192 On the Composition of the Photographic Image. 
usually employed in the practice of photography. The resulting 
darkened surfaces were found to be capable of restoration by 
renewed exposure to gaseous chlorine. These experiments were 
pursued no further, as they did not appear to present so close a 
parallel to the ordinary application of chloride of silver as the 
system of preparation already described. 
From a general examination of the products obtained by the 
action of light upon several of the more definite compounds of 
silver, it appeared to me that the oxalate would be likely to pre- 
sent the most indubitable evidence of reduction to the metallic 
state, and from its ready decomposability be well fitted for com- 
parison with the results afforded by the examination of the 
altered chloride. Some oxalate of silver was therefore prepared 
by precipitation from nitrate of silver and oxalate of ammonia 
(the former‘in slight excess), washed by decantation, and exposed 
under pure distilled water to the direct rays of the sun. The 
white oxalate soon changed colour, becoming reddish brown, 
and was seen to evolve small gas bubbles, which proved to be 
carbonic acid by the water having now the property of precipi- 
tating basic acetate of lead, producing a milkiness easily soluble 
in acetic acid. That szlver was, on the other hand, the product 
of reduction, became evident on repeating the experiment with 
a more finely divided condition of the substance, and by re- 
moving the large proportion of unaltered material, which in this 
instance also remained in admixture with the blackened parti- 
cles. The same transition of colour from dark purple to grey, 
attended the withdrawal of the undecomposed oxalate by dilute 
hyposulphite of soda solution, and the metal was left in a state 
of purity. Hence the decomposition will be expressed by 
AgO, C2? 08=Ag +2C02, 
Passing in review the results obtained in the foregoing expe- 
riments, it will probably be considered that the weight of evi- 
dence tends to show that the metal is the ordinary product of 
the chemical action of light upon chloride of silver; and that 
the principal difficulty which has stood in the way of accepting 
this conclusion has in a great measure to be accounted for by the 
often varying shades of colour presented by the reduced metal, 
and more especially the transition observed at the moment of 
removing the unaltered portion of material by the application of 
the fixing agent. If in these several stages the change in phy- 
sical condition be considered in its proper connexion, and due 
allowance be made for the very important influence known to be 
exercised over the light-reflecting capacity of these minutely 
divided particles by very slight modifications in their state of 
aggregation (quite irrespective of change in chemical constitu- 
