282. Occurrence and Reduction.--Silver is found uncombined, and combined, as Ag2S, argenite, and AgCl, horn silver. It occurs usually with galena, PbS. It is abundant in the Western States, Mexico, and Peru. Silver is separated from galena by melting the two metals. As they slowly cool, Pb crystallizes, and is removed by asieve, while Ag is left in the liquid mass. The principle is much like crystallizing NaCl from solution and leaving behind the salts of Mg, etc., in the mother liquor. When, by repeating the process, most of the Pb is eliminated, the rest is oxidized by heating in the air. Pb + O = PbO. Ag does not oxidize, and is left in the metallic state. Another mode of reduction is to change the silver salt to its chloride, and then remove the Cl with Fe. Roasting with NaCl makes the first change, 2 NaCl + Ag2S = Na2S + 2 AgCl, and with Fe the second, 2 AgCl + Fe = FeCl2 + 2 Ag. Ag is separated from the other products by adding Hg, with which it forms an amalgam. By distilling this, Hg passes over and Ag remains. This is the amalgamating process. 283. Salts of Silver are much employed in organic chemistry, and AgCl, AgBr, and AgNO3 are used in photography. AgNO3 is a soluble, colorless crystal, and is the basis of the silver salts. It blackens when in contact with organic matter. Stains on a photographer's hands are due to this substance, and the use of AgNO3 in indelible inks depends on the same property. This may be due to a reduction of AgNO3 to Ag4O. Stains can be removed from the skin or from linen by a solution of Kl, or of CuCl2 followed by sodium hyposulphite. Lunar caustic is made by fusing AgNO3 crystals, and is used for cauterizing (burning) the flesh. Much AgCN finds use in electroplating. Experiment 128.--Put 5 cc. AgNO3 solution in each of three t.t. To the first add 3 cc. HCl, to the second 3cc.NaCl solution, and to the third 3 cc. KBr solution. Write the reaction for each case, and notice that the first two give the same ppt., as in fact any soluble chloride would. Filter the second and third, on separate filter papers, and expose half the residue to direct sunlight, observing the change of color by occasionally stirring. Solar rays reduce AgCl and AgBr, it is thought, to Ag2Cl and Ag2Br. Try to dissolve the other half in Na2S2O3, sodium thiosulphate solution. This experiment illustrates the main facts of photography. CHAPTER LI. PHOTOGRAPHY. 284. Descriptive.--The silver halogens, AgCI, AgBr, AgI, are very sensitive to certain light rays. Red rays do not affect them; hence ruby glass is used in the "dark room." Photography involves two processes. The negative of the picture is first taken upon a prepared glass plate, and the positive is then printed on prepared paper. The negative shows the lights and shades reversed, while the positive gives objects their true appearance.