When the solar rays are passed through a print and thrown upon prepared paper, the unshaded parts are slowly copied; but the lights transmitted by the shaded parts are seldom so definite as to form a distinct resemblance of them by producing different intensities of colour.

Professor Davy has found, that the images of small objects produced by means of the solar microscope may be copied without difficulty on prepared paper. This will probably be a useful application of the method; that it may be employed successfully, however, it is necessary that the paper be placed at a small distance from the lens.

With regard to the preparation of the solution, this gentleman found the best proportions those of one part of nitrate to about ten of water. In this case, the quantity of the salt applied to the leather or paper will be sufficient to enable it to become tinged, without affecting its composition, or injuring its texture.

Count Rumford has made some valuable experiments concerning the chemical properties that have been attributed to light.* This philosopher observed, that white ribands wetted in a diluted aqueous solution of muriate of gold, and exposed wet to the action of the sun's direct rays, soon become tinged of a beautiful crimson colour. Magnesia wetted with a solution of muriate of gold, and exposed wet to the action of the sun's rays, soon changes to a purple, and afterwards to a deep crimson colour. But if the riband or magnesia, after being so wetted, be dried in a dark place, and then exposed in a dry state to the sun's rays, their colour will not be sensibly changed by the action of the light..

1

The following method of silvering ivory, which we shall transcribe from the Count's paper, is extremely beautiful, and may become a valuable acquisition to the arts.

EXPERIMENT III.

Take a slip of ivory, immerse it in a diluted solution of pure nitrate of silver, and suffer it to be immersed till the ivory has acquired a bright yellow colour; then take it out of the solution, and immerse it in a tumbler of pure

* Rumford's Philosoph. Fapers, vol. 1, 1802, p. 22.

†l. c. p. 363.

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water, and immediately expose it, in the water, to the direct rays of a bright sun. After the ivory has been thus exposed for about two or three hours to the action of the sun's light, it will appear black; but on rubbing it a little, the black surface will become changed into a metallic one, resembling a slip of fine silver.

Although this coating of revived metal is extremely thin, yet if the ivory be well impregnated with the nitrate of silver the solution will penetrate to a considerable depth, and as fast as the silver wears off from the surface of the ivory, the oxyd below it becoming uncovered, and exposed to the light, a new coating of revived metal will be formed to replace it, and the surface of the ivory will not lose its metallic appearance.

The Count further observes, that this method of silvering ivory, which is not only expeditious, but very economical, might, no doubt, be employed with advantage in many cases, for ornamental purposes. The process is certainly curious and beautiful, when considered merely as a philosophical experiment.

All metallic oxyds, but especially those of mercury, bismuth, lead, silver, and gold, become of a deeper colour, by exposure to the sun; some of them become perfectly revived, others only partially, as may be seen by observing various painters' colours, preserved in powder, and exposed to the light, in the windows of colour-shops. The yellow oxyd of mercury, the acetate and muriate, are very soon affected by light.

The black oxyd of mercury, obtained by pouring ammonia copiously into a neutral solution of sulphate of mercury (or rather on dry sulphate) exposed to the rays of light, becomes readily converted into fluid quicksilver. The yellow oxyd of tungsten, if exposed to light, loses weight and becomes blue.

Green prussiate of iron exposed to the solar light becomes also blue.

The rose-coloured solution of sulphate of manganese becomes colourless when kept exposed to the light, and regains its colour when removed into the dark.

4. EFFECTS OF LIGHT ON ACIDS.

EXPERIMENT I.

Decomposition of nitric acid by light.

If a white glass bottle be half filled with concentrated nitric acid, and exposed to the rays of the sun, at the end of some days the white acid will be converted into an orange-coloured and fuming one, and the bottle will become filled with red vapours. If the bottle be now carried into a dark place and suffered to remain there for some time, the red vapours will gradually diminish, and the acid return to its primitive or colourless state.

Nitric acid, as we shall see hereafter, consists of the basis of two aeriform fluids, called nitrogen and oxygen. The acid in this instance loses part of its oxygen, which unites with the light, and forms oxygen gas; hence the acid becomes more volatile, and fuming.

This experiment succeeds best with highly concentrated acid, exposed to the light in bottles only half filled.

