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HISTORICAL SURVEY 11
than Niepce if a fortunate accident had not worked in his favour.
He made experiments with iodide of silver plates, which he produced by exposing silver plates to the vapour of iodine, a peculiar and very volatile chemical element. Under this treatment, the silver plate assumes a pale yellow colour, which is peculiar to the compound of iodine and silver. These plates of iodide of silver are sensitive to light, they take a brown colour when exposed to it, and thus an image is produced upon them when they are exposed to the action of light in the camera. A very long exposure to light, however, is necessary to accomplish this end"; and the thought could scarcely have arisen of taking the likeness of any person in this manner, since he would have been obliged to remain motionless for hours.
One day Daguerre placed aside as useless, in a closet in which were some chemical substances, several plates that had been exposed too short a time to the light, and therefore as yet showed no image. After some time he happened to look at the plates, and was not a little astonished to see an image upon them. He immediately divined that this must have arisen through the operation on the plates of some chemical substance which was lying in the closet. He therefore proceeded to take out of the closet one chemical after the other, and placed there plates which had been exposed to the light, when, after remaining there some hours, images were again produced upon them. At length he thought that he had removed in succession all the chemical substances from the closet ; and still images were produced upon the plates. He was now on the point of believing the closet to be bewitched, when he discovered on the floor a dish containing mercury, which he had hitherto overlooked. He conceived the notion that the vapour from this substance — for mercury gives off vapour even at an ordinary temperature — must have been the magic power which produced the image.