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THE

BRITISH ENCYCLOPEDIA.

PHO PHOSPHITES, are salts formed of the phosphorous acid, with alkalies, earths, &c. In several of their properties they resemble the phosphates; but may be distinguished from them, by appearing luminous when heated with the blow pipe, and by affording, by distillation, a small quantity of phosphorus. They detonate, too, with oxy-muriate of potash, and precipitate gold from its solution in a metallic state. By exposure to the air, they pass into phosphates. PHOSPHORESCENCE,? See LIGHT.

PHOSPHORI,

PHOSPHORIC acid. When phosphorus undergoes combustion in oxygen gas, a great quantity of white fumes are produced, which are deposited in white flakes. These are phosphoric acid; so that it is a compound of phosphorus and oxygen. The phosphoric acid was first shewn to be distinct from all other acids, in the year 1743, by Margraaff. He found that it existed in the salts which were taken from human urine, and that phosphorus could only be obtained from this acid, as well as that it could be converted into phosphoric acid. This acid was found to exist in some vegetable substances, although it was formerly supposed to be peculiar to animal matters. Phosphoric acid may be obtained, not only by the method just mentioned, but also by transmitting a current of oxygen gas through phosphorus melted under water. The acid, as it is formed, combines with

PHO

the water, from which it may be obtained in a state of purity by evaporation. The specific gravity of this acid varies according to the different states in which it exists. In the liquid state it is 1.4; in the dry state it is 2.7; in the state of glass 2.85. It changes the colour of vegetable blues to red; has no smell, but a very acid taste. When it is exposed to the air it attracts moisture, and is converted into a thick viscid fluid, like oil. It is very soluble in water. When in the form of dry flakes, it dissolves in a small quantity of this liquid, producing a hissing noise like that of a red hot iron plunged into water, with the extrication of a great quantity of heat. The component parts of this acid have been accurately ascertained by Lavoisier, and it consists of,

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very heavy. It forms a great bed in the province of Estremadura in Spain. In appearance it resembles curved, lamellar, heavy spar; but it is harder and lighter than this kind of heavy spar.

PHOSPHOROUS acid, is obtained by the slow combustion of phosphorus at the common temperature of the air. If phosphorus, in small pieces, be exposed to the air in a glass funnel placed in a bottle, it attracts the oxygen and moisture from the atmosphere, and runs down into the bottle. This is the phosphorous acid. By this process, about three times the weight of the phosphorus is obtained. It is then in the form of a thick liquid, adhering to the sides of the vessel. It varies in consistence according to the state of the air. Its specific gravity is not known. It has an acid pungent taste, not different from phosphoric acid. It also reddens vegetable blue colours. The phosphorous acid is not altered by light. When exposed to heat in a retort, part of the water combined with it is first driven off, and when it is concentrated, bubbles of air suddenly rise to the surface, and collect in the form of white smoke, and sometimes inflame, if there be any air in the apparatus. If the experiment be made in an open vessel, each bubble of air, when it comes to the surface, produces a vivid deflagration, and diffuses the odour of phosphorated hydrogen gas. This acid is composed of the same constituent parts as the phosphoric, and is considered by some as the phosphoric acid holding in solution a small quantity of phosphorus. Phosphorous acid forms compounds with alkalies, earths, and metallic oxides, which are known under the name of phosphites.

PHOSPHORUS. This singular substance was accidentally discovered in 1677 by an alchymist of Hamburgh, named Brandt, when he was engaged in searching for the philosopher's stone.Kunkel, another chemist, who had seen the new product, associated himself with one of his friends, named Krafft, to purchase the secret of its preparation; but the latter, deceiving his friend, made the purchase for himself, and refused to communicate it. Kunkel, who at this time knew nothing further of its preparation than that it was obtained by certain processes from urine, undertook the task and succeeded. It is on this account that the substance long went under the name of Kunkel's phosphorus. Mr. Boyle is also considered as one of the discoverers of phosphorus. He communicated the se

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cret of the process for preparing it to the Royal Society of London in 1680. It is asserted, indeed, by Krafft, that he discovered the secret to Mr. Boyle, having in the year 1678 carried a small piece of it to London, to show it to the royal family but there is little probability that a man of such integrity as Mr. Boyle would claim the discovery of the process as his own, and communicate it to the Royal Society, if this had been the case. Mr. Boyle communicated the process to Godfrey Hankwitz, an apothecary of London, who for many years supplied Europe with phosphorus, and hence it went under the name of English phosphorus. In the year 1774, the Swedish chemists, Gahn and Scheele, made the important discovery, that phosphorus is contained in the bones of animals, and they improv ed the processes for procuring it.

