where it was discovered. This substance is now formed, chiefly, during the decomposition of animal substances in high temperatures. Three parts of blood, evaporated to dryness in an iron dish, are to be mixed with one part of subcarbonate of potash, (common pearlash) and calcined in a crucible, which should be only twothirds filled by the materials, and covered with a lid. The calcination must be continued, with a moderate heat, as long as a blue flame issues from the crucible; and when it becomes faint, and likely to be extinguished, the process must be stopped. Throw the mass, when cold, into ten or twelve parts of water; allow it to soak a few hours, and then boil them together in an iron kettle. Filter the liquor, and continue pouring hot water on the mass as long as it acquires any taste. To this solution, add one composed of two parts of alum, and one of sulphate of iron, in eight or ten of boiling water, and continue the mixture as long as any effervescence and precipitation ensues. Wash the precipitate several times with boiling water. It will have a green colour; but on the addition of a quantity of muriatic acid, equal to twice that of the sulphate of iron which has been used, it will assume a beautiful blue colour. Wash it again with water, and dry it in a gentle heat. In this state it is the pigment, called Prussian blue, which consists of a mixture of prussiate of iron with alumine. From prussiate of iron, the prussic acid may be separated by the following process: mix two ounces of red oxide of mercury, prepared by nitric acid, with four ounces of finely powdered Prussian blue, and boil the mixture with twelve ounces of water in a glass vessel, shaking frequently.Filter the solution, which is a prussiate of mercury, while hot, and when cool, add to it, in a bottle, two ounces of iron filings, and six or seven drachms of sulphuric acid; shake these together, decant the clear liquor into a retort, and distill off one-fourth of the liquor. The distilled liquor is the prussic acid, which combines with alkalies and earths, and has many of the properties belonging to the other acids. It has a sweetish taste, and a smell resembling that of bitter almonds; it does not redden blue vegetable colours. It precipitates sulphurets, and curdles soap. It separates alumine from nitric acid. Oxygenized muriatic acid entirely decomposes it. It does not appear to have a strong affinity for alkalies, nor does it take them from carbonic acid, for no effervescence arises on adding it to a so

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lution of alkaline carbonates; on the contrary, its combinations with alkalies and earths are decomposed by exposure to carbonic acid, even when highly diluted, as in atmospheric air. It readily combines, however, with pure alkalies, destroys their alkaline properties, and forms crystallizable salts. It does not precipitate iron blue, but green, and this green precipitate is soluble in acids. The rays of light render the green precipitate blue, as does also the addition of metallic iron, or sulphurous acid.

PSIDIUM, in botany, guava, a genus of the Icosandria Monogynia class and order. Natural order of Hesperida. Myrti, Jussieu. Essential character: calyx five-cleft, superior; petals five; berry one-celled, many-seeded. There are eight species, natives of the East and

West Indies.

PSITTACUS, the purrot, in natural history, a genus of birds of the order Pica. Generic character: bill hooked, upper mandible moveable; nostrils round in the base of the bill, and sometimes covered with a cere; tongue fleshy, broad, and blunt at the end, head large, crown flat; toes formed for climbing. These abound within the tropics, and live on seeds and fruits in their natural state, but in confinement will eat both flesh and fish. They often appear in flocks, yet are in such cases generally somewhat separated into pairs. They are noisy, mimetic, singularly capable of articulating human sounds, extremely docile, and long lived. They breed in the hollows of trees, with out constructing any nest, and use their feet as hands to convey food to their mouths. Latham notices one hundred and thirty-three species, and Gmelin no fewer than one hundred and sixty-nine. The general division is regulated by the evenness or unevenness of the tails. The following are the principal species.

P. macoa, or the red and blue maccaw, is as large as a capon, and inhabits South America. With its bill it breaks a peach stone with the most perfect ease. These birds lay their eggs in decayed trees, and often enlarge the hollow for this purpose with their bills. They are used for food in vast numbers in Cayenne. They are, in common with many species, exposed

to fits when confined.

