198 ON A NEW PROCESS OF PHOTOGRAPHY WITHOUT SILVER. 



brought it before the Dublin Chemical Society as a novelty, and, more- 

 over, as a discovery of bis own. But this author has had recourse to nitrate 

 of silver to finish his proofs, whereas, in my process no silver is used at all. 



It is well known that light has a peculiar action (a reductive action) upon 

 many organic salts of iron, more especially upon the oxalate of pet oxide of 

 iron, which it reduces to the state of oxalate of protoxide. The first of 

 these salts forms beautiful emerald green, prismatic crystals, extremely 

 soluble in water, and decomposable by light ; the latter salt is yellow, 

 insoluble in water, and not influenced by light. 



The first thing to be done is to prepare a concentrated solution of 

 oxalate of peroxide of iron. For this purpose I take a solution of chloride 

 of iron, and having precipitated the peroxide of iron by ammonia, 3 collect 

 it upon a filter and wash it with boiling water, after which this oxide is 

 dissolved in a warm concentrated solution of oxalic acid. A beautiful 

 emerald green solution is thus obtained, which must be concentrated a little 

 by evaporation, and then set aside in a dark place for use. If this solution 

 be exposed to the sunlight, microscopic yellow crystals of oxalate of pro- 

 toxide of iron are deposited, more or less rapidly, until the solution con- 

 tains no more iron, and has become perfectly colourless. Upon this remark- 

 able decomposition is founded the process of which I speak. The paper, 

 destined to receive the photographic image, is floated for about ten minutes 

 upon the green solution of oxalate of peroxide in a flat dish or caps ule, to 

 which a certain quantity of oxalate of ammonia has been added ; the 

 whole, of course, being kept away from the day light, and at the expiration 

 of that time, the paper is taken out of the solution and hung up by one of 

 its corners to dry. 



Let us suppose, for example, that a positive proof is required -: the 

 paper thus prepared is placed behind the negative and exposed to the light 

 for ten or twenty minutes, according to the weather. It is then well washed 

 with distilled water or with rain water (spring water will not answer, on 

 account of the lime it contains which decomposes the image by forming 

 oxalate of lime). All the non-decomposed oxalate of peroxide of iron is 

 thus washed from the proof, and a feeble yellow image of oxalate of pro- 

 toxide, scarcely visible, is left upon the paper. 



This image can be transformed into Prussian blue, or into sulphide of 

 iron, by plunging it into ferricyanide of potassium, acidulated with a few 

 drops of nitric acid, or into sulphide of ammonium. In the first case, the 

 image, though very permanent, is blue, a colour not generally desirable ; in 

 the second, it is jet black, but not permanent, for the damp air destroys it, 

 and turns it brown or red. 



The best means I have hitherto discovered of developing the image, and 

 obtaining proofs equal in tone, colour, and vigour, to those obtained with 

 salts of silver, is as follows : 



The faint yellow proof, obtained as above, is plunged for a little time 

 into a solution of permanganate of potash, to which a few drops of ammo- 

 nia have beejk- added. In this bath the image soon becomes brown and dis- , 



