M. C. Lea—Photobromide and Photortodide of Silver. 491 
ment, the contrast between its action on the normal haloids 
and on the photosalts is very striking. Ifa rose-colored photo- 
bromide and some normal bromide are. exposed side by side 
the normal bromide (formed in presence of excess of alkaline 
bromide) darkens but very slowly while the photobromide is 
quickly acted on. I have seen deep purple photobromide 
change to brown all over its surface with less than five minutes’ 
exposure to diffuse light in the middle of a room, an amount 
of exposure which would scarcely have produced a visible 
effect on normal bromide formed in presence of excess of KBr. 
When photobromide is exposed to the spectrum, it shows 
the same difference in sensitiveness and darkens more rapidly 
than photochloride. But it gives little indication of color. 
When exposed under colored glass photobromide gave dis- 
tinct indications of reproducing colors, but much less favorably 
than photochloride. Under green glass it became bluish, under 
blue greenish, under yellow glass it bleached and under red 
glass the red of the photobromide remained unchanged. 
Photoiodide of Silver. 
The most characteristic color of this substance seems to be a 
fine rich purple. It is obtained in much the same way as the 
corresponding chloride and bromide, with this limitation, that 
an excess of alkaline haloid must not be present, as the photo- 
iodide is quickly destroyed by it. The iodine salt differs much 
more from the bromide than does the bromide from .the 
chloride and two striking distinctions are, its easy decomposi- 
tion by its own alkaline salt, and its action with ammonia, as 
will appear beyond. 
A very easy and satisfactory method of obtaining the photo- 
iodide is the following: Silver is to be reduced from the nitrate 
or chloride, in fine powder in any convenient way; I have usu- 
ally employed Levol’s method. Toa solution of potash, iodine 
is to be added until the liquid becomes almost black. This 
iodine.solution is to be diluted and poured over the silver by 
degrees, keeping the silver constantly agitated, until the whole 
mass becomes clear bright purple. 
Any excess of silver present may be removed by boiling 
with dilute nitric acid, but this operation requires far more 
circumspection than in the case of the corresponding chlorine 
and bromine products. The acid (1°36 was used) must be 
diluted with twelve or fifteen times its bulk of water, and the 
boiling must be very short, otherwise the photoiodide is con- 
verted into normal yellow iodide. 
Another method by which it may be obtained is to add 
ammonia to silver nitrate in considerable excess and to pour 
