A Manual Of Photography

EAKLY RESEARCHES ON THE SOLAR RAYS. 5 that all the effects produced upon metallic solutions by bright sunshine can be obtained by a prolonged exposure to a tempe- rature of 210° Fahrenheit. We are now, however, in a position to show that the chemical effects produced by rays of dark heat are of a very different character from those usually attributed to light, but which it will be shown have no illuminating power. Mr. Kobert Harrup, in a communication to Nicholson’s Journal in 1802, refuted the experiment of Count Rumford, showing that several salts of mercury were reduced by light alone, and not heat. .I11 1801, Ritter proved the existence of rays a considerable distance beyond the visible spectrum, which had the property of speedily blackening chloride of silver. These researches excited the attention of the scientific world: M. Berard, Seebeck, Ber- thollet, and others, directed their attention to the peculiar con- dition of the different rays in relation to their luminous and chemical influences ; while Sir William Herschel and Sir Henry Englefield investigated the calorific powers of the coloured rays, and were followed in these investigations by Seebeck andWunsch. Dr. Wollaston pursued and published an interesting series of experiments on the decomposition effected by light on gum guaiacum. He found that paper washed with a solution of this gum in spirits of wine, had its yellow colour rapidly changed to green by the violet rays, while the red rays had the property of restoring the yellow hue. Sir Humphry Davy observed that the puce-coloured oxide of lead became, when moistened, red by exposure to the red ray, and black when exposed to the violet ray ; that hydrogen and chlorine entered into combination more rapidly in the red than in the violet rays, and that the green oxide of mercury, although not changed by the most refrangible rays, speedily became red in the least refrangible. These, and some curious observations by Morichini and Con- figliachi, M. Berard and Mrs. Somerville, on the power of the violet rays to induce magnetism in steel needles, are the prin- cipal points of discovery in this branch of science, previously to the announcement of the Daguerreotype. Seebeck and Berzelius investigated this involved subject: it lias again and again engaged the attention of experimentalists ; but to the present time it may be regarded as an unsettled point, whether magnetism can be induced in steel by the solar rays. A statement has been made by the French, to the effect that M. Charles was in possession of a process by which portraits could be obtained by the agency of sunlight, producing a dark impression upon a prepared surface. This is, however, exceed- ingly doubtful, and even the Abbé Moigno, in his Répertoire,