M. C. Lea— Development of the Latent Photographic Image. 481 
least not to any useful extent, it becomes necessary to find an 
appropriate solvent. The most available solvents are solu- 
tions of ammonium and potassium 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 tartrate they are apt 
gradually to be precipitated. 
errous oxalate is a powerful Spas pal the question 
immediately presented itself whether the developing Dome 
exhibited, for instance by ferrous oe dissolved in 
monium oxalate might not be due to the formation of ferrous 
chi But several reactions contradict this supposition. 
cooling, and this aie net is not ferrous oxalate but ferrous 
So 
as I find that ferrous tarthate has itself aereneie 8 properties. 
But as ferrous phosphate is to some extent soluble in a solu- 
tion of ferrous sulphate, and as ferrous sulphate (in the form 
of development here under consideration, namely: in the 
absence of soluble silver salt) is wholly without develop- 
And it proved that a solution obtained by adding one of 
disodic aot al to one of ferrous sulphate until a permanent 
precipitate began to form, undoubtedly possessed developing 
powers, though in a less degree. 
The number of ferrous salts capable of developing the latent 
image is very considerable. Singular anomalies are often 
compounds n nearly allied do not exhibit ve in this 
respect. For example: ferrous phosphate and ferrous meta- 
ue el are active developers, while ferrous pyrophosphate 
as no similar power. 
Among other tavous salts possessing more or less develop- 
ing power, may be mentioned ferrous hyposulphite (hydrosul- 
phate), ammonio- eu a acetate, antimonio-tartrate, etc. Fer- 
rous formiate, which might naturally be expected to be a 
powerful develo er, is ie though not entirely, destitute a 
the property. he most active agents found were ferrous 
rate, phosphate, fatphite and oxalate, respectively dissolved, 
the phosphate in neutral ammonium oxalate, the others in 
neutral potassium oxalate, 
May, 1880. 
