ee Be) 
pom v. — Observations on haere processes ; py Wasa 
wk ‘gee “SF oCnanninde ts 
In the apiansdeericic preparations of silver, the agency oak light 
is confined to a change in the arrangement of the particles, or to 
a partial decomposition. As in galvanic decomposition; it is as- 
sisted by coinciding chemical affinities, and these may: be brought 
to act-on the salt of silver either in the preparation before expo- 
sure to light, or afterwards to bring out the impression. In the 
Daguerreotype, mercury seems to act mechanically in pi 
apparent the molecular change wrought in the iodide of silver by. 
the action of light. In the recent photographic processes of 'Fal- 
bot, however, a chemical agent is used subsequently to complete 
the-decomposition which light had begun. 
» Avsensitive preparation of silver is one in which “the elements 
are so delicately balanced as to be overturned by the slightest 
cause.”* The fulminating compounds of silyer are examples of 
such a composition and may form hereafter preparations very sen- 
sitive to light. None of the more simple salts of silver are imme- 
diately reduced by light, though it may afterwards be made appa~ 
rent by chemical. means that they have undergone a change of 
structure or that a subsalt has been formed. Thus chloride of 
silver is apparently insensible to light, unless, sang organic 
matter containing 2 
| tion: Piet cnkomsihipanl “the oxide of silver im many of its 
oxy-salts. Electro-positive bodies therefore, which are associated 
with the salt of silver; conti caataiha ly. to decompose it 
by removing the electro-negative rings ode with the 
silver, while an excess of electro-negative elements, oxygen, 
ehlorine, &c. as in the perchlorate, chlorate‘and hypochlorate of 
silver; retards decomposition. ‘Thus compounds such as phos- 
phorus, tannic, gallic, crenic; fulminic acids, which may be called 
agents, all absorbing oxygen, and also another series 
actirig*mechanically, such as mereury, deserve attention, first in 
— a a preparation, and second, in bringing out 
apf Sento anny. 
Vol. XLII, No. . 1 April-June, 1842. 10 