P. rufirostris, or the long-tailed green parrakeet, is of the size of a blackbird, extremely clamorous, and highly imitative. These birds are seen in large flocks, and alighting on certain trees, can with dif ficulty be distinguished, in consequence

of the similar colour of their plumage to that of the leaves. They inhabit various parts of America, are used for food, and are extremely fat. The above have tails uneven at the end.

P. Meluccensis, or the Molucca cockatoo, inhabits the Moluccas, is about fifteen inches long, and is regarded by Buffon as one of the most docile and interesting birds of the tribe.

P. pullarius, or the red-headed Guinea parrakeet, is of the size of a lark, and is extremely common in many parts of Africa. These birds are peculiarly distinguished by their mutual affection. They are exported from Africa in considerable numbers, for their beauty and attachment, and not on account of any power of arti culation or enchantment of melody, their sounds being harsh and grating. Few, however, survive the voyage. They are kept in cages, in pairs, and the attentions of the male to the female are highly tender, elegant, and interesting. He extricates the seeds from their husks, and presents them to her in this prepared state, and appears restless and miserable on the slightest separation. Indeed, the attachment is reciprocal, the sadness of one always producing distress in the other; and the death of either involving the survivor, generally, in fatal as well as fruitless grief.

P. Carolinensis, the Carolina parrot, is about thirteen inches long, yellowish green, head and fore part of the neck yellow; front, cheeks, and edge of the shoulders rufous; bill and orbits whitish; feet pale flesh colour, claws black; greater wing-coverts and primaries yellow, with a tinge of green; shafts of most of the feathers black. Tail long, cuneiform; knees and vent fulvous. Inhabits the southern parts of the United States, as far north as Maryland, but on the rivers Mississippi and Ohio it is of frequent Occurrence much farther northwardly. It is the only species found in the United States.

lar to the sound of a child's trumpet, and, being easily domesticated, will often follow the person to whose care it is committed, through the streets, making this singular noise. It may be fed on bread and fish. It runs fast, aided by the expansion of its wings. When confined with poultry, it often annoys both common fowls and turkeys, and, indeed, occasionally destroys them. It will follow the negroes in the West Indies, and catch at their legs, not unfrequently producing blood. Their flesh is esteemed a considerable delicacy.

PSORALEA, in botany, a genus of the Diadelphia Decandria class and order-Natural order of Papilionacea or Legumi

nosæ.

Essential character: calyx besprinkled with callous dots, the same length with the legume, which has only one seed in it. There are thirty-three species, chiefly natives of the Cape of Good Hope.

PSYCHOTRIA, in botany, a genus of the Pentandria Monogynia class and order. Natural order of Stellatæ. Rubiacex, Jussieu. Essential character: calyx five-toothed, crowning; corolla tubular; berry globular; seeds two, hemispherical, grooved. There are thirty-nine species.

PTELEA, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of Terebintaceæ, Jussieu. Essential character; calyx four-parted, inferior; corolla four-petalled; stigmas two; fruit with a roundish membrane, having one seed in the middle. There is but one species, viz. P. trifoliata, threeleaved ptelea, or shrubby trefoil, a native of North America.

PTERIS, in botany, a genus of the Cryptogamia Filices class and order. Natural order of Filices or Ferns. Generic character: fructifications in an uninterrupted marginal line; involucre from the margin of the frond turned in, uninterrupted, separating on the inner side. There are thirty-four species.

PTEROCARPUS, in botany, a genus of the Diadelphia Decandria class and order. Natural order of Papilionacea or Luguminosa. Essential character: calyx five

ricose; seeds few, solitary. There are six species, found chiefly in South America and the West-Indies.

PTERONIA, in botany, a genus of the Syngenesia Polygamia Equalis class and order. Natural order of Compositæ Discoideæ. Cinarocephala, Jussieu. Essential character: receptacle with many

parted bristles; down subplumose; calyx imbricate. There are eighteen species, all found at the Cape of Good Hope.

