the Photographic Image. 187 
becoming therefore reduced to the state of metal ; while according 
to a second view, the progress of this reducing action is limited 
to an intermediate stage, whereby a compound is produced con- 
taining less chlorine by one-half than the original substance, 
and to which the name and formula, subchloride of silver 
(Ag? Cl), have been applied. As a contribution towards a fuller 
explanation of the chemical changes involved, I beg to submit 
the following results of a series of experiments, made at inter- 
vals of leisure during the summers of 1857, 1858, and 1859, 
which would appear strongly to favour the first-mentioned hy- 
pothesis. 
Preliminary experiments upon the freshly precipitated chlo- 
ride of silver, as ordinarily prepared, having demonstrated the 
difficulty of effecting more than a mere superficial decomposition 
by exposure to sunlight, a process of preparation was adopted 
whereby an exceedingly finely divided condition of the substance 
was ensured, and its exposure conducted under circumstances 
favourable to its thorough decomposition. For this purpose 
highly dilute solutions were prepared, both of nitrate of silver 
and chloride of sodium, in proportions so adjusted that equal 
bulks represented amounts of chlorine and of silver in the ratio 
of their chemical equivalents. 
(5°85 grains of pure rock-salt, on the one hand, and 17 grains 
of fused and neutral nitrate of silver, were dissolyed each in 
two gallons of pure distilled water.) 
When equal measures of these solutions were mixed in an 
obscurely illuminated apartment, the white chloride of silver 
was precipitated in a form so finely divided that an opalescence 
only, without visible particles, was at first apparent. By dif- 
fused daylight this became quickly darkened, and in the course 
of time subsided into a very small purple-grey deposit. But in 
order to ensure full decomposition, it was the general practice to 
employ the silver solution in excess and to add the salt water 
under the full action of sunshine, the liquid being contained in 
three, and sometimes four, pale glass flasks, each of nearly two 
gallons capacity, which were placed on the roof of one of the 
buildings in the Royal Arsenal, Woolwich, and in such a posi- 
tion that the solar rays had uninterrupted access to their con- 
tents, almost from sunrise to sunset. Under favourable circum- 
stances it was then frequently impossible to observe the formation 
of the white chloride of silver on mixing the two solutions, so 
rapidly was it converted into the dark coloured product. At the 
expiration of the day’s action the small precipitate had become 
completely darkened, and in the same time had subsided, so that 
on the following morning the supernatant liquid could be drawn 
off through a siphon, and a fresh charge introduced, the pro- 
