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SCHEELE'S EXPERIMENTS.

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it does not appear that experimental evidence of any value was obtained. A large amount of curious facts was collected-effects depending on very diversified causes were recorded, all of which were attributed to that element which the author was inclined to regard as the most important and active in his scheme of creation, to the rejection of every other power.

(13.) A more correct record of observations now opens before us. In 1722 Petit showed that solutions of saltpetre (nitrate of potash) and sal-ammoniac (hydro-chloride of ammonia) crystallised more readily in the Light than they did in darkness.* These experiments continued, for some time, to excite considerable attention; and in 1788 we find the "vegetation of salts the subject of Chaptal's inquiry, and in the following year M. Dizé published a paper upon the crystallisation of salts by the action of Light.

(14.) Charles William Scheele of Stralsund, in Swedish Pomerania, one of the most brilliant geniuses of his or of any age, was the first who analysed the action, and studied the influences, of the differently-coloured rays of Light. The investigations of this chemist are so complete of their kind, and so little known, that I shall give a general view of his experiments. I am induced to this more particularly from the circumstance that much discussion has arisen on a subject,-the change which takes place in the salts of silver employed in Photography,carefully settled by Scheele in 1777, although his object was to prove that the inflammable principle "phlogiston" existed in Light.

"It is well known," says Scheele, "that the solution of silver in acid of nitre (nitrate of silver), poured on a piece

*Sur la Végétation des Sels. Mém. de Paris, 1722.

† Observations sur l'Influence de l'Air et de la Lumière dans la Végétation des Sels. Mémoirs de l'Acad. Royale des Sciences de Toulouse, vol. iii.

Scheele, Traité de l'Air et du Feu.

of chalk, and exposed to the beams of the sun, grows black. The Light of the sun reflected from a white wall has the same effect, though more slowly. Heat without Light has no effect on the mixture. Should the black colour not be real silver?" "I put some silver earth (oxide of silver) on a piece of porcelain, and exposed it in the focus of a burning glass; immediately the surface of this earth was reduced to silver."

The following experiments are instructive.

"I precipitated a solution of silver by sal-ammoniac; then I edulcorated and dried the precipitate, and exposed it to the beams of the sun for the space of two weeks, when the surface of the white paper grew black; after which I stirred the powder and repeated the same several times. Hercupon I poured some caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This menstrum dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed, was, for the greater part, dissolved by a pure acid of nitre, which, by the operation, acquired volatility. This solution. I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun's Light, and likewise the solution of silver poured on chalk, is silver by reduction."

"I mixed so much of distilled water with well edulcorated horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the sun, and shook it several times each day; the other half I set by in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the luna cornua grown already black; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver."

Scheele continued his experiments on the chloride of gold, and some of the salts of manganese, obtaining similar results.

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Again, Scheele writes, "Fix a glass prism at the window, and let the refracted sunbeams fall on the floor; in this coloured light put a paper strewed with luna cornua, and you will observe that this horn silver grows sooner black in the violet ray than in any of the other of the other rays, i. e. that the calx of silver separates the phlogiston sooner from the violet ray of light than from any other colour."

This is the first intimation we have of the dissimilar action of the refracted rays- the interpretation given was of course discarded with the theory of phlogiston; but the fact observed, has led the way to the discovery of all the phenomena which depend on the chemical agency of the solar rays.

Senebier, who repeated the experiments of Scheele, states that he found the violet ray to darken the chloride of silver in fifteen minutes, while the red rays required twenty minutes, and the other colours various intermediate periods.* The bleaching of wax by exposure to the solar rays became also the subject of Senebier's investigations.

It became about this time, the latter part of the eighteenth century, a question whether the observed chemical changes were produced by the Light, properly so called, of the solar rays, or by the heat which accompanied it.

(15.) Count Rumford, in a Memoir "On the Propagation of Heat in Fluids," published in the Philosophical Transactions, states, that he saw reason to doubt the existence of those chemical properties in Light which had been attributed to it; and, to conclude, that all those visible changes produced in bodies by the action of the sun's rays were effected merely by the heat which is generated or excited by the Light which is absorbed by them. In 1798 this philosopher communicated to the Royal Society a Paper, entitled "An Inquiry concerning the Chemical Properties that have been attributed to Light." The experimental in

* Senebier sur la Lumière, tom. iii. p. 199.

vestigations recorded in this memoir are important, as marking the progress of the inquiry.

Count Rumford found that gold and silver might be melted by the heat-invisible to sight-which exists in the air at the distance of more than an inch above the point of the flame of a wax candle. He then proceeded to examine what would be the effect of this heat on the oxides of these metals. Having wetted a piece of taffeta riband with a saturated solution of the chloride of gold, it was held, stretched horizontally, over the clear bright flame of a wax candle, the under side of the riband being kept at the distance of about an inch and a half above the point of the flame: that part of the riband which was directly over the point of the flame began almost immediately to emit steam in dense clouds; and, in about ten seconds, a circular spot, about three quarters of an inch in diameter, having become nearly dry, a spot of a very fine purple colour, approaching to crimson, suddenly made its appearance in the middle of it, and, spreading rapidly on all sides, became, in one or two seconds more, nearly an inch in diameter. The hue was not uniform, but varied from a light crimson to a very deep purple, approaching to a reddish brown. No traces of revived gold could be discovered, but the riband had all the appearance of being covered with a thin coating of the most beautiful purple enamel, which, in the sun, had a degree of brilliancy that was sometimes quite dazzling.

Whatever material was saturated with the solution of gold, the same stain was produced by the agency of a strong heat; and it was found that solutions of silver, under similar circumstances, imparted stains of a deep orange colour. It was ascertained by several trials that the light of a candle produced no change, the decomposition of the salts of gold and silver in these experiments being solely effected by the agency of heat.

(16.) Count Rumford next proceeded to examine what influence was exerted by the direct solar rays. Pieces of

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riband were wetted, and parcels of magnesia moistened with a solution of gold: those which were exposed to the strong Light of the sun gradually changed colour, and in a few hours acquired a fine purple hue, whilst those preserved in darkness remained unchanged. It was also found, that the decomposition was more readily brought about, when the riband or the magnesia were exposed in a damp state, than if the solution was permitted to dry on them previous to exposure.

(17.) Arguing from the facility with which most of the metallic oxides are reduced, by means of charcoal when exposed to a high temperature, that gold might be revived from its solutions in the same manner, if the solution and the charcoal were equally exposed to a sufficient degree of heat, Count Rumford instituted some experiments to put this idea to the test. He found, by putting small pieces of charcoal into a glass tube filled with solutions of gold or of silver, and exposing it to a temperature of 210° Fahrenheit for two hours in the dark, that revived gold adhered to the surface of the charcoal. Similar tubes filled with solution of gold or of silver and fragments of charcoal were exposed to the direct rays of a very bright sun, and in less than half an hour small specks of revived gold in all its metallic splendour, appeared on the surface of the charcoal. Etherial solutions of gold were found to be decomposed by the solar rays, and even by prolonged exposure to diffused Light. Spirits of turpentine and olive-oil were mixed with aqueous solutions of gold and silver, and exposed to heat in a steam apparatus: both of them assisted in reviving the metals, but spirits of wine had no such influence, notwithstanding the presence of carbon, which enters nearly as largely into its composition as it does into the composition of the other two. From these results Count Rumford concludes, that heat is generated by the absorption of the sun's rays; that at the moment of its generation it exists in almost infinitely small spaces; and consequently it is only in bodies that are inconceivably

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