to large stones, with the most extraordinary firmness. They have a wonderful tenacity to life, and various parts of the body long continue to move after it is separated from the head: and the head itself will adhere to a rock for hours after the greater part of the body is cut away. The Severn is the river in this country, in which lampreys are usually found, and they are highly prized when they first arrive from the sea. The Petromyzon fluviatilis, or lesser lamprey, is very abundant in the Thames; many thousands are caught in the year, sometimes as many as half a million, which are exported for the Dutch cod and turbot fishery at the rate of

about 40 shillings a thousand. These fishes wil live many days out of water. In Russia they are taken from beneath the ice, packed in snow, exported to great distances, and will often recover themselves, when thrown into the water.

and

PETUNSE, in natural history, one of the two substances of which porcelain is made; a coarse kind of flint. The other is KAOLIN. They both consist of silex, alumine, and lime, and when mixed, they, together, give a compound of silex and alumina, with less than 5 per cent. of lime.

PEWTER, an artificial metal, compounded of tin, lead, and brass, in the following proportions: tin, one hundred weight; lead, fifteen pounds; and brass, six pounds.

pearance.

PHENOMENON, a Greek word, signifying an ap In philosophy, it is used to denote any appearance in nature, whether according to the usual course of things, or uncommon, or drawn forth by experiment.

PHALENA, the moth, in entomology, a genus of insects of the lepidoptera order. These insects are nocturnal, and fly abroad only in the evening and during the night, feeding on the nectar of the flowers. The larva is active and quick in motion, mostly smooth, more or less cylindrical, and preys on the leaves of various plants. Of all the moths the Phalana mori is by far the most important. This is a whitish moth, with a broad pale brown bar across each of the upper wings. The caterpillar or larva, known by the popular name of the silk-worm, is, when full grown, nearly three inches long. It feeds on the leaves of the white mulberry, but in defect of these on the leaves of