a name given to certain substances which shine in the dark without emitting heat. By this circumstance they are distinguished from the pyrophori, which though they take fire on being exposed to the air, are yet entirely destitute of light before this exposure.

Phosphori are divided into several kinds, known by the names of Bolognian phosphorus, Mr Canton's phosphorus, Baldwin's phosphorus, phosphorus of urine, &c., of which the last is by far the most remarkable both with respect to the quantity of light which it emits, and its property of taking fire and burning very fiercely upon being slightly heated or rubbed. For the method of preparing these, see Chemistry, No. 193, 476—486.

Besides these, however, it has been found that almost all terrestrial bodies, upon being exposed to the light, will appear luminous for a little time in the dark, metals only excepted. This points out a general division of the phosphori into two classes; namely, such as require to be exposed to the light either of the sun, or of some artificial fire, before they become luminous; and such as do not. Of the former kind are the Bolognian phosphorus, Mr Canton's phosphorus, the phosphori from earths, &c. Of the latter kind are rotten wood, the skins of fishes, and the phosphorus of urine. To these we may add some other substances which become luminous in another way; viz., the maws which remains after the distillation of volatile sal ammoniac with chalk, loaf-sugar, and the phosphorus of urine dissolved in spirit of wine. The first, which is a composition of the marine acid of the sal ammoniac with the chalk, after being fused in a crucible, becomes luminous when struck with any hard body; white sugar is luminous, when grated or scraped in the dark; and the solution of phosphorus in spirit of wine is luminous only when dropped into water, and even then the light is only perceived where the drops fall into the liquid. One part of phosphorus communicates this property to 600,000 parts of spirit of wine.

There is a remarkable difference between the light of rotten wood, fishes, and that of phosphorus of urine, even when it is not in an ignited state; for this last does not cease to be luminous even when included within an exhausted receiver, the contrary of which happens to rotten wood and fishes. If air is strongly blown upon this phosphorus from a pair of bellows, it will extinguish its light for some time, which is not the case with the other kinds. When kept in water, and placed in a warm air, the phosphorus of urine discharges such large and bright flames into the air above it, as are apt to surprise and even frighten those who are unacquainted with it. These eruptions are contracted in their passage through the water, but expand as soon as they get above it; however, the experiment can only be tried to advantage in warm weather, and in a cylindrical glass not above three quarters filled with water.

The phenomena exhibited by the earthy phosphorus are very curious; both on account of the singular circumstances in which they exhibit their light, and the varieties observed in the light itself. All these, as has been already mentioned, emit no light till they have been first exposed to the light of the sun, or some other luminous body. After that, they are luminous in the dark for a considerable time; but by degrees their light dies away, and they emit no more till after another exposure to the sun. But if this happens to be too long continued, they are then irrecoverably spoiled. The same thing will happen from being too much heated without any exposure to light. Indeed, if a phosphorus, which has just ceased to be luminous, be heated, it will again emit light without any exposure to the sun; but by this its phosphoric quality is weakened, and will at last be destroyed. Indeed these phosphori are so tender, and impatient either of light or heat, that the best method of rendering them luminous occasionally is by discharging an electric bottle near them. The light of the flash immediately kindles the phosphorus, and it continues luminous for a considerable time, after which it may again be revived by another flash, and so on. However, with all the care that can be taken, these phosphori are very far from being perpetual; nor has any method been yet fallen upon to render them so.

The singularities in the light of the phosphori abovementioned are, that they emit light of many different and most beautiful colours. This difference of colours seems to be natural to them; for some will at first emit a green, others a red, others a violet, &c., at their formation. However, the best kinds agree in this strange property, that if they are exposed to a red light, they emit a red light in the dark; and the same of other colours. But this must not be understood without limitation; nor is the phosphoreal light at any time so bright as the luminous body, whatever it was, by which it was kindled. Neither are we to imagine, that any particular phosphorus has a particular kind of light appropriated to it; for the same phosphorus. Phosphorus which at one time emits a purple light, will at another perhaps emit a green, or a light of some other colour.

