M. C. Lea—Sensitiveness of Silver Haloids. 97 
sitiveness followed. After a time it was noticed that certain 
: : : 
appeared to take, to some extent, the place of the silver salt ° 
removed, and that the silver haloid was distinctly more sensitive 
when in contact, for example, with tannin, than when isolated. 
In explanation of the fact, a theory was published by M. 
. 
lodine came into play in aid of the action of light. Shortly 
after, Dr. H. Vogel published the same theory. 
light in the presence of tannin, because the affinity of tannin for 
the other side to combat, or experiments to discuss, as I can 
find none. I shall, therefore, state what I conceive to be the 
true p pacstion, and endeavor to support it adequately by 
proof. 
of increasing the sensitiveness of the silver haloids, we notice 
that the one property which they possess in common is, not an 
affinity for the halogen, as the Poitevin—Vogel theory would 
ead us to expect, but that they are ail reducing agents. 
Betty of pyrogallol was added and the mixture was expose 
Sunlight for fifteen minutes in contact with water. The 
istinetly acid reaction, increasing with continued exposu 
