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THE PHOTOGRAPHIC IMPORTANCE OF THE CHROMIUM COMPOUNDS
Numerous attempts have been made to substitute other sensitive materials for the expensive salts of silver, and some of these attempts have been crowned with success. It is true that no substance has been found permitting a negative to be prepared in the camera as easily as can be done by means of silver salts. But positives from existing negatives can be successfully produced by the help of many other metallic combinations. The results obtained by employing some of these are important, for they admit of multiplication in the printing press without the help of light. The compounds of chromium are the most useful in this respect, and the development of some of the principal photographic processes in which these substances are used will be next considered.
The Chromium Compounds. — A black mineral called chrome iron ore occurs in nature, especially in Sweden and America. If this is fused with carbonate and nitrate of potash, a beautiful orange-red salt is formed, which dissolves in water and readily c^stallizes on evaporation. This orange-red salt is bichromate of potash. It consists, as implied by the name, of chromic acid and potash. The latter is the chief component part of the soap-boiler's ley ; the former is a combination of the metal chromium, with oxygen. Chromium and ox}7gen combine together in several proportions, thus :
28 parts chromium with 8 parts oxygen form protoxide of chromium. 28 ,, ,, 12 ,, ,, sesquioxide of chromium.
28 ,, ,, 16 „ „ dioxide of chromium.
28 „ „ 24 „ „ chromic acid.
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