partially exposed, will, when breathed upon, or presented to the action of mercurial vapour, show that a disturbance has been produced upon the portions which were illuminated, whereas no change can be detected upon the parts which were kept in the dark. It was thought, until lately, that a few chemical compounds, such as the iodide of silver, the material employed in the Daguerreotype and Calotype,--chloride of silver, the ordinary photographic agent,--a few salts of gold, and one or two of lead and iron, were the only materials upon which these very remarkable changes were produced. We now know that it is impossible to expose any body, simple or compound, to the sun’s rays, without its being influenced by this chemical and molecular disturbing power. To take our examples from inorganic nature, the granite rock which presents its uplifted head in firmness to the driving storm, the stones which genius has framed into forms of architectural beauty, or the metal which is intended to commemorate the great acts of man, and which in the human form proclaims the hero’s deeds and the artist’s talent, are all alike destructively acted upon during the hours of sunshine, and, but for provisions of nature no less wonderful, would soon perish under the delicate touch of the most subtile of the agencies of the universe. Niepce was the first to show that all bodies which underwent this change during daylight possessed the power of restoring themselves to their original conditions during the hours of night, when this excitement was no longer influencing them. Resins, the Daguerreotype plate, the unprepared metal tablet, and numerous photographic preparations, prove this in a remarkable manner.[121]