98 M. C. Lea—Sensitiveness of Silver Haloids. 
acid) because the former is perfectly neutral to test paper, and 
- cannot confuse the reaction as would be the case with tannin, 
which of itself reddens blue litmus paper]. 
Again: if the Poitevin-Vogel theory were correct, it would 
afford a criterion as to what substances should, and what should 
not, exalt the sensitiveness of the silver haloids. Substances 
aving an affinity for iodine should increase the sensitiveness, 
and substance not possessing such affinity, should have no such 
action. This is not the fact. There are substances which ex- 
the iodide to darken.. 
8. Starch, which has a well-known affinity for iodine, does 
not appear to increase the sensitiveness of silver iodide by con- 
tact with it. 
a strong support to the view that I have here expressed. 
I conclude, therefore, that such organic bodies as increas? 
