480 M. C. Lea—Development of the Latent Photographic Image. 
ArT. LXVIL—On Substances possessing the Power of Developing 
the Latent Photographic Image; by M. Carry Lea, Phila- 
del phia. 
Axout three years since, I communicated to this Journal the 
results of a long series of studies on development. At the time 
when these were undertaken there were but four substances 
known to possess the power of development: ferrous sulphate, 
gallic acid, and pyrogallol, which had been long known to 
have this property, and heematoxyline, which [ had some years 
before added to the number 
The studies made three years ago prove that the power of 
development, so far from being possessed by this small number 
of substances only, extends to a large number of chemical 
compounds, and is exhibited by many cuprous salts, by 
several vegetable acids, glucosides, etc. But the most curious 
result obtained was with ferrous salts. It was known that 
ferrous sulphate, though a powerful developer in the so-called 
“ wet devaldnmeat,” i. e., development 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 develop- 
ment was to be made at the expense of the film itself. I was 
able to show that ferrous oxide combined with almost any 
organic acid, possessed this power of forming a visible image 
at the expense of the film. So that a solution of ferrous sul- 
hate by mixing with one of an alkaline oxalate, succinate, 
salicylate, etc., 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 develop- 
ing was not limited to the organic salts of ferrous oxide, but 
was possessed by many of its inorganic compounds. It. cer- 
tainly has never been suspected that such compounds as ferrous 
phosphate, ferrous borate, ferrous sulphite, ferrous hypophosphite, 
etc., possessed the power of development, but this they un- 
doubtedly do, and not in any uncertain way. On the contrary, — 
some of these compounds are among the most powerful of all 
known developing agents, equalling, or possibly even exceed 
ing, ferrous oxalate in this respect, so that it is far from 1m- 
possible that some of them may pass into technical use 1 
preference to those now employed. 
ome of these ferrous salts, especially the phosphate, sul- 
phite and borate, are, like the oxalate, insoluble in water, and 
therefore need to be got into solution. As these salts are not, 
like the oxalate, soluble in the corresponding alkaline salt, at 
