ie Observations on Photographic Processes. 
on paper is soon decomposed, so that gallic acid discolors it. The 
iodized paper will keep along while, unless there is much excess 
of nitrate of silver. It is best, however, to use it soon after it is 
made. ‘ 
It is important to the sensibility of all these preparations, that 
the nitrate of silver solution*should be neutral, have no excess of 
acid, the electro-negative ingredient; and also that there should 
be no excess of iodide of potassium, another electro-negative ele- 
ment, on the paper, but rather the reverse. 'The brushes used 
should be made without metal, and no metal should touch the 
paper during its preparation. All the parts of the operation should 
be kept distinct.to ensure a perfect result. : 
_ The analogy in these processes to the Daguerreotype is very 
interesting, iodide of silver being brought out in both cases after — 
the action of light. Though the Daguerreotype cannot be brought 
out by gallic acid, yet Balard noticed a year ago* that the vapor 
of mercury would bring out an impression on. prepared paper. It 
is curious to hold one half of a piece of iodized paper, which has 
received an impression over mercury heated in a capsule, and 
wash the other half with gallic acid. The first brings out a faint 
negative sketch, the last a strong negative picture. © preparing 
a black carbonaceous paper with iodide of silver, the vapor of 
mercury brings out a sort of positive picture which may deserve 
some attention, > pi 
The solution of galls. seems to furnish much that is wanted in. 
a secondary reducing agent. - Still the field is open for inquiry. 
A solution of common tea, from its tannic acid, brings out an im- 
pression imperfectly well. Crenic acid, or a solution of peat, which 
also abstracts oxygen during its decomposition, seems also to have 
this property. Migs nu chy 
There is no science which is now advancing so rapidly as pho» 
tography. These processes will soon undoubtedly be supersed- 
ed; they may, however, be of interest and use. at the present 
moment. mes: si ae ee 
muary 28,1842... 
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