( 740 ) 
with the subhaloids poorer in halogen, and with the exception of a 
single case mentioned below, it is ‘therefore unfit for controlling 
purposes. So the best plan is to start from silver haloid only, which 
owing to its white or light yellow colour has little or no influence 
upon the colour of the subhaloids that are formed. 
The binding substance. In most cases gelatine was used for this 
purpose. In a perfectly dry condition it does not absorb free halogen ; 
it does, however, in the presence of water. 
The chemical sensitizers. As it appeared desirable to me, in connec- 
tion with these experiments, to obtain as much as possible results 
that could Le compared with each other, I have made as little as 
possible use of different chemical sensitizers. Besides alkali-haloid 
and moist gelatine, silver nitrate was selected as being a powerful 
sensitizer. The photo-chemical reduction in the presence of gelatine 
has, as appeared from a parallel experiment without silver haloid, 
no disturbing influence upon the determination of the colours of the 
subhaloids. 
Description of the experiments. 
The silver subiodides. The cause why hitherto nobody has succeeded — 
in demonstrating the iodine liberated photo-chemically, is no doubt 
to be put down to the very unfavourable conditions of diffusion 
(high atomic weight and low vapour tension of iodine), and to 
absorption by silver iodide with the formation of an absorption 
compound **). Moreover the visible photo-chemical decomposition is 
always very slight, notwithstanding the high sensitiveness of silver 
(sub)iodide, to which I have already referred more than once, in 
other words, only very little iodine is liberated, which has been 
associated by H. Lueain**) with the low equilibrium potential of 
silver iodide. This makes silver iodide together with iodo-silver 
bromide, mentioned above, the most suitable silver haloids for 
daguerreotypy, in which the destruction of the «-subhaloïd, upon 
which the reducibleness depends, would otherwise be much stronger 
as compared with the emulsion processes. Consequently it is unneces- 
sary to emulsionize the silver iodide. 
If silver iodide with an excess of potassium iodide is precipitated 
from a silver nitrate solution, then a very slight discoloration appears 
on exposure*’), without a subiodide colour becoming observable. The 
cause is probably that during the precipitatien of silver iodide potas- 
sium iodide is separated out as well. Now in photo-chemical decom- 
position the liberated iodine is absorbed in a less degree by silver 
iodide, but largely by potassium iodide, potassium tri-iodide being 
