362 M. C. Lea—Chloride, Bromide and Lodide of Silver. 
from dull dark gray to crimson) is due to three concurrent 
actions: that of the mere heat, the removal of the silver, and 
the breaking up of uncombined subchloride. 
It is not possible to dissolve out the normal chloride by a 
solvent like ammonium chloride from the photochlorides, leav- 
ing the subchloride behind. When red chloride is boiled with 
successive portions of strong solution of ammonium chloride in 
large excess, the material gradually diminishes until, if the 
operation is continued long enough, there remains a small resi- 
due of a warm gray color, which consists of metallic silver and 
dissolves without residue in nitric acid. 
If sodic chloride is substituted for ammonium chloride, the 
same result follows, except that the operation is greatly more 
tedious. If persevered in until the hot solution no longer 
removes traces of silver chloride, the residue consists of noth- 
ing but metallic silver. 
Action of Light on Normal Silver Chloride. 
When silver chloride precipitated with excess of HCl is 
exposed to light, it becomes with time very dark. Cold, strong, 
nitric acid 1°36 sp. gr. extracts a trace only of silver. 
The principal action of light on AgCl (precipitated in pres- 
ence of excess HCl) consists in the formation of a small quan- 
tity of subchloride which enters into combination with the 
white chloride not acted upon, forming the photochloride, and 
thus is able to withstand the action of strong nitric acid. At 
the same time a trace is formed, either of metallic silver or of 
uncombined subchloride, it is impossible to say which. After 
a certain very moderate quantity of photochloride is formed, 
the action of light seems to cease. This cessation has been 
noted by many observers, perhaps most exactly by Dr. Spen- 
cer Newbury. 
The nature of the product formed by the continued action 
of light on silver chloride seems to support the conclusion that 
the subchloride is combined with the whole of the normal 
chloride after the manner of lakes rather than in equivalent 
proportions. If the latter were the case it seems probable that 
the continued action of light would extend to much greater 
decomposition than it is found to do. 
The action of light in the formation of the so-called latent 
image will be examined in the second part of this paper. 
PHOTOBROMIDE AND PHOTOIODIDE. 
It has been already mentioned that bromine and iodine form 
with silver combinations in all respects analogous to those of 
chlorine. A more particular account of the bromine and iodine 
