BICHROMATE OF POTASSA. KO, 2 CT Os. MS salt is Largely manufactured for calico-printers. Its solution should not be brought into contact with the sldn, as it sometimes causes sores difficult to heal. One ounce in ten of cold water forms it saturated solution; it is much more soluble in hot water. It easily givas up one atom of oxygen to deoxidizing bodies, becoming neutral chromate and binoxide of chromium. The binoxide has a tendency to be reduced still further to sesquioxide. When the chromic acid is set free from the potassa by sulphuric acid, it becomes a strong oxidizer, by the facility with which it parts with oxygen ; and such a mixture of bichromate and sulphuric acid will determine whether a print has been toned with gold or sulphur, by the greater resistance which gold offers to its oxidizing influence. Light will also act upon the bichromate in the presence of organic matter, and precipitate insolu ble oxide of chromium in combination with that matter. This property has been taken advantage of as the basis of several photo graphic processes, and the chemical action is stiictly analogous to that of the use of nitrate of silver alone on paper. A vi.sible imag,e is impressed on the paper, on which reduce,d silver and other metals (iron, &c.) may be afterwards precipitated by suitable developing
solutions. When protosulphate of iron and gallic acid are employed, the picture, or its chemical composition, resembles writing ink. The iron and other metallic solutions are sometimes presented, with the bichromate in the paper, to the action of light, and the variety of modifications appears to be infinite. The insoluble compounds of chromium, formed by light, and the action of the light itself, have both been used in the art of dyeing, the chromium forming a mordant on the textile fabric, in parts exposed to light through a perforated pattern, on which mordant the colours are subsequently applied ; or the colours may be put on with the bichromate, and subsequently washed out from the parts not acted on by the light, just as in a similar photographic process on paper. Papers prepared with bichromate of potassa and nitrate of silver have also been found to give images varying in colour from red to gre,en, and blue. We believe, before the publication of the talbotype and dag-uerreotype processes, if not before that of Niepce's, fixed pictures were taken by Mr. Mongo Ponton, by means of this salt, and the process published. See " Printing."