Chemistry for Photographers

s investigated by Sir John Herschel, and he indeed invented, in 1840, one of the simplest photographic printing processes, using salts of iron, namely, the "blue print" process. Experiment 33. Coat a piece of sized paper with a little ten per cent solution of ferric ammonium citrate, either by floating the paper upon the liquid or by applying the solution with a brush or a wad of absorbent cotton. When the coated paper has become dry, expose it for one or two minutes to sunlight under a negative. Develop the print in a little dilute potassium ferricya- nide solution and wash it thoroughly. Note that the shadows are blue and the highest lights white. Coat another piece of paper with the same ferric ammonium citrate solution, expose it similarly under a positive transparency, such as an unmounted lantern slide, and develop with dilute potassium ferrocyanide and wash. Observe in this case also that a blue positive has been obtained, but from a positive instead of from a negative. By the action of light the ferric citrate, in contact with the organic matter of the sizing, has been reduced to ferrous citrate. This same reaction of course takes place in both parts of the experiment. Since ferrous salts interact with ferricyanide to form an insoluble blue compound, "TurnbulPs blue," the application of potassium ferricyanide to the first print, from the negative, forms blue ferrous ferricyanide wherever any ferric salt has been reduced, viz., in the shadows and hal...