Page:Popular Science Monthly Volume 1.djvu/678

662 The more securely chemistry had established its household, the more willingly were its services offered to its neighbors. In investigating the composition of minerals, an exact science was created out of our collections of specimens. What meaning had a mineral whose constituents were unknown, in which nothing was observable but what every layman could perceive, viz., color, hardness, and form? Owing to chemistry, mere knowledge about minerals ripened into the science of mineralogy; she induced basalt and granite to yield a glass of water, and taught the process of their formation.

Casual observations had shown that certain substances changed their color on exposure to light. This was especially the case with several silver compounds. The attempt to utilize this property resulted in the invention of photography. A film of albumen or collodion on a glass plate contains a material which, together with silver, makes up a substance sensitive to the action of light. Thus prepared, the glass plate is immersed in a glass of water containing an argentic solution. When the plate is exposed in an optical apparatus to the action of the luminous rays of an object, the result of this action is an image produced on the plate, though invisible to the eye. In the places acted upon by the light-rays, the connection between the constituents of the sensitive argentic substance is not dissolved entirely, but rendered very unstable. The additional action of an oxidizing agent, such as a ferrous salt or pyro-gallic acid, causes opaque metallic silver to be formed on the lighted parts of the image, and a reversed picture, the so-called negative, is produced. The plate is now dipped again into a glass of water, containing a substance which removes the last traces of the sensitive coating, leaving the darkened picture behind. In this way the negative is protected from any further influence of light. Of course, the picture is not recognizable, for the lights and shadows are reversed, but by the same process they can be reversed a second time. A sheet of paper is covered with albumen and thereby sensitized, then laid under the negative and exposed to the action of the sun. The parts of the paper under the darkened portions of the negative remain unchanged; those under the lighted portions are changed by the sunlight. On their withdrawing, by means of hyposulphite of soda, the sensitive substance remaining on the paper, a real picture of the object, the so-called positive, is obtained. This wonderful process of sun-drawing was also involved in the discovery of oxygen, albeit that accident and planless searching had much to do with it. Accident, however, was unthinkable, had not chemistry first defined the substances and their properties. In photography great use is made of iodine, an element existing in the ocean. An eminent chemist, Gay-Lussac, investigated and described this body, without which no photograph can be made. Who would have thought at that time that the violet vapors of iodine contained both an invaluable medicine and photography?