PTEROSPERMUM, in botany, a genus of the Monadelphia Decandria class and order. Essential character: calyx single, five-parted; corolla five-petalled; filaments fifteen, with five ligules, one between every three filaments; capsule five-celled, with the cells two-valved; seeds many, winged. There are two species, viz. P. suberifolium and P. acerifolium, both natives of the East Indies.

PTEROTRACHEA, in natural history, a genus of the Vermes Mollusca class and order. Generic character: body detached, gelatinous, with a moveable fin at the abdomen or tail; two eyes placed within the head. There are four species.

PTINUS, in natural history, a genus of insects of the order Coleoptera. Generic character: antennæ filiform, the last joints larger; thorax nearly round, not margined, receiving the head. There are about forty species, divided into sections: A. feelers clavate, lip entire: B. feelers filiform, lip bifid. Of the former section is P. pulsator, or death-watch, which is of a dusky colour, with irregular grey brown spots. This insect is found in various parts of Europe, in old wooden furniture, makes a peculiar ticking with the fore part of its head, resembling the beating with the nail upon a table: this is done in several distinct strokes in the night time, and has been considered by the common people as prophetic of some fatal occurrence in the family, but is nothing more than the call of one sex to the other. This must not be confounded with a much smaller insect of a very different genus, which makes a sound like the ticking of a watch, and continues for a long time without intermission. This belongs to a different order, and is the Termes pulsatorium of Linnæus. But the real death-watch of vulgar superstition is the ptinus. P. pertinax is brown, imma. culate; thorax compressed. It inhabits Europe, and is very destructive to wooden furniture and books. When touched, it draws in its head and legs, and becomes immoveable. An insect strongly allied to this species inhabits the United States, and has generally been considered as the same, but from some of its characters we should suppose it to be specifically distinct.

PTOLEMAIC, or PTOLEMÆAN system of astronomy, is that invented by Claudius Ptolemy. This hypothesis supposes the

earth immovably fixed in the centre, not of the world only, but also of the universe and that the sun, the moon, the planets, and stars, all move about it from east to west, once in twenty-four hours, in the order following, viz. the Moon next to the Earth, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, the fixed stars, the first and second crystalline heavens, and above all the fiction of their primum mobile. This system, or hypothesis, was first invented and adhered to, chiefly because it seemed to correspond with the sensible appearances of the celestial mo. tions

PTOLEMY, (CLAUDIUS,) in biography, a very celebrated geographer, astronomer, and mathematician, among the ancients, was born at Pelusium, in Egypt, a bout the seventieth year of the Christian era, and died, it has been said, in the seventy-eighth year of his age, and in the year of Christ 147. He taught astronomy at Alexandria, in Egypt, where he made many astronomical observations, and composed his other works. It is certain that he flourished in the reigns of Marcus Antoninus and Adrian; for it is noted in his Canon, that Antoninus Pius reigned twenty-three years, which shows that he himself survived him he also tells us in one place, that he made a great many observations upon the fixed stars at Alexandria, in the second year of Antoninus Pius; and in another, that he observed an eclipse of the moon in the ninth year of Adrian; from which it is reasonable to conclude, that this astronomer's observations upon the heavens were many of them made between the year 125 and 140.

Ptolemy has always been reckoned the prince of astronomers among the ancients, and in his works he has left us an entire body of that science. He has preserved and transmitted to us the observations and principal discoveries of the ancients, and at the same time augmented and enriched them with his own. He corrected Hipparchus's catalogue of the fixed stars; and formed tables, by which the motions of the sun, moon, and planets might be calculated and regulated. He was, indeed, the first who collected the scattered and detached observations of the ancients, and digested them into a system, which he set forth in his " Μεγάλη Zuvrage, sive Magna Constructio," divided into thirteen books. He adopts and exhibits here the ancient system of the world, which placed the earth in the centre of the universe; and this has been

called from him the Ptolemaic System, to distinguish it from those of Copernicus and Tycho Brahe.

