The Chemistry of Photography

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1892 edition. Excerpt: ... CHAPTER XIII. CHEMICALS EMPLOYED IN PHOTOGRAPHY (CONTINUED). Iodine. Symbol, I: Combining weight, 127. The elementary body iodine was discovered by Cuurtois, at Paris, in 1812. IIe obtained it from kelp--the ashes of certain seaweeds which contain the iodides of sodium and magnesium. Iodine is prepared in a precisely similar way to bromine and chlorine, its fellow-halogens, by heating an iodide--usually potassium iodide--with sulphuric acid and black oxide of manganese. Iodine is slightly soluble in water, more so in alcohol. A few drops of tincture of iodine added to the hydroquinone developer have a powerful accelerating effect, and reduce contrasts. 2KI]MnO2 + 2H2S04= I, K2SO4+MnSO4 + 2H2O. Iodine is usually seen as bluish-black scales, having a somewhat metallic lustre. It melts at 239 deg. Fahr., and at 4uO deg. Fahr. is converted into a beautiful violet-colored vapor. Iodine is but very slightly soluble in water, but readily dissolves in alcohol, in carbon-bisulphide, or in chloroform. Free iodine forms a blue compound with starch, and this furnishes a well-known test. To make this test, a drop of potassium iodide solution is added to some very dilute starch-paste. It' a drop or two of chlorine water is then added to the mixture, some iodine will be liberated, and will unite with the starch to form a blue compound. This blue compound, once formed, is itself a delicate test for ' hypo," the latter substance discharging the blue color. A solution of iodine in water can be obtained, if to the water is first added some iodide of potassium. This solution has been recommended by Vogel and by Chapman Jones as a hypo-eliminator. "After fixing as usual, wash the prints in three or four changes of water, and then place them in water colored...