lat 
i 
M. C. Lea on the action of Light upon Iodid of Silver. 199 
iodid of silver. If the iodid of silver, after exposure to light 
in the presence of free nitrate, is carefully washed, the free ni- 
of course dissolve who olly i in nitric acid. But as just ‘said, this 
it does not do, but leaves behind yellow iodid of silver perfectly 
soluble in hyposulphite of soda. It is clear, therefore, that iodid 
of silver is reduced b ors reese = light, when free nitrate of 
silver is present, to acbdiod sub-iodid is pares Py 
nitric acid into nitrate of — tie iodid a2 oe And I 
found the same to be the case when tan zn is subsite 
for free nitrate of silver, though the actibd’ is areniy slower: 
in the former case a sub-iodid is formed. 
There is a question, however, far more difficult than these, to 
answer, and it is this: Does reduction of some sort invaria ly 
accompany the action of light upon iodid of silver? Is, or is 
not that action, in its essence a chemical action 
Before proceeding to investigate that question, another pre- 
sents itself, demanding solution. It had been long held as an 
indisputable fact, which none had attempted to controvert, that 
perfectly pure iodid of silver was insensitive to light, and that 
sensitiveness only a peared when free nitrate of silver, tannic 
acid, or other “sensitizer” was presen 
I soon satisfied myself that this asserted fact (for the we gee 
= of which long discussions had taken place) had ist- 
nee whatever, and that t pure iodid of silver was ane mk 
tive to light. The long series of experiments, made with the 
out difteatay The demonstration was so convin 
had the pleasure of seeing those who were the most janes sup- 
Porters of the old view, abandon it entirely. 
x return then to the main question, which is: Deus chemieak 
company the production of - im- 
r Ssion upon iodid of yn? In my opinion 2 does not. I 
old that: 1. When perfectly pure iodid of silver, rool J is 
exposed to light, it receives a physical impression only. 2. Bat 
that when certain other pean for se nitrate of sil- 
ver, tannic acid, and perhaps many others, are , then 3 
ical action, a reduction, a or may, take 