About the year 827, this work was translated by the Arabians into their language, in which it was called "Almagestum," by order of one of their kings; and from Arabic into Latin, about 1230, by the encouragement of the Emperor Frederic II. There were also other versions from the Arabic into Latin; and a manuscript of one done by Girardus Cremonensis, who flourished about the middle of the fourteenth century, Fabricius says, is still extant, in the library of All Souls College, in Oxford. The Greek text of this work began to be read in Europe in the fifteenth century, and was first published by Simon Grynæus, at Basil, 1538, in folio, with the eleven books of Commentaries by Theon, who flourished at Alexandria, in the reign of the elder Theodosius. In 1541, it was reprinted at Basil, with a Latin version by George Trapezond; and again at the same place in 1551, with the addition of other works of Ptolemy, and Latin versions by Camerarius. We learn from Kepler, that this last edition was used by Tycho.

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Of this principal work of the ancient astronomers, it may not be improper to give here a more particular account. general it may be observed, that the work is founded upon the hypothesis of the earth's being at rest in the centre of the universe, and that the heavenly bodies, the stars and planets, all move round it in solid orbs, whose motions are all directed by one, which Ptolemy calls the primum mobile, or First Mover, of which he discourses at large. But to be more particular, this great work is divided into thirteen books.

In the first book, Ptolemy shows that the earth is in the centre of those orbs, and of the universe itself, as he understood it; he represents the earth as of a spherical figure, and but as a point in comparison of the rest of the heavenly bodies: he treats concerning the several circles of the earth, and their distances from the equator; as also of the right and oblique ascension of the heavenly bodies in a right sphere.

In the second book he treats of the habitable parts of the earth; of the elevation of the pole in an oblique sphere, and the various angles which the several oircles make with the horizon, according to the different latitudes of places; also of the phenomena of the heavenly bodies depending on the same.

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In the third book he treats of the quantity of the year, and of the unequal motion of the sun through the zodiac: he here gives the method of computing the mean motion of the sun, with tables of the same; and likewise treats of the inequality of days and nights.

In the fourth book he treats of the lunar motions, and their various phenomena; he gives tables for finding the moon's mean motions, with her latitude and longitude; he discourses largely concerning lunar epicycles; and by comparing the times of a great number of eclipses mentioned by Hipparchus, Calippus, and others, he has computed the places of the sun and moon, according to their mean motions, from the first year of Nabonazar, king of Egypt, to his own

time.

In the fifth book he treats of the instrument called the astrolabe; he treats also of the eccentricity of the lunar orbit, and the inequality of the moon's motion according to her distance from the sun; he also gives tables and an universal canon for the inequality of the lunar motions; he then treats of the different aspects or phases of the moon, and gives a computation of the diameter of the sun and moon, with the magnitude of the sun, moon, and earth, compared together; he states also the different measures of the distance of the sun and moon, according as they are determined by ancient mathematicians and philosophers.

In the sixth book he treats of the conjunctions and oppositions of the sun and moon, with tables for computing the mean time when they happen; of the boundaries of solar and lunar eclipses; of the tables and methods of computing the eclipses of the sun and moon, with many other particulars.

In the seventh book he treats of the fixed stars, and shows the methods of describing them, in their various constellations, on the surface of an artificial sphere or globe; he rectifies the places of the stars to his own time, and shows how different those places were then, from what they had been in the times of Timocharis Hipparchus, Aristillus, Calippus, and others he then lays down a catalogue of the stars in each of the northern constellations, with their latitude, longitude, and magnitudes

In the eighth book he gives a like catalogue of the stars in the constellations of the southern hemisphere, and in the twelve signs or constellations of the zo. diac. This is the first catalogue of the

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stars now extant, and forms the most vaJuable part of Ptolemy's works. He then treats of the galaxy, or milky way; also of the planetary aspects, with the rising and setting of the sun, moon, and stars.

