850 IL. C. Lea—Chloride, Bromide and Lodide of Silver. 
According to the one, the first effect produced by light is 
simply a physical change, predisposing the elements of the 
silver haloid to dissociation, so that when a reducing agent is 
applied, the molecules so affected, yield more quickly to its 
influence. According to the other theory, the invisible image 
is formed of a sub-salt (sub-chloride, ete.). Observations 
which I published many years ago led me strongly to the first- 
mentioned of these theories. But of late years, results have 
been obtained not easily reconcilable with it. On the other 
hand, the theory that the latent image is formed of subsalt is 
opposed to striking facts. Silver subchloride for example is 
an unstable substance, quickly destroyed by dilute nitric acid. 
But I have formed a latent image on silver chloride and after 
exposing it for five minutes to the action of strorg nitric acid 
(sp. gr. 1°36) have developed the image without difficulty: the 
same with silver bromide. Evidently these images, which so 
strongly resisted the action of undiluted acid could not be 
formed of simple subchloride and subbromide of silver, sub 
stances quickly destroyed by it. 
In the desire to find a satisfactory explanation of the nature 
of the image based on adequate chemical proof, I have devoted 
nearly three years of laboratory work to this and to closely 
allied subjects. JI am led to the conclusion that neither of the 
older views is correct. A truer theory seems to be deducible 
from the result of some experiments which I published in 1885, 
to the effect that the silver haloids were capable of uniting with 
certain other substances, much in the same way that alumina 
forms lakes. When a silver haloid was precipitated in the 
presence of certain coloring matters they combined with it, and 
though soluble in water, they could not be subsequently 
washed out. They had formed a somewhat stable compound, 
although the proportion of coloring matter was very small in 
comparison with the haloid; evidently much too small to 
represent a stoichiometrical composition. 
Now'I find that a silver haloid may in the same way, unite 
with a certain proportion of its own subsalt, which by this 
union quite loses its characteristic instability and forms a com- 
pound of great permanence. 
Another explanation is possible: the subsalt may combine 
with the normal salt, not in the manner above described, but 
in stoichiometrical proportion, and this compound may be dif- 
fused through ordinary silver haloid. I have not been able to 
find any reaction decisive between these explanations,* but the 
general behavior of the substance seems rather to indicate the 
* Silver chloride may be dissolved out by hot solutions of sodium or ammonium 
chloride, but the subchloride is at the same time decomposed. See beyond under 
head of ‘ Reactions.” 
