Intelligence and Miscellaneous Articles. 71 



glucosides, &c. But the most curious result was obtained with 

 ferrous salts. It was known that ferrous sulphate, though a pow- 

 erful developer in the so-called " wet development " (i. e. develop- 

 ment in presence of a soluble silver-salt), had no power whatever 

 for those developments in which no soluble silver-salt was present, 

 and where the development was to be made at the expense of the 

 film itself. I was able to show that ferrous oxide combined with 

 almost with any organic acid possessed this power of forming a 

 visible image at the expense of the film. So that a solution of fer- 

 rous sulphate, by mixing with one of an alkaline oxalate, succinate, 

 salicylate, &c, immediately acquires the power of development. 

 Ferrous oxalate exhibits the power of development to a degree so 

 remarkable that it seems likely to displace the older methods. 



The study of the subject was resumed during the past winter, 

 and with the result of ascertaining that this power of development 

 was not limited to the organic salts of ferrous oxide, but was pos- 

 sessed by many of its inorganic compounds. It certainly has never 

 been suspected that such compounds as ferrous phosphate, ferrous 

 borate, ferrous sulphite, ferrous hyposulphite, &c. possessed the power 

 of development ; but this they undoubtedly do, and not in any un- 

 certain way. On the contrary, some of these compounds are 

 among the most powerful of all known developing agents, equal- 

 ling, or possibly even excelling, ferrous oxalate in this respect ; so 

 that it is far from impossible that some of them may pass into tech- 

 nical use in preference to those now employed. 



Some of these ferrous salts, especially the phosphate, sulphite, 

 and borate, are, like the oxalate, insoluble in water, and therefore 

 need to be got into solution. As these salts are not, like the oxa- 

 late, soluble in the corresponding alkaline salt, at least not to any 

 useful extent, it becomes necessary to find an appropriate solvent. 

 The most available solvents are solutions of ammonium and potas- 

 sium oxalate, and of ammonium and sodium tartrate. Of these, 

 the first have the material advantage that the ferrous salts remain 

 permanently in solution, whereas with ammonium and sodium tar- 

 trate they are apt gradually to be precipitated. 



As ferrous oxalate is a powerful developer, the question imme- 

 diately presented itself whether the developing-power exhibited, 

 for instance, by ferrous phosphate dissolved in ammonium oxalate, 

 might not be due to the formation of ferrous oxalate. But several 

 reactions contradict this supposition. "When a hot solution of am- 

 monium (neutral) oxalate is fully saturated with ferrous phosphate, 

 a precipitate separates in cooling ; and this precipitate is not ferrous 

 oxalate but ferrous phosphate. Again, ferrous phosphate exhibits 

 powerful developing-properties when dissolved in sodic or amnionic 

 tartrate. This reaction, however, is not in itself decisive, inas- 

 much as I find that ferrous tartrate has itself developing-properties. 

 But as ferrous phosphate is to some extent soluble in a solution of 

 ferrous sulphate, and as ferrous sulphate (in the form of develop- 

 ment here under consideration, namely in the absence of a soluble 



