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the yellow and green rays afforded the greatest quantity of Light, and that the violet ray had the least. In conclusion, Dr. Herschel asks, “May not the chemical properties of the prismatic colours be as different as those which relate to heat and Light ?"

(26.) Seebeck, in 1810, made some important observations on the influences of the solar radiations, the production of colours on chloride of silver being among the most remarkable. He tells us the violet rays rendered it brown, the blue rays produced a shade of blue, the yellow preserved it white, and the red rays constantly gave a red colour to that metallic salt. Berzelius noticed some of the peculiar conditions, described in a future page, in the action of the sun's rays on the salts of gold. Fischer also pursued some researches of a similar character to those of Seebeck, whose results he generally confirmed. *

(27.) Gay Lussac and Thenard, when engaged on some investigations on chlorine, observed that when mixed with hydrogen it did not combine in the dark; that they combined slowly in diffused light, and with an explosion in the sunshine. Seebeck collected chlorine over hot water; and combining it with hydrogen, he placed equal quantities of the mixture in a yellowish red and in a blue glass. In the blue glass, combination took place instantly when the mixture was exposed to daylight, but without explosion.

The mixture in the red glass was exposed by twenty minutes without apparent change. When, however, the mixture was transferred from the red vessel to a white one it combined instantly in sunshine, without any explosion. This evidently indicates some physical change, such as we include under the term Allotropism, which will receive attention in its proper section.

(28.) Sir Henry Englefield, at the suggestion of Sir Humphry Davy, tried several experiments with respect to the power of the several coloured rays in rendering

* Philosophical Magazine, vol. vii. 2 ser. p. 462.

Canton's phosphorus luminous. It was found that the blue rays possessed that power in the highest degree, and there was reason to suspect that this power extended beyond the visible violet ray.

(29.) About this period Dr. Blackburne put forth a theory that Light was a compound of caloric and oxygen; but it does not appear to have found any supporters. In the consideration of the very remarkable phenomena connected with the changes produced by Light, it will be evident that the heat-giving rays, often very materially modify the results; hence the above sketch of the very interesting discoveries of Dr. Herschel will be found to be perfectly in place.

(30.) By far the most important series of researches, however, which were undertaken were those of M. Berard, in 1812, which were examined and reported on by Chaptal, Berthollet, and Biot. In their report they say, "M. Berard found that the chemical intensity was greatest at the violet end of the spectrum, and that it extended, as Ritter and Wollaston had observed, a little beyond that extremity. When he left substances exposed for a certain time to the action of each ray, he observed sensible effects, though with an intensity continually decreasing in the indigo and blue rays. Hence we must consider it as extremely probable, that if he had been able to employ reactions still more sensible he would have observed analogous effects, but still more feeble, even in the other rays. To show clearly the great disproportion which exists in this respect between the energies of different rays, M. Berard concentrated, by means of a lens, all that part of the spectrum which extends from the green to the extreme violet; and he concentrated, by means of another lens, all that portion which extends from the green to the extremity of the red. This last pencil formed a white point so brilliant that the eyes were scarcely able to endure it; yet the muriate of silver remained exposed more than two hours to this brilliant point of light, without undergoing any sensible alteration. On the



other hand, when exposed to the other pencil, which was much less bright and less hot, it was blackened in less than six minutes." This is the earliest intimation we have of any indication that the luminous and chemical powers may be due to dissimilar agencies. On this, the Commissioners remark:—“ If we wish to consider solar light as composed of three distinct substances, one of which occasions light, another heat, and the third chemical combinations ; it will follow that each of these substances is separable by the prism into an infinity of different modifications, like light itself; since we find by experiment, that each of the three properties, chemical, colorific, and calorific, is spread, though unequally, over a certain extent of the spectrum. Hence we must suppose, on that hypothesis, that there exists three spectrums one above another; namely a calorific, a colorific, and a chemical spectrum. We must likewise adınit that each of the substances which compose the three spectrums, and even each molecule of unequal refrangibility which constitutes these substances, is endowed, like the molecules of visible light, with the property of being polarized by reflection, and of escaping from reflection in the same positions as the luminous molecules, &c.” Some other objections to M. Berard's views are then urged.

