a“ 
204 M.C. Lea on the action of Light upon Iodid of Silver. 
alls, imposes no more difficulty than the dissimilarity of the 
nature of the glass from that of the precipitate which is attached 
to those parts of the glass which have been pressed by the rod 
in stirring. 
In fact it may be truly said that whenever one body is pressed 
against another, the particles of the body pressed against tend 
to have their attraction for each other diminished, and their at- 
traction for external bodies, whether homogeneous or heteroge- 
neous, increased. 
I have endeavored in this brief review of a subject replete 
with difficulties, to show that the action of light upon pure iodid 
of silver isolated cannot be a chemical reduction: 
1st. Because that effect, even when carried many hundred 
thousand times further than in the ordinary ott Ba pro 
cesses, perfectly disappears in a few hours, spontaneously, under 
circumstances which render it impossible to suppose that jodine 
have been restored to replace that which (had reduction 
taken place) must have been disengaged. 
. Because, even where the action of light is prolonged many 
hundred thousand fold the ordinary time, no reduced silver nor 
sub-iodid can be detected as present. 
3d. I have shown that another metal, mercury, is capable of 
sil 
developing these images as well as silver. 
