( 741 ) 
formed, which with subiodide gets into a state of photo-chemical 
equilibrium, which is nearer to silver iodide than that of subiodide 
and adsorbed iodine. The sensitiveness of potassium iodide, by which 
iodine is photo-chemically liberated, may also have some influence. 
If silver iodide with an excess of silver nitrate -be precipitated by 
potassium iodide, it is free from potassium iodide. The photo-chemical 
decomposition now proceeds much further; a greyish green colour 
sets in, which points to a high amount of «-subiodide. This seems 
at variance with the observations of Hrrscnen’s effect, and the 
solarisation in the case of the silver iodide daguerreotype plate, where 
with.much shorter exposures a decrease of reducibleness undoubtedly 
sets in, owing to the photo-chemical splitting up of «-subiodide into 
B-subiodide and iodine. This contradiction is solved, if it is borne in 
mind that, to render the exposed silver haloid reducible, the presence 
of the «-subhaloid alone is not sufficient, but that, moreover, it must 
be at the surface of the silver haloid, so that through molecular 
attraction the reduction products of the dissolved silver salt or the 
mercury vapour may be able to settle on it. The greyish green 
discoloration may, therefore, be attributable to a high amount of 
a-subiodide, but it may itself be covered by an extremely thin layer 
of subiodide poorer in halogen. 
If we endeavour to remove the halogen pressure of iodine altogether 
by conducting the photo-chemical decomposition under a silver nitrate 
solution, then the reductions become very complicated. The acceleration 
of the visible photo-chemical decomposition appears to be hardly 
appreciable, while the substance assumes a grey colour, and yet in 
the silver iodide collodion process silver nitrate behaves as an excel- 
lent chemical sensitizer. On a former occasion **) I already pointed 
out that im the light silver nitrate must exercise a strongly oxidizing 
influence upon subiodide, silver oxide being formed, and accounted 
for the more rapid increase in the reducibleness (i.e. more rapid rise 
of the amount of a-subiodide) by this oxidation, which can act only 
then in such a way that d-subiodide is more rapidly oxidized into 
a-subiodide than the latter into silver iodide. Consequently when 
silver iodide in a silver nitrate solution is exposed, two opposite 
actions take place: a progressive reaction, the photo-chemical decom- 
position, and a regressive reaction, the oxidation, the former taking 
place a little more rapidly than the latter, which reactions probably 
even with the subiodides poorer in halogen perfectly neutralize each 
other. With oxidation, however, silver oxide is separated out; I 
therefore surmise that the grey discoloration mentioned before must 
be ascribed to this silver oxide. 