In the ninth book he treats of the order of the sun, moon, and planets, with the periodical revolutions of the five planets; then he gives tables of the mean motions, beginning with the theory of Mercury, and showing its various phenomena with respect to the earth.

The tenth book begins with the theory of the planet Venus, treating of its greatest distance from the sun; of its epicycle eccentricity, and periodical motions; it then treats of the same particulars in the planet Mars.

The eleventh book treats of the same circumstances in the theory of the planets Jupiter and Saturn. It also corrects all the planetary motions; from observations made from the time of Nabonazar to his own.

The twelfth book treats of the retrogressive motion of the several planets, giving also tables of their stations, and of the greatest distances of Venus and Mercury from the sun.

The thirteenth book treats of the several hypotheses of the latitude of the five planets; of the greatest latitude or inclination of the orbits of the five planets, which are computed and disposed in tables; of the rising and setting of the planets, with tables of them. Then follows a conclusion or winding up of the

whole work.

This great work of Ptolemy will always be valuable, on account of the observations he gives of the places of the stars and planets in former times, and according to ancient philosophers and astronomers that were then ex ant; but principally on account of the large and curious catalogue of the stars, which, being compared with their places at present, we thence deduce the true quantity of their slow progressive motion, according to the order of the signs, or of the precession of the equinoxes.

Another great and important work of Ptolemy was his Geography, in seven books; in which, with his usual sagacity, he searches out and marks the situations of places according to their latitudes and longitudes; and he was the first that did so. Though this work must needs fall far short of perfection, through the want of necessary observations, yet it is of considerable merit, and has been very useful to modern geographers. Cellarius, in

deed, suspects, and he was a very com petent judge, that Ptolemy did not use all the care and application which the nature of his work required; and his reason is, that the author delivers himself with the same fluency and appearance of certainty, concerning things and places at the remotest distance, which it was impossible he could know any thing of, that he does concerning those which lay the nearest to him, and fell the most under his cognizance. Salmasius had before made some remarks to the same purpose upon this work of Ptolemy. The Greek text of this work was first published by itself at Basil, in 1533, in quarto: after. wards with a Latin version, and notes, by Gerard Mercator, at Amsterdam, 1605; which last edition was reprinted at the same place, 1618, in folio, with neat geographical tables, by Bertius.

Other works of Ptolemy, though less considerable than these two, are still extant. As, "Libri quatuor de Judiciis Astrorum," upon the first two books of which Cardan wrote a Commentary; "Fructus Librorum suorum," a kind of supplement to the former work; "Recentio Chronologica Regum;" this, with another work of Ptolemy, "De Hypothesibus Planetarum," was published in 1620, 4to., by John Bainbridge, the Savilian Professor of Astronomy at Oxford, and Scaliger, Petavius, Dodwell, and the other chronological writers, have made great use of it; "Apparentiæ Stellarum Inerrantium;" this was published at Paris by Petavius, with a Latin version, 1630, in folio; but from a mutilated copy, the defects of which have since been supplied from a perfect one, which Sir Henry Saville had communicated to Archbishop Usher, by Fabricius, in the hird volume of his Bibliotheca Græca; "Elementarum Harmonicarum libri tres," published in Greek and Latin, with a commentary, by Porphyry, the philosopher, by Dr. Wallis, at Oxford, 1682, in 4to; and afterwards reprinted there, and inserted in the third volume of Wallis's works, 1699, in folio.

Mabillon exhibits, in his German Travels, an effigy of Ptolemy looking at the stars through an optical tube; which effigy, he says, he found in a manuscript of the thirteenth century, made by Conradus a monk. Hence, some have fancied, that the use of the telescope was known to Conradus. But this is only matter of mere conjecture, there being no facts or testimonies, nor even probabilities, to support such an opinion.