(31.) Being desirous of including within this general notice, all the phenomena of chemical change which had been observed as being produced by the solar rays, the experiments of Wedgwood and Davy, as belonging especially to photography, have been passed over in the order of time. To a consideration of these, however, it is necessary now to return.

In June, 1802, Mr. Thomas Wedgwood, the son of the celebrated porcelain manufacturer, published, in the Journals of the Royal Institution, “ An Account of a Method of Copying Paintings upon Glass, and of making Profiles by the Agency of Light upon Nitrate of Silver; with Observations by H. Davy." This was certainly the first published account of any attempt to produce images by the decom

posing powers of Light. It does indeed appear, that nearly about the same time M. Charles, in his lectures at Paris, proposed to make use of a prepared paper, to produce black profiles by the action of Light, but he died without disclosing the preparation which he employed ; indeed his countryman the Abbé Moigno admits that Charles left “no authentic document to attest his discovery.” Mr. Wedgwood made use of white paper, or white leather, moistened with a solution of nitrate of silver. Notwithstanding the imperfect character of his process, it is so very interesting, as the first attempt at producing pictures by Light, that I shall copy the author's description of it, and some of the remarks, from the Memoir:

(32.) “ White paper, or white leather, moistened with solution of nitrate of silver, undergoes no change when kept in a dark place, but on being exposed to the daylight it speedily changes colour, and after passing through different shades of grey and brown, becomes at length nearly black. The alterations of colour take place more speedily in proportion as the Light is more intense. In the direct beams of the sun, two or three minutes are sufficient to produce the full effect; in the shade several hours are required, and Light transmitted through different coloured glasses, acts upon it with different degrees of intensity. Thus it is found that red rays, or the common sunbeams passed through red glass, have very little action upon it; yellow and green are more efficacious, but blue and violet Light produce the most decided and powerful effects. * * * * * * * * When the shadow of any figure is thrown upon the prepared surface, the part concealed by it remains white, and the other parts speedily become dark. For copying paintings on glass, the solution should be ap. plied on leather, and in this case it is more readily acted on than when paper is used. After the colour has been once fixed on the leather or paper, it cannot be removed by the application of water, or water and soap, and it is



in a high degree permanent. The copy of a painting or the profile, immediately after being taken, must be kept in an obscure place; it may, indeed, be examined in the shade, but in this case the exposure should be only for a few minutes; by the Light of candles or lamps, as commonly employed, it is not sensibly affected. No attempts that have been made to prevent the uncoloured parts of the copy or profile from being acted upon by Light have as yet been successful. They have been covered with a thin coating of fine varnish, but this has not destroyed their susceptibility of becoming coloured; and even after repeated washings, sufficient of the active part of the saline matter will still adhere to the white parts of the leather or paper, to cause them to become dark when exposed to the rays of the sun. Besides the applications of this method of copying that have just been mentioned, there are many others; and it will be useful for making delineations of all such objects as are possessed of a texture partly opaque and partly transparent. The woody fibres of leaves and the wings of insects may be pretty accurately represented by means of it; and in this case it is only necessary to cause the direct solar Light to pass through them, and to receive the shadows upon prepared leather. * * * * * * The images formed by means of a camera obscura have been found to be too faint to produce, in any moderate time, an effect upon the nitrate of silver. To copy these images was the first object of Mr. Wedgwood, in his researches on the subject; and for this purpose he first used the nitrate of silver, which was mentioned to him by a friend, as a substance very sensible to the influence of Light, but all his numerous experiments as to their primary end proved unsuccessful. In following these processes, I have found that the images of sınall 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

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