76 Observations on Photographic Processes. 



on paper is soon decomposed, so that gallic acid discolors it. The 

 iodized paper will keep a long 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. By 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. 



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. 



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. 



Boston, February 28, 1842. 



* Coraptes Rendus, February, 1841. 