The most convenient process for obtaining phosphorus seems to be that recommended by Fourcroy and Vauquelin, which we shall transcribe. Take a quantity of burnt bones and reduce them to powder. Put 100 parts of this powder into a porcelain or stone-ware basin, and dilute it with four times its weight of water. Forty parts of sulphuric acid are then to be added in small portions, taking care to stir the mixture after the addition of every portion. A violent effervescence takes place, and a great quantity of air is disengaged. Let the mixture remain for twenty four hours, stirring it occasionally, to expose every part of the powder to the action of the acid. The burnt bones consist of the phosphoric acid and lime; but the sulphuric acid has a greater affinity for the lime than the phosphoric acid. The action of the sulphuric acid uniting with the lime, and the separation of the phosphoric acid, occasion the effervescence. The sulphuric acid and the lime combine together, being insoluble, and fall to the bottom.Pour the whole mixture on a cloth filter, so that the liquid part, which is to be received in a porcelain vessel, may pass through. A white powder, which is the insoluble sulphate of lime, remains on the filter. After this has been repeatedly washed with water, it may be thrown away; but the water is to be added to that part of the liquid which passed through the filter. Take a solution of sugar of lead in water, and pour it gradually into the liquid in the porcelain basin. A white powder falls to the bottom, and the sugar of lead must be added so long as any precipitation takes place.

The whole is again to be poured upon a filter, and the white powder which remains is to be well washed and dried. The dried powder is then to be mixed with one-sixth of its weight of charcoal powder. Put this mixture into an earthen-ware retort, and place it in a sand bath, with the beak plunged into a vessel of water. Apply heat, and let it be gradually increased, till the retort becomes red hot. As the heat increases, air-bubbles rush in abundance through the beak of the retort, some of which are inflamed when they come in contact with the air at the surface of the water. A substance at last drops out similar to melted wax, which congeals under the water. This is phosphorus. To have it quite pure, melt it in warm water, and strain it several times through a piece of shammy leather under the surface of the water. mould it into sticks, take a glass funnel with a long tube, which must be stopped with a cork. Fill it with water and put the phosphorus into it. Immerse the funnel in boiling water, and when the phosphorus is melted, and flows into the tube of the funnel, then plunge it into cold water, and when the phosphorus has become solid, remove the cork, and push the phosphorus from the mould with a piece of wood. Thus prepared, it must be preserved in close vessels, containing pure water. When phosphorus is perfectly pure, it is semi-transparent, and has the consistence of wax. It is so soft that it may be cut with a knife. Its specific gravity is from 1.77 to 2.03. It has an acrid and disagreeable taste, and a peculiar smell, somewhat resembling garlic.

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When a stick of phosphorus is broken, it exhibits some appearance of crystallization. The crystals are needle shaped, or long octahedrons; but to obtain them in their most perfect state, the surface of the phosphorus, just when it becomes solid, should be pierced, that the internal liquid phosphorus may flow out, and leave a cavity for their formation. When phosphorus is exposed to light, it becomes of a reddish colour, which appears to be an incipient combustion. It is therefore necessary to preserve it in a dark place. At the temperature of 99° it becomes liquid, and if air be entirely excluded, it evaporates at 219°, and boils at 554°. At the temperature of 43° or 44°, it gives out white smoke, and is luminous in the dark. This is a slow combustion of the phosphorus, which becomes more rapid as the temperature is

raised. When phosphorus is heated to the temperature of 148° it takes fire, burns with a bright flame, and gives out a great quantity of white smoke. Phosphorus enters into combination with oxygen, azote, hydrogen, and carbon. Phosphorus is soluble in oils, and when thus dissolved, forms what has been called liquid phosphorus, which may be rubbed on the face and hands without injury. It dissolves too in ether, and a very beautiful experiment consists in pouring this phosphoric ether in small portions, and in a dark place, on the surface of hot water. The phosphoric matches consist of phosphorus extremely dry, minutely divided, and perhaps a little oxygenized. The simplest mode of making them is to put a little phosphorus, dried by blotting paper, into a small phial; heat the phial, and when the phosphorus is melted turn it round, so that the phosphorus may adhere to the sides. Cork the phial closely, and it is prepared. On putting a common sulphur match into the bottle, and stiring it about, the phosphorus will adhere to the match, and will take fire when brought out into the air.

PHOSPHURETS, in chemistry, are substances formed by an union with phosphorus; thus we have the phosphuret of carbon, which is a compound of carbon with phosphorus: we have also the phosphuret of lime, hydrogen, &c.

