492. M. C. Lea—Photobromide and Photoiodide of Silver. 
this into solution of ferrous sulphate. Potassie iodide is dis- 
solved in very dilute sulphuric acid and added till the mixture 
has a sharp acid reaction. It is necessary to observe that. the 
KI added must be somewhat short of a proportion equivalent 
to that of the silver. Any excess of silver may be removed in 
the manner already explained. 
Different specimens of the purple product in this way ob- 
tained gave various amounts of Ag,I from 0°64 per cent to 
4-638. The same remark made as to the meaning of these deter- 
minations in reference to the other haloids, applies to this. 
The method of roasting silver oxide until it is black and 
acting on it with the hydrogen acid of the halogen, which 
works well in the case of the chloride, does not answer well for 
the iodide. : 
When silver iodide is boiled with solution of sodium hypo- 
phosphite, it gives a brown product, evidently indicating that 
reduction to some extent has taken place; the hypophosphite 
solution may or may not show traces of iodine. The color of 
the silver iodide may show a very marked darkening, and yet 
the solution may give no trace of iodine by the most delicate 
reagents. 
This was very difficult to explain until I found that silver 
iodide has the property of taking up and retaining small por- 
tions of iodine, a reaction not very surprising in view of the 
tendency I have found in silver baloids to take up foreign sub- 
stances of very various natures, and also of the facility with 
which iodine is taken up by alkaline iodides. This property 
in silver iodide was verified by shaking up portions of freshly 
precipitated and still moist AgI with iodine solutions. Alco- 
holic solution of iodine diluted until it has a pale sherry wine 
color is quickly decolorized by AgI, and the same thing hap- 
pens with a very dilute solution of iodine in KI, which ina 
few minutes becomes as colorless as water. 
This reaction I found particularly interesting, for it not only 
explained the action of hypophosphite in the case just men- 
tioned, but also gave a clue to the cause of a phenomenon I 
observed more than twenty years ago, and which then and 
long afterwards seemed to me an unanswerable argument in 
favor of the physical nature of the latent image. 
At the time referred to I formed films of pure silver iodide 
entirely isolated from foreign matter, by reducing metallic sil- 
ver on plates of ground glass, iodizing them with alcoholic 
solution of iodine, or with Lugol’s solution, then washing most 
thoroughly under a tap for hours. When these films of silver 
iodide were exposed to light, they received an invisible image 
which could be developed. But these invisible images, if the 
plates were laid aside in the dark, had the property of fading 