The explanation of the principal phenomena of phosphorus is deducible from what has been shown concerning the nature of fire, compared with what is mentioned under the article Quicklime. Under this last article it is shown, that when calcareous earths are deprived of their fixed air, a proportionable quantity of active fire is absorbed by them; that is, the etherial fluid, which pervades all bodies, has a violent tendency to expand itself, or to act all around every particle of the calcined earth, as from a centre. Of consequence, if this tendency was not counteracted by some other power, these substances would emit a perpetual flame. This power, however, is found in our atmosphere; which has already been shown either to be the positive principle of cold, or to contain it*. Hence, the latent fire in these substances is checked, and cannot act, excepting within the very substance itself. But if any other body comes in contact with the calcined earth, in which the principle of cold is less vigorous than in the atmosphere, the active fire in the quicklime immediately shows itself, and the body either becomes hot, or is consumed as if by fire. Hence it will follow, that if a very inflammable body is touched by quicklime, it ought to be set on fire. But of this we have no instance, because it is impossible for the quicklime to part with any of its fire, unless it receives something in exchange. This indeed it might receive from the atmosphere; which could supply it either with more fire, if it was in a state of ignition; or with fixed air, if any substance was at hand to receive the fire. But the atmosphere refuses to part with the fire which it contains, because the effort of the fire in the quicksilver is not sufficiently strong to overcome the opposition it meets with in other bodies; and, on the other hand, the effort of the fire in the quicklime is sufficient to keep the earth from attracting fixed air out of the atmosphere. But when water, for instance, is poured on the quicklime, the dry earth absorbs it very greedily, and parts with a proportionable quantity of its latent fire, which the water also absorbs much more readily than the atmosphere. Hence the mixture becomes so exceedingly hot as sometimes to fire combustible bodies. Now, if instead of water we suppose the lime to be mixed with oil, this also will absorb the fire; but not with such force as the water; neither is the heat by any means so considerable; because oil is capable of containing a vast quantity of heat in a latent state, the only consequence of which is an increase of its fluidity, without any very perceptible change of temperature. At the same time, however, we must remember, that if the oil is in very small quantity, and intimately combined with the quicklime in that peculiar state which we call phlogiston, it is easy to conceive, that it may be so much saturated with fire, as to be unable to contain any more without being ignited. In this case, if more fire is forced into the compound, a quantity of the phlogistic matter which it contains will be decomposed; and of consequence, the fire which it has imbibed will be thrown out, as in the common ignition of vapour; and in proportion to the degree of heat thus communicated, will the degree of ignition and the continuance of it be. If the quantity of heat is very great, the phlogiston will be dissipated all at once; but if otherwise, the ignition will continue for a much greater length of time, as is the case with a common fire.

To apply this to the accension of phosphori, we must consider, that these substances are all formed by calcining calcareous substances, and combining them with some portion of phlogistic matter. Baldwin's phosphorus is made by dissolving chalk in nitrous acid, afterwards evaporating the solution, and driving off most of the acid. The consequence of this is, that the earth is left in an exceedingly caustic state, as the acid expels the fixed air more completely than could be done almost by any calcination whatever; at the same time that any phlogistic matter which might have been contained in the mixture is most accurately diffused through it, and combined with it. The Bolognian phosphorus is composed of a gypseous earth, which contains a quantity of vitriolic acid; and as no mineral is to be found perfectly free from phlogistic matter, the vitriolic acid unites with it during the calcination into an exceedingly inflammable sulphur; for the greater the quantity of acid there is in proportion to the phlogiston, the more inflammable is the compound†. Thus the Bolognian, as well as Baldwin's phosphorus, is a compound of quicklime and inflammable matter; and the case is still more plain with regard to Mr Canton's, where the quicklime is mixed with sulphur, and both calcined together.—Neither are the phosphori made by calcining oyster-shells without addition to be accounted any way different from those already mentioned; since the shells always contain some portion of inflammable matter, which, being reduced to a coal by the action of the fire, furnishes a quantity of phlogiston, and imparts it to the whole of the calcareous matter.

