190 Mr. J. Spiller on the Composition of 
metallic residue, after repeatedly washing with dilute iodide of 
potassium solution and finally with water, was found to consist 
of silver without any admixture either of chlorine or iodine. 
Nitric acid in a cold and diluted form was imactive ; but more 
concentrated acid effected the removal of the reduced silver by 
converting it into nitrate (with evolution of red nitrous fumes), 
and left insoluble the white chloride of silver; ammonia then 
added dissolved completely this latter substance. 
On the other hand, the darkened product was reconyerted 
into its original white condition, with varying degrees of rapidity, 
by treatment with chlorine-water, nitro-hydrochloric acid, the 
brown solution of bichloride of manganese, and by a mixture of 
hydrochloric acid and chlorate of potassa. An acid solution of 
the green chloride of copper had also the power of reconverting, 
although more slowly, the darkened chloride into its primitive 
condition ; and a similar change appeared to be brought about 
by digesting in a cold saturated solution of chloride of mereury, 
but in this instance the conversion was attended with a reduc- 
tion of the mercury salt to the state of subchloride, so that a 
black’ residue, derived from the calomel, remained on afterwards 
attempting to dissolve the chloride of silver in ammonia. 
It was in the next place thought desirable to prepare for com- 
parison a sample of altered chloride which had not been so fully 
acted upon by the light, and to restrict the excess of nitrate of 
silver employed, in order to ascertain whether at an earlier stage 
a more partial reduction, attended with the formation of an in- 
ferior chloride, could possibly occur, On a cloudy day in Sep- 
tember 1857, a purple product was obtained, which differed from 
the former samples by containing a much larger proportion of 
unchanged chloride ; and in consequence of the more marked 
physical change in the state of aggregation of the particles 
attending the removal of this larger quantity of unaltered matter, 
the colour of the substance exhibited a more striking transition 
from purple to grey on treatment with hyposulphite of soda and 
other solvents already enumerated. Neither in this instance was 
any chlorine detected in the insoluble residual portion, nor evi- 
dence furnished of its having been removed from a state of weak 
chemical combination. 
It will be perceived that the results now recorded bear refer- 
ence to a series of experiments from which the interfering in- 
fluences of organic matter, and all other chemical agents, ex- 
cepting only water and the nitrates of silver and soda, have been 
intentionally excluded. The motive for such a course rests on 
the belief that the full and accurate determination of the action 
of light i its simplest phase must precede other considerations 
likely to inyolve secondary and more complex reactions, which 
