2

HISTORY OF niOTOGRAHlY.

by tlio action of light. However this may have been, it does not appearthat they know any thing of such a process in the present day.

The alchemists, amidst the multiplicity of their manipulations,dropped on the combination of silver and muriatic acid, to which whitesalt they gave the name of caustic silver, or, when fused, of horn silver,aud they noticed that it was blackened by light; but, appearing to pro-mise nothing to their cupidity, it attracted but little of their attention.

The illustrious Sclieele, in his admirablo Traite de l’Air et du Feu,gave us the first philosophical examination of this peculiar propertyof the salts of silver, and showed the dissimilar powers of the differentrays of light in effecting this change. In 1801, Hitter proved the ex-istence of rays a considerable distance beyond tlio visiblo spectrum,which had the property of speedily blackening chloride of silver. Theseresearches excited tlio attention of the scientific world, and M. Berard,Lubcck and Bertliollet, Sir William Ilerscliel, Sir Henry Engleficld,and others, directed their attention to the peculiar condition of the dif-ferent rays in relation to luminous, calorific, and chemical influences.Dr. Wollaston pursued and published an interesting series of experi-ments on the changes effected by light on gum guaiacum. lie foundthat papers washed with a solution of this gum in spirits of wine, hadits yellow colour rapidly changed to green by the violet rays, while thered rays had the property of restoring the yellow hue. Sir Humphry Davy observed, that the puce-coloured oxide of lead became, whenmoistened, red, by exposure to the red ray, and black when exposed tothe violet ray: that hydrogen and chlorine entered into combinationmore rapidly in the rod, than in the violet rays, and that the greenoxide of mercury, although not changed by tlio most refrangible rays,speedily became red in tlio least refrangible.

The revival of gold and silver from their oxides, by the action of the' sun’s light, also occupied tlio attention of Count Ilumford, who com-municated two valuable papers on this subject to the Royal Society .These, and some curious observations by Morichini and Configliaclii,M. Berard and Mrs. Somerville, on the powor of the violet rays to in-duce magnetism in steel needles, are tlio principal points of discoveryin this branch of photometry. Indeed, with regard to the last men-tioned, Berzelius states, from a review of the experiments of Seobcch,that he concludes the fact announced by Mrs. Somerville rests on anillusion; which agrees with the opinions of Snow Harris, who endea-voured to produce polarity in needles exposed in vacuo to'tlie influenceof violet light, but failed in every instance.* These researches led tlioway to the establishment of the art, on tlio consideration of which womust now more particularly enter. It being admitted on all hands thatthe first attempts to delineate objects by light were those of Wedg-

* The Rev. Thomas Knox, and G. J. Knox, Esq., have very recently repeated those ex-periments; they have, however, only proved that the induced magnetism is dependent uponthe oxidation of the steel, which takes place more rapidly in the violet, than in any of theother rays .—See the Reports of the Royal Irish Academy .