490 M. C. Lea—Photobromide and Photoiodide of Silver. 
may be dissolved in ammonia and be treated first with ferrous 
sulphate and then with dilute sulphuric acid. This method, 
which is very good with the chloride, is less available for the 
bromide because of the less solubility of the normal bromide in 
ammonia, so that although the product is good it is small in 
quantity. 
Very good results are obtained by dissolving silver phos- 
phate, nitrate and probably almost any other salt of silver in 
ammonia, adding ferrous sulphate and after two or three min- 
utes, hydrobromic acid. 
Potash bromide and cupric sulphate may be made to act on 
metallic silver in fine powder, but the product is contaminated 
with much copper, difficult to get rid of. 
When AgBr is treated with sodium hypophosphite a brown 
or brownish purple form of photobromide is obtained which 
seems to be more easily decomposed by nitric acid than most 
other forms of this substance. 
Potash or soda with oxidable organic substances, made to 
react on silver nitrate and then treated with HBr gives the 
photobromide. With soda and milk sugar or aldehyde a 
rose-colored or pink product is generally obtained. 
Reactions.—In strong solution of potassic iodide it dissolves 
and this solution by dilution lets fall pale yellow normal bro- 
mide. With a weaker solution it becomes somewhat lighter in 
color. 
With acid ferric sulphate there is no action in the cold, but 
with a few minutes boiling the photobromide is converted into 
bright colored normal bromide. 
In sodium hyposulphite it dissolves, leaving a little black 
residue of silver. 
With ammonia the action at first seems slower than is the 
case of the corresponding chloride and if the ammonia is poured 
over the photobromide in small quantity, it may seem to be 
without effect. But the photobromide shaken well up in a 
test tube with a large excess of ammonia is almost instantly 
blackened. 
Exposed to light, the red shades of photobromide darken 
with great rapidity. Placed along side of the corresponding 
chloride, the contrast is very striking:—a strong image forms 
on the bromide long before anything appears on the chloride; 
nevertheless, if these two films are thrown into a developing 
solution, the chloride on which nothing could be seen, gives a 
vigorous positive development, while the bromide which already 
showed a strong positive image develops a negative one. The 
details of this reverse development have been already con- 
sidered in another paper. 
As respects the direct effect of light unaided by develop- 
