AMMONIO-CITRATE OF IRON. This neutral double salt of citrate of peroxide of iron, and citrate of ammonia, dissolved in water, and mixed with solution of ferro-cyanide of potassium, gives a purple solution nearly black. Papers prepared with this are bleached in the light, but darken again in the dark. Salts of cyanogen, and salts of iron, are both subject to actinolyitc action ; but the iron is here the one attacked. This paper may be used to take evanescent pictures of many objects successively. The ammonio-citrate of iron itself, is a beautiful pharmaceutical preparation. It is a salt of the peroxide of iron, which is partially reduced by the chemical rays to protoxide. Papers prepared with a solution of it, change by insola tion from a bright yellow to an ochrey grey, and may be developed before the picture becomes visible by ferridcyanide of potassium, (red prussiate of potash), which proves the reduction to the state of protoxide, by the formation of prussian blue. It is known to all
photographers that the protosalts of iron, as the protonitrate, the protacetate, the protocitrate, &c. are developers, and their formation by light is a curious instance in which the usual order of operations in taking a picture is reversed. First, the paper is prepared with an iron persalt, which, by solar action, becomes a developer in certain parts only of the paper, and the picture is then produced by the action of this local developer on any salts applied uniformly to the paper which it is capable of deoxidizing. Bichromate of potass, nitrate of silver, and many others may be thus applied, and appear to develope the print themselves, but in truth only supply material to the invisible developer, which pre-existed on the siuface.