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SILVER SALTS.
There are four chemical elements either of which combined with a metal forms a compound resembling sea-salt. These four elements are Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I). They are termed in chemistry "halogens" (salt-producers), and the compounds which they form are called "haloids." When they are combined with silver they make silver haloids, or salts of silver. Three of these salts, silver chloride, silver bromide, and silver iodide, are the substances most quickly affected by light, and are most important agents in making a photographic image.
Silver chloride is often found native in silver mines, and is called by the miners "horn-silver." As early as the sixteenth century it was observed that this "horn-silver" turned dark when it was brought up from the mines into the sunlight, but it was not until the year 1777 that it was found this darkening of the silver chloride was due to the chemical effect of light. This discovery was made by a Swedish chemist, Charles William Scheele. Silver chloride was the first salts of silver used in photography, and the first picture made on a sensitive surface by means of a lens was made by that famous chemist Sir Humphry Davy. His lenses were taken from his solar microscope. By coating paper with silver chloride and exposing it for a long time in the camera he obtained pictures of small objects. These pictures were positives, not negatives. An English chemist by the name of Wedgwood worked with him; but though they succeeded in making pictures, they could not "fix" the image, so that all their pictures were kept in portfolios away from the light, and only examined by candle-light.
Silver chloride is used in making photographic printing-paper, not by coating the paper with the silver chloride, but by producing it upon the paper itself by means of two solutions with which the paper is coated. The chemical formula for silver chloride is AgCl, meaning that a molecule of silver chloride contains one atom of silver and one atom of chlorine. (The chemical name for silver is argentum, and the symbol is Ag.) This chloride was used by Davy for coating the paper on which he made his pictures, but the paper was not very sensitive to light, it taking from a half-hour to two hours to make a picture. By repeated experiments, Fox Talbot, an Englishman, succeeded in making a paper which was very sensitive to light. He first coated the paper with a solution of common salt, sodium chloride (NaCl), and dried it. This salted paper was then brushed over with a solution of nitrate of silver, which combined with the sodium chloride (salt), and formed silver chloride.
In preparing the paper the nitrate of silver solution was made strong enough so that there might be a little left on the paper in addition to that which combined with the sodium chloride to form the silver chloride. (Sodium nitrate is also produced, but it has no effect on the paper.) Silver nitrate is very largely used in photography in all sensitive preparations. In surgery it is known as "lunar caustic," and is used to cauterize or burn the flesh to prevent the spreading of disease. It is produced in the separation of gold from silver in the refining process. It is produced chemically by dissolving pure silver in an equal part of nitric acid. The chemical formula for it is AgNO3. (Nitrate of silver is very poisonous.)