20 THE POPULAR SCIENCE MONTHLY. 



making the impressions lasting. In 1827 he sent a paper to the Royal 

 Society, accompanied with si)ecimens; but, as he kept the process a 

 secret, the communication could not be received. The process, how- 

 ever, he named heUogra})hy, or sun-drawing, a term by wliich 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 befoi'e the work was matured, and Daguerre, 

 very naturally, reaped the honor of it. The French Government 

 bought his secret, paying wdth a life-pension, and promulgating it to 

 the world, without restriction of patent, in August, 1839. The new 

 pictui-es were at once known as dagtierreotypes, and the mode of 

 making them the daguerreotype process. These uncouth terms en- 

 dured for a while, but were at length supplanted by the word 2^hotog- 

 raphy, or light-drawing, which has become esiablished. Yet the ap- 

 pellation is incorrect, and the error is as broad as the difference b<. - 

 tween 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 sulphur- 

 ous principle of the sun's rays," they concluded that this change, 

 effected by light, was the commencement of the process by which sil- 

 ver was to be transmuted into gold. 



It was in 1777 that the illustrious Swedish chemist, Scheele, pub- 

 lished the fii'st 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 after- 

 ward 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. 



