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662 THE PHOTOGRAPHIC NEWS. [September 18, 1891. are, in fact, potential reducing agents. This can be done in test tubes or flasks in the first place, and then on paper films, leading to the ordinary cyanotype and blue printing processes. A few hints for the carrying out of the experiments may be found servicable :— 1. A solution of ferric chloride (2-3 p.c.), mixed with a solu tion of oxalic* acid will, of course, on testing with potassium ferricyanide, give no blue colouration. Some of the same solu tion, exposed for five minutes or so to strong light, will be found to contain ferrous salt on again testing with ferri cyanide :—• 2FeCl 3 + H,C,0, = 2FeCl 2 + 2HC1 + 2CO a . 2. By using ferricyanide with the ferric salt, and exposing to light, the reduction is made visible by the formation of Turnbull’s blue. This can be done by adding ferricyanide to the foregoing, or preparing two solutions : one containing 8 grammes of potassium ferricyanide in 50 c.c. of water, and the other containing 10 grammes of ammonio-ferric citrate in 50 c.c. of water. The solutions are mixed before use, and then exposed to light, first in a test-tube, and then on paper coated with the solution, and allowed to dry in the dark. The practical application of this method for copying and printing will be obvious. The chief point of general theoretical importance brought out by such experiments as these is, that light only reduces the ferric salts in the presence of oxidisable compounds of suf ficient instability. It is advisable, at this stage, to introduce the notion of aensitisers, and to point out that oxalic acid, citric acid, alcohol, &c., may be regarded in this light in the experiments referred to. The demonstrations with ferric salts may, of course, be extended in many directions, and made the basis of numerous practical exercises and lessons in the applica tion of general chemical principles to special cases. All that has to be borne in mind is, that a surface of an organic salt exposed to light under a stencilled design (or a picture) gives ferrous salt on the exposed portions, leaving the unexposed portions unchanged. Various reagents may then be used to reveal the chemical difference in the two portions, the subject of photographic development being thus introduced, and the changes evolved being explained by ordinary chemistry. By way of example :— A design printed on paper coated with ammonio-ferric citrate is developed by ferricyanide. The exposed (reduced) portions come out blue, owing to the formation of Turnbull’s blue. Supposing ferrous citrate to be formed :— Fe", (C 6 H 6 O 7 ) 2 + K,Fe,Cy = Fe, (Fe 3 Cy 1S ) + 2K, (C 6 H S O 7 ). The blue design, after being well washed, can be made the subject of many further experiments, all instructive as illus trating chemical principles with which the student should be familiar. Thus, on treatment with a dilute solution of caustic soda, the blue is at once decomposed, with the formation of Fe,O, which remains on the paper. We have thus a faintly visible brownish design, which can again be developed by taking advantage of the property possessed by the oxides of iron of forming coloured compounds with organic substances, such as gallic acid, alizarin, nitro-sophenols, &c. It may be pointed out that the photo-chemical reduction of ferric salts, although practically useful for printing purposes, takes place too slowly to enable these compounds to be used at present for the production of camera pictures. But there is no reason why the rate of photo-chemical reduction—i.e., the sen sitiveness of these compounds—should not be increased by admixture with some easily oxidisable substance, and a sensi tive film prepared by this means, which would cheapen photo graphic processes by dispensing with the use of silver salts. In the same way that the photo-chemistry of iron is studied, the other sensitive metallic compounds may be dealt with. The reduction of uranic salts, and the development of uranium prints by various reagents, will naturally be connected with the analogous ferric salts. The photo-chemical reduction of chromates in the presence of organic substances, such as gum, albumen, and gelatine, will lead to the numerous practical applications of chromated gelatine in the processes of etching, * The addition of some alcohol increases the sensitiveness of the mixture. pigment printing, collotype, &c. In these processes practical instruction may be given at this stage as far as thought desir able. The salts of mercury and copper may also be studied with advantage, as illustrating the nature of photo-chemical decomposition. The well-known greenish mercurous iodide is easily prepared by decomposing freshly-precipitated and washed mercurous chloride with a solution of potassium iodide. Some of this salt, washed by decantation, and exposed under water to the action of strong light, rapidly darkens, owing to the libera tion of metallic mercury. The action of light on the salts of copper forms a con venient introduction to the photo-chemistry of the silver salts. Thus, cuprous chloride darkens, on exposure to light, with as great a rapidity as silver chloride. When discussing photo-chemical decomposition, it should be pointed out that indirect results are often obtained by using a mixture of substances of which one of the constituents is not directly affected by light, but is altered by contact with the product resulting from the photo-chemical decomposition'of the other constituent of the mixture. For example, paper coated with ferric chloride, and exposed to the light, gives a surface of ferrous chloride by photo-chemical reduction, the size or cel lulose of the paper acting as the sensitiser (chlorine absorbent). But if the surface is coated with a mixture of ferric chloride and cupric chloride, the ferrous chloride which is formed reduces the cupric salt with which it is in contact:— 2FeCJ 2 4- 2CuCl, = Cu,CI, + 2FeCl. The picture is thus formed in cuprous chloride instead of in ferrous chloride, and, by treatment with potassium thiocyanate, cuprous thiocyanate is formed, which, on subsequent treat ment with potassium ferricyanide, leads to the development of a brown print. This method of utilising a mixture of ferric and cupric salts is the basis of Obernetter’s process. There can be no doubt that this principle of indirect decom position is destined to play a very important part in the pho tography of the future. It has already come into prominence in the well-known platinotype process of Willis, in which a surface is coated with a mixture of ferric oxalate and potassium chloro platinite. On exposure to light ferrous oxalate is formed, while the chloroplatinite is not directly reduced. On treatment with a hot solution of potassium oxalate the ferrous oxalate is dissolved out, and at the moment of solution reduces the chloroplatinite to finely divided platinum in situ. In the direct printing platinotype process we have a surface of potassium chloroplatinite, sodium oxalate, and sodio-ferric oxalate. In this case the reduction of the ferric salt by light is accompanied by the indirect reduction of the chloroplatinite by means of the ferrous salt thus formed. In the cold platinotype of Willis the operations are separated, the ferric surface being first exposed in the usual way, and then development being effected by immersion in a cold solution of potassium chloroplatinite containing potassium oxalate and phosphate. {To be continued.) Photographic Club.—Subject for Sept. 23rd, “Enlarging”; Sept. 30th, “Mounting and Finishing.” Outing Saturday next (19th) Perivale ; train from Paddington to Castle Hill at 2.35 p.m. The last outing of the season (Hampstead) has been postponed to Saturday, October 3rd. M. DAGREVE, in Les AnnalsPhoto'jraphiques, suggests the fol lowing method of silvering old plated or copper articles, which may be useful to any chemist who has some old fixing baths and some worn electro-plate. The articles to be plated are well washed in soap and water, and then immersed in an old fixing bath which has been used for fixing plates. In a very short time a deposit of silver forms, and then the article should be taken out, rinsed, polished with soft leather, and again im mersed till the deposited silver is thick enough. When an extra thick coating is desired, a piece of wire is affixed to the object, and at the other end of the wire a sheet of zinc, allowing one square centimetre of zinc to every square deci metre of the article to be plated. It is not advisable to use old print fixing baths for this purpose, as the silver has a pecu liar yellow tinge ; but chloride of silver which has not been exposed to light may be used,