If oxygenated muriatic acid be exposed to the light, oxygen gas may easily be obtained, in the following manner, in considerable quantity.

EXPERIMENT II.

Decomposition of oxygenated muriatic acid by light.

Put concentrated oxygenated muriatic acid into a bottle, and fit to it a bent glass tube, one end of which pierces the cork of the bottle, and the other end reaches under a bell or receiver, filled with and inverted in a basin of water, care being taken that the tube does not touch the acid; the light falling directly on the acid, decomposes it, and a part of the oxygen of the acid becomes disengaged in the gaseous form, and passes into the receiver.

Thus, there remains no doubt that light acts chemically on various substances. We have seen that incombustible bodies, such as oxyds of metals, become combustible when exposed to the light; that acids are decomposed by its contact; that various other substances change their nature; that vegetables acquire colour, odour, taste, and in

flammability: whereas, on the contrary, deprived of light they remain pale and insipid, and are what is called etioluted, or blanched.

Light therefore enters into the composition of bodies; and its effects are so considerable, that the skilful operator ought to be constantly aware of the influence it may have upon the products submitted to his investigation.

SECT. VI.

COMBINATION OF LIGHT WITH DIFFERENT BODIES.

Philosophers have ascertained that various bodies, if exposed to light either at a high or low temperature, combine with it and emit it again, without alteration, under certain circumstances.

Substances of this kind have been called SOLAR PHOSPHORI, or substances which shine in the dark without emitting heat, after having been exposed to the light.

The world is stocked with an immense variety of bodies of this kind. All terrestrial objects not only possess a power of absorbing light, but they likewise emit it again. The eyes of cats, owls, and several other animals, are constructed so as to collect light, to enable them to find their food in the dark. Water condensed into the form of ice or snow, emits light remarkably, This accounts for the light afforded by snow, even when the heavens are involved in extreme darkness. The snow on the ground absorbs light, by being exposed to it all the day, and emits a considerable quantity again in the dark.

The principal bodies of this kind are the following: BOLOGNIAN PHOSPHORUS....The process for preparing this phosphorus is as follows: pieces of sulphate of barytes are first made red-hot, for a few minutes, in a covered crucible, placed in the middle of a fire, and then left to cool. When cold they are pulverized in a stone mortar, and sifted. This powder is formed into a paste with a

little mucilage of gum arabic, and divided into cakes, cylinders, or pieces of the thickness of a quarter of an inch. Those pieces are dried in a moderate heat, and then exposed, by degrees, to a more violent heat, among charcoal, in a strong drawing wind-furnace. As soon as the coals are half consumed, the furnace must be filled a second time, and the phosphorus be left undisturbed. When the coals are quite consumed, the ashes must be carefully blown off with a pair of bellows, and the phosphorus is found at the bottom of the grate.

These phosphoric stones exposed for a few minutes to the light, shine when taken into the dark like burning coals. Even immersed in water, they emit the same glowing light. Not only this phosphorus, but likewise all other phosphori of this class, lose their luminous property gradually, which they however regain on being heated a second time.

The

RATIONALE.... During the process of preparing the Bolognian phosphorus, a partial decomposition of the sulphuric acid of the sulphate of barytes takes place. ignited charcoal, at a high temperature, has a greater affinity to the oxygen of the sulphuric acid than the sulphur has; it therefore attracts it, and forms carbonic acid gas; part of the sulphur of the sulphuric acid becomes thus separated; it unites to the pure barytes, and forms sulphuret of barytes, which seems in this union more disposed to decompose oxygen gas than when single. The phenomenon is therefore to be considered analogous to a slow combustion of the sulphur; the extrication of caloric, during each instant of observation, being too inconsiderable to be appreciable either by the senses or by the thermometer.

CANTON'S PHOSPHORUS....To prepare this phosphorus, take some oyster shells; calcine them, by keeping them in a good coal-fire, for about an hour. Then let the purest part of the lime so obtained be broken into pieces (not powdered.) To three parts of it, add one of flowers of sulphur, and place the whole in layers, in a crucible, tightly. Let it then be placed in the middle of a fire, where it must be kept red hot for an hour at least ; it may then be removed to cool. When it is cold, select the brightest part, which, if good, will shine in the dark. This is to be kept in a wide-mouthed glass phial, well

closed.