PHOSPHURETTED hydrogen, phos phorus dissolved in hydrogen gas; which may be done by introducing phosphorus into a glass jar of hydrogen gas standing over mercury, and then melting it by means of a burning glass; the gas dissolves a large proportion of it. The compound has a very fetid odour, something like that from putrid fish. When it comes into contact with common air, it burns with great rapidity, and if mixed with that air is detonates violently. Oxygen gas produces a still more rapid and brilliant combustion than common air. When bubbles of it are made to pass up through water, they explode in succession as they reach the surface of the liquid; a beautiful column of white smoke is formed. This gas is the most combustible substance known. Its combustion is the combination of its phosphorus and hydrogen with the oxygen of the atmosphere, and the products are phosphoric acid and water. These substances, mixed or combined, constitute the white smoke.

PHOTOMETER, an instrument intended to indicate the different quantities of

light, as in a cloudy or bright day, or between bodies illuminated in different degrees. The ratio of the intensities of two luminous objects has been attempted to be measured by placing them at different distances from a given object, until that object cast two shadows of equal darkness; or by observing when two equal objects appeared to be equally illuminated each by one of the luminous objects: for then, by a well known and established law, the proportion of the intensities of their light was supposed to be as the squares of the distances. Thus if two equal objects appear to be equally illuminated, when one of them is three feet from a tallow candle,and when the other is nine feet from a wax candle, then it is inferred that the intensity of the light of the former candle is to that of the latter as nine to eighty-one. Mr. Leslie has more recently invented an instrument of this kind, the essential part of which consists of a glass tube like a reversed syphon, whose two branches should be equal in height, and terminated by balls of equal diameter; one of the balls is of black enamel, and the other of common glass, into which is put some liquid.

The motions of the liquor, which is sulphuric acid tinged red with carmine, are measured by means of a graduation: the zero is situated towards the top of the branch that is terminated by the enamelled ball. The use of this instrument is founded upon the principle, that when the light is absorbed by a body, it produces a heat proportional to the quantity of absorption. When the instrument is exposed to the solar rays, those rays that are absorbed by the dark colour, heat the interior air, which causes the liquor to descend at first with rapidity in the corresponding branch. But as a part of the heat which had introduced itself by means of the absorption, is dissipated by the ra diation, and as the difference between the quantity of heat lost and that of the heat acquired goes on diminishing, there will be a point, where, these two quantities having become equal, the instrument will be stationary, and the intensity of the incident light is then estimated by the number of degrees which the liquor has run over. The author of this ingenious instrument has pointed out its advantages in determining the progressive augmentation undergone by the intensity of the light, and the gradation in a contrary sense, which succeeds to that progress, both from the beginning of day to its end, and from the winter solstice to the end of the succeeding autumn. With the help

of such an instrument one might also compare the action of rays of light in different countries, of which some dart with sufficient constancy from a fine and serene sky, while others seem to be covered with a veil, which dims and obscures their lustre. Mr. Leslie, having proposed to himself to measure the energy of the several coloured rays which compose the solar spectrum, caused a beam of light to pass through a prism of flint glass; and the indications of the photometer presented successively to the different parts of the spectrum have furnished, nearly, for the relation between the degrees of force of the blue, green, yellow, and red rays, that of the numbers 1, 4, 9, 16; a relation which, considered in the two extreme terms, is more than quadruple that which was substituted for it by Dr. Herschel, who has made experiments for the same purpose.

PHRYGANEA, in natural history, a genus of insects of the order of Neuroptera. Generic character: mouth with a horny short curved mandible; feelers four; three stemmata; antennæ setaceous, longer than the thorax; wings equal, incumbent, the lower ones folded. There are nearly sixty species, in two divisions. A. Tail with two truncate bristles. B. Tail without bristles. The insects of this genus are seen in a summer's evening floating in l.e air in large masses, and are eagerly devoured by swallows. They resemble moths, particularly the division called Tinea; but may readily be distinguished by their feelers, and also by the stemmata situated at the top of the head. The phryganeæ proceed from aquatic larvæ of a lengthened shape, residing in tubular cases, which they form by agglutinating various fragments of vegetable substances, &c. These tubular cases are lined within by a tissue of silken fibres, and are open at each extremity. The included larvæ, when feeding, protrude the head and fore parts of the body, creeping along the bottom of the waters they inhabit, by means of six short and slender legs; on the upper part of the back is a sort of prop, preventing the case, or tube, from slipping too far forwards during the time the animal is feeding. One of the largest species is the P. grandis, (see Plate IV. Entomology, fig. 2). This insect is about an inch in length, very like a phalena; the upper wings are grey, marked by various darker and lighter streaks and specks, and the under wings yellowish brown, and semi-transparent. The larva of this genus is known by the

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