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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 79
The last combination, chromic acid, is the best known of all. It may be prepared by adding sulphuric acid to a solution of chromate of potash : it crystallises in red needles, which readily part with their oxygen. Thus alcohol, dropped upon chromic acid, takes fire ; the chromic acid instantaneously gives up part of its oxygen to the alcohol, and is changed into a green substance, sesquioxide of chromium. The sesquioxide of chromium forms salts with acids — for example, sulphate of chromium. This unites again readily with sulphate of potash to form a double salt, which is known by the name of chrome alum, and is sold crystallised in very beautiful dark violet octahedra (fig. 19). It is employed in dyeing, together with chromate of potash.
If chromate of potash be mixed with a solution of protosulphate of iron (green vitriol), a brown precipitate of peroxide of chromium is formed, the iron salt having removed a part of the oxygen from the chromate. This is often formed by the action on chromic acid or its salts of substances absorbing oxygen.
Chromic acid is of special interest to us, because both it and its salts are sensitive to light. Neither pure chromic acid, nor chromate of potash, are changed by light ; they can be exposed for years to the sunlight without any decomposition being perceived. But as soon as a body is present that can unite with oxygen — for example, wood-fibre, paper, etc. — the light immediately produces its effect. This fact was published by Mungo Ponton, in the New Philosophical Journal, in the same year as the discovery of photography, that is in 1839. He writes :—
Experiments of Ponton. — " Paper immersed in bichromate of potash is powerfully and rapidly acted upon