Having thus seen that the phosphori of which we now speak are all composed of pure calcareous earth and phlogiston, we are next to consider, that the phlogiston must be in such a state as it is when saturated with fire and ready to inflame. It is not indeed in the state of vapour, because this would require a quantity of fire detached from any other substance, and interposed between the particles of the vapour, in order to keep them at a distance, or to give it elasticity. But the fire which ought to do this is confined by the calcareous earth, which also detains the phlogiston itself. As long therefore as the balance is thus preserved, the phosphorus cannot shine; but as soon as a fresh quantity of light is discharged upon it, then more light or fire (for they are the very same in this case), enters the quicklime than it can contain. The consequence of this is, that the quantity which cannot be retained by the earth, exerts its force upon the phlogiston; which having already as much as it can hold, not only the superfluous quantity is discharged, but also part of that which the phlogiston had absorbed before. The burning indeed is very slow and weak, because the phlogiston is obstinately retained by the earth, which both impedes the ignition, and prevents the dissipation of the phlogiston in vapour. However, as soon as the lime has by its action impeded the farther extrication of the phlogiston, the balance is restored, the fire goes out, and the phosphorus ceases to be luminous. Heat will kindle it again; but thus a larger quantity of phlogistic phlogitic matter is dissipated, and the phosphorus is soon destroyed. Light does the same, but in a much more moderate degree; and therefore the phosphorus may be frequently rekindled by means of light, and will continue its splendor for a long time. But if the light is too long continued, or too violent, it will produce the same consequence whether it is attended with perceptible heat or not.

With regard to the phosphorus of urine, the case is the same; only, instead of the calcareous earth, we have here an acid joined with phlogiston. The latter is in exceeding small quantity, and of consequence so loaded with fire that the least additional heat, rubbing, or alteration in the weather, forces more fire upon it than it can bear, and therefore part of it is continually flashing off in those curiosities formerly mentioned. The reason why this phosphorus flashes like lightning, and the others give only a steady light like coals, is, that the compound is very volatile. It requires indeed a violent fire to distill it at first; but in the distillation so much fire is imbibed, that it seems ever afterwards ready to evaporate spontaneously; and therefore phosphorus, when once made, is easily redistilled in close vessels.

It now remains only to show the reason why the phosphorus of urine and some others will shine under water, or in an exhausted receiver, while rotten wood, &c. will not. This seems to arise from the quantity of fire which they have internally, and which requires no supply from the external air as in the case of common fire; and hence the phosphorus of urine shines more briskly in vacuo than in the air; because the pressure of the atmosphere is then taken off, and the evaporation of the phlogistic matter promoted. The light of fishes and rotten wood seems to be of an electric nature; and therefore ceases when the air is exhausted, as on this fluid all the phenomena of electricity are found to depend.

With regard to the various colours of phosphoric light, some have imagined that the earthy substance was capable of imbibing a certain quantity of light, and emitting it afterwards in the very same state, and having the same colour which it had before. But this is now known to be a mistake, and the light of the phosphorus is found to be owing to a true accension, though weak, as in other burning bodies. Hence it is very probable that the colour of the light depends upon the degree of accension; for we see that even in common fires the colour depends in a great measure on the strength of the flame. Thus the flame of a candle, where it is not well kindled at bottom, always appears blue. The flame of a small quantity of sulphur, or of spirit of wine, is blue; but if a large quantity of either of these substances be set on fire, the flame will in many places appear white. A strong flame mixed with much smoke appears red. A weak one in similar circumstances appears brown, &c. Hence if the phosphorus is weakly kindled, it will emit a brown, violet, blue, or green flame; if strongly, a red or white one.

