252 RECENT IMPROVEMENTS IN PHOTOGRAPHY. 
1st. Wash a piece of the iodized paper with the gallo-nitrate; 
expose it to daylight for a second or two, and then withdraw 
it. The paper will soon begin to darken spontaneously, and 
will grow quite black. 
2d. The same as before, but let the paper be warmed. 
The blackening will be more rapid in consequence of the 
warmth. 
3d. Put a large drop of the gallo-nitrate on one part of 
the paper, and moisten another part of it more sparingly, then 
leave it exposed to a very faint daylight ; it will be found 
that the lesser quantity produces the greater effect in darken- 
ing the paper, and in general it will be seen that the most 
rapid darkening takes place at the moment when the paper 
becomes nearly dry ; also, if only a portion of the paper is 
moistened, it will be observed that the edges or boundaries 
of the moistened part are more acted on by the light than 
any other part of the surface. 
4th. If the paper, after being moistened with the gallo-ni- 
trate, is washed with water, and dried, a slight exposure to 
daylight no longer suffices to produce so much discoloration ; 
indeed, it often produces none at all. But by subsequently 
washing it again with the gallo-nitrate, and warming it, the 
same degree of discoloration is developed as in the other 
cases (experiments 1 and 2.) The dry paper appears, there- 
fore, to be equal or superior in sensitiveness to the moist : 
only with this difference, that it receives a virtual instead of 
an actual impression from the light, which it requires a subse- 
quent process to develope. 
