662 THE POPULAR SCIENCE MONTHLY. 



The more securely chemistry had established its household, the 

 more willingly were its services offered to its neighbors. In investi- 

 gating the composition of minerals, an exact science was created out of 

 our collections of specimens. What meaning had a mineral whose con- 

 stituents 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 re- 

 sulted 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 so- 

 lution. When the plate is exposed in an optical apparatus to the ac- 

 tion 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 con- 

 stituents 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 be- 

 hind. 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 hypo- 

 sulphite of soda, the sensitive substance remaining on the paper, a real 

 picture of the object, the so-called positive, is obtained. This wonder- 

 ful process of sun-drawing was also involved in the discovery of oxy- 

 gen, albeit that accident and planless searching had much to do with 

 it. Accident, however, was unthinkable, had not chemistry first de- 

 fined 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 invaluabU 

 medicine and photography ? 