It has already been mentioned, that almost all terrestrial bodies have a phosphoric quality; however this, in most of them, is extremely weak, and continues only for a very short time. Signor Beccaria, who discovered this property, in order to find out what substances were phosphoric, and what were not, had a machine contrived like a dark lanthorn, in which he included himself, in order to perceive with the greater facility any small quantity of light which might be emitted by the substances which he designed to examine. In the fide of the machine was a cylinder capable of being turned about without admitting any light. Upon this were pasted the substances he designed to examine, and by turning the cylinder he immediately brought them from the light of the sun into intense darkness; in which situation there were but few substances which did not afford a sufficient quantity of light to render themselves visible. This phenomenon, however, is evidently similar to an optical illusion by which we are made to see what is not present before us; for if we look very intensely upon any thing for some time, suffering no more light to enter our eyes than what is reflected from that object, we will imagine that we still see it, though we remove into the dark or shut our eyes. The reason of this is, that the nervous fluid being once put into motion after a certain manner, continues that motion for a short space of time after the moving cause is removed. In like manner, as the light is partly reflected from bodies, and partly penetrates them, when any body is exposed to the light, and then is suddenly brought into a dark place, the etherial fluid within its substance being once put into motion does not cease to move immediately, but for a time produces that vibration which we call light; for the substance of light is present in the most intense darkness as well as in sunshine. Hence almost all substances are capable of emitting light in the dark, after being exposed to a vigorous sunshine; though the reason of their doing so may be very different from that by which the phosphorus become luminous.

Many entertaining experiments may be made with the various kinds of phosphorus, especially with that of urine. This last, however, is sometimes dangerous on account of the violence with which it burns. If dissolved in oil of cloves, it loses this property, but continues to be as luminous as before; so that this mixture, called liquid phosphorus, may be used with safety. As on some occasions it may be wished to have it in powder, it is proper to observe that this may be done with safety by pouring some hot water upon the phosphorus in a glass mortar. The compound melts, and while in a soft state is easily reducible to powder of any degree of fineness.

Acid of Phosphorus. This acid, called also the microcosmic acid, has already been described, and the method of procuring it from urine shown, under the article Chemistry, no. 307, 308. Since that article was written, however, it has been discovered by Mr Schele, that an acid capable of making phosphorus is producible from calcined bones or hartshorn, and the vitriolic acid. The process for procuring this acid recommended by that gentleman was to dissolve the bones in nitrous acid; afterwards to precipitate the earth by means of the vitriolic acid; to filter and evaporate the liquor to dryness; and, after driving off the nitrous acid, the phosphoric acid remains. This process, however, is expensive on account of the waste of nitrous acid; and is likewise very inconvenient, because a great deal of the earthy matter continues dissolved, Phosphorus even after the vitriolic acid is poured in, and therefore the phosphoric acid is never to be obtained pure; for which reason, the following process is preferable.

Take of calcined bones or hartshorn, one pound; oil of vitriol, 14 ounces. Let the bones be reduced to fine powder; then pour on the acid undiluted, and rub both together till they are as accurately mixed as possible. Having let them remain for some hours in this situation, pour on as much water, stirring and dissolving the lumps into which the mass will now be concreted, till it is all equally distributed through the liquid, and has the consistence of thick gruel. Let it remain 24 hours, and then pour it into a canvas cloth in order to let the liquid drain from it. This is a very tedious operation, as fresh water must continually be pouring on till all the saline matter is washed off. When this is done, pour into the liquid a quantity of caustic volatile alkali, which will occasion a copious precipitation; for the earth of bones is much less strongly attracted by acids than even the caustic volatile alkali. The liquid being now filtered a second time, which will be done with sufficient ease, and afterwards evaporated, there remains a mass composed of phosphoric acid, and vitriolic sal ammoniac. By increasing the fire, the latter is dissipated in vapour; and if the process has been successful, four ounces or more of pure phosphoric acid will remain.

With regard to the properties of this acid, it is not yet ascertained whether they are exactly the same with the microcolmic acid or not. Indeed, as far as yet appears, they seem to be different; and there are very strong reasons for supposing that the phosphoric acid thus produced is no other than the vitriolic altered by its combination with the earth of bones. See the article Bones in the APPENDIX.