( 730 ) 
Chemistry. — “A contribution to the photo-chemistry of silver (sub-) 
haloids’. By Mr. A. P. H. Trivets. (Communicated by 
Prof. 5. HooGewerFr). 
(Communicated in the meeting of January 30, 1909). 
Introduction. 
Previous investigations into the photo-chemical decomposition of 
silver haloids led to the following formula: 
2 AgHal — Ag,Hal — 2 Ag. 
It is true the great variability in the chemical composition of the 
photo-chemically formed subhaloid has given rise to doubts as to the 
correctness of the formula Ag,Hal, so that J. M. Epmr') even thinks 
it possible that the subhaloid has the formula Ag,Hal,, Ag,Hal, or 
perhaps Ag,,Hal,,, but a satisfactory explanation of these deviations 
was found by Gunrz*) in the partial and only superficial photo- 
chemical decomposition, so that when quantitative determinations 
were made, the subhaloid dealt with was never pure, but always 
contaminated with AgHal and Ag. It is a faet that with continued 
light action the photo-chemical decomposition of the silver chloride 
never reaches the formula Ag,Cl, but approaches it *). Thus Ricur *) 
found, after silver chloride had been exposed to the action of light 
for 1'/, years, a composition corresponding to the formula Ag,Cl,. 
In 1895 ©. Wierer *) discovered the colour formation of the 
Seebeck-Poitevin photo-chromics by mechanical colour accommodation, 
which can only set in, if there are a number of mutually different 
subhaloids. E. Bauer ®) took these to be four “modifications” of one 
and the same subhaloid. A little more than a year afterwards 
J. M. Eper?) in his excellent investigations as to the substance of 
the latent image came to the conclusion that there exist a number 
of subhaloids, which consecutively originate photo-chemically one 
from another, react differentiy on developers, sodium thiosulphate, 
ammonia, ete, and are formed anew by oxidation (nitrie acid) in 
the opposite order (as compared with the photo-chemical deeompositon). 
These results, however, require a slight correction, because J. M. 
Eper started from the idea then still generally held, but now anti- 
quated, that the process of development was nothing but a simple 
reduction of the unstable subhaloid-bearing silver haloid grain, whereas 
according to the more recent view, enunciated by W. Osrwarp 5), 
K. Scuaum and W, Braun’), and confirmed microscopically by 
