making the impressions lasting. In 1827 he sent a paper to the Royal Society, accompanied with specimens; but, as he kept the process a secret, the communication could not be received. The process, however, he named heliography, or sun-drawing, a term by which it was truthfully characterized. M. Daguerre, another Frenchman, had been working at the same problem, and in 1829 these two men, with a common purpose, formed a partnership to carry on their researches jointly. Neipce died before the work was matured, and Daguerre, very naturally, reaped the honor of it. The French Government bought his secret, paying with a life-pension, and promulgating it to the world, without restriction of patent, in August, 1839. The new pictures were at once known as daguerreotypes, and the mode of making them the daguerreotype process. These uncouth terms endured for a while, but were at length supplanted by the word photography, or light-drawing, which has become established. Yet the appellation is incorrect, and the error is as broad as the difference between light and darkness. It is not light that makes the picture, but dark radiations that are associated with it, and that have the peculiar effect of producing changes in certain chemical compounds.
Although photography, in its wonderful development as an art, belongs to the past generation, yet the knowledge of the chemical effects ascribed to light is as old as chemical science. The subject began to be inquired into, experimentally, about 100 years ago. In fact, like most other modern chemical results, it had not escaped the notice of the alchemists, but, like every thing else they discovered, it was subordinated to their mystical speculations. In the multiplicity of their manipulatory processes they stumbled upon a combination which they called luna cornua, or horn-silver, and which is now known as silver chloride. The alchemists knew nothing of its composition, but only that there was silver in it which had undergone a change. They noticed, however, that when this horn-silver was exposed to light it underwent a blackening, and, as they taught that "silver only differed from gold in being mercury interpenetrated by the sulphurous principle of the sun's rays," they concluded that this change, effected by light, was the commencement of the process by which silver was to be transmuted into gold.
It was in 1777 that the illustrious Swedish chemist, Scheele, published the first results of investigations upon the subject undertaken simply for the extension of chemical knowledge. He found that, when powdered horn-silver is spread over paper, and the colors of the solar spectrum are made to fall upon it, the powder in the violet ray turns black sooner than that exposed to the other colors. Senebier afterward showed that the silver chloride was darkened in the violet ray in fifteen seconds to a shade which required the action of the red ray for twenty minutes; that is, the chemical intensity of the violet ray was eighty times greater than the red.
