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MM. OC. Lea—Photobromide and Photoiodide of Silver. 489 
ArT. XLVIII—On Photobromide and Photoiodide of Silver ; 
by M. Carey Lea, Philadelphia. 
Photobromide of Silver. 
THIS substance is formed for the most part by the same reac- 
tions as the corresponding chlorine salt. 
It is not however to be supposed that all reddish-brown sub- 
stances resulting from the action of reducing agents on silver 
bromide are the photosalt. By reduction, AgBr may yield a 
brownish colored form of silver, which, mixed with unreduced 
AgBr may form a substance resembling the photosalt, but hav- 
ing none of its properties. The two are easily distinguished 
by the action of cold nitric acid which added to a brown mix- 
ture of AgBr and Ag quickly dissolves the silver leaving 
AgBr. On the photosalt it has no action. 
A beautiful variety of photobromide is easily obtained by 
dissolving silver nitrate in ammonia and adding it to ferrous 
sulphate previously mixed with solution of soda. Then KBr 
is added dissolved in dilute sulphuric acid, until the mixture 
has a strong acid reaction. 
Sometimes this method gives immediately a fine purple, 
sometimes a brown product. But in either case, after washing 
and cautiously heating with dilute nitric acid, a beautiful 
purple results. Much care is needed in the nitric acid treat- 
ment or particles of yellow bromide will form. 
A specimen obtained in this way gave figures indicating 
7-25 per cent of subbromide. Hach specimen however varies 
in composition, often very materially. 
I subsequently found it desirable in some degree to vary the 
method and to determine the best proportions in which the 
materials were used, to obtain a constant product. That which 
I prefer to use is as follows: 
Six grams of silver nitrate are to be dissolved in 200 centi- 
meters cube of water and ammonia added until the precipitated 
oxide re-dissolves easily. Twelve grams of ferrous sulphate 
are dissolved in 200 c.c. of water and the silver solution is 
poured into this. Then four grams pure caustic soda dissolved 
in 50 c¢. c. of water are added, let stand a few minutes, then 
five grams of KBr dissolved in a little water. Finally, dilute 
sulphuric acid until the whole has a strong acid reaction. 
This product, well washed and then heated cautiously with 
nitric acid 1°36 diluted with five times its bulk of water, gives 
photobromide of a shade of royal purple, extremely beautiful. 
Notwithstanding its fine color it proved to contain but little 
subbromide, not quite one per cent (0°98 p. c.). 
Various other methods may be employed. Silver bromide 
