74 Observations 07i Photographic Processes. 



an impression after the action of light. There are singular reac- 

 tions, however, constantly observed in photographic chemistry, 

 which cannot be comprehended or explained by any general 

 statement. 



In a recent process of Hunt,* a paper is first prepared with 

 iodide of silver and then washed with ferrocyanide of potassium 

 and used moist. The ferrocyanide contains carbon, potassium, 

 iron and nitrogen without oxygen. Mixed in powder with sub- 

 stances abounding in oxygen, such as nitrate or chlorate of potash, 

 it explodes by heat or percussion. In this case it is easy to see 

 that if water were present a slight cause would determine its 

 oxygen to the ferrocyanide and its hydrogen to the iodine of the 

 silver. Water in the form of water of crystallization, hygro- 

 metric moisture, or artificially applied, often thus performs an 

 important part in these preparations. The cyanide of potassium 

 is equally or more efficacious than the ferrocyanide. Besides the 

 inconvenience of using a moist paper, this is not so sensitive as 

 Talbot's calotype paper,f founded on his important discovery of 

 the properties of gallic acid. 



For this the paper is washed successively with nitrate of sