sa 
198 M. C. Lea on the action of Light upon Iodid of Silver. 
prea that amount crisis to the sulphuric acid fan 
by analysis was deducted and the remainder set down as sul- 
bear The carbonic acid was determined by adding weighed 
amounts of the water to chlorid of calcium that had been freed 
previously from carbonic aci 
The ammonia was determined by Boussingault’s method. 
general, the methods given in Fresenius’s quantitative Re = 
were followed in the several determinations. 
Art. XXIX.—On the Nature of the Action of Light upon Iodid 
of Silver; by M. Carey Lea, Philadelphia.’ 
Muc# difference = ra teak has long existed in respect to the 
explanation of certain phenomena of photographic action. In 
the vast majority of eases the action of light is a reducing one. 
Salts of iron, of uranium, and of other metals are reduced from 
a higher to a lower stage of oxydation, and the same is the case 
with the combinations of certain metallic acids, such as bichro- 
mates and ee These phenomena present n o difficulty. 
It is oy en we come to the silver haloids that obscurity 
comm 
It is Peacrally held, and there seems no reason to doubt it, i 
that chlorid and bromid of silver undergo reduction when ex- te 
posed to light. I shall therefore pass over anes compounds, : 
and discuss only the action of light upon the i 
n respect to this, two opposite opinions saat divided those 
are who have seriously occupied themselves with the sub- 
Some believe the action of light on the iodid to be purely 
physic antes hold it to be connected with an absolute chemi- 
cal change; some again holding this chemical change to be a 
reduction to a sub- iodid, others to metallic silver 
ed 
to light in the presence of free nitrate of silver, it undergoes re- 
uction. An examination of this reduced substance showed that 
it still contained iodine; when treated with nitric acid, a solu- 
tion of nitrate of silver was obtained, with a product of yellow 
1 Tn the following series of investigations it has been the object me the writer to 
to fix, with greater exactness, the obscure chemical and p' phenom- 
pty oa ge Rifle Mgt een 
pu in the journals devoted sugges” 
ted to him to make a brief aro f of these studies in a essentially chemical 
and physical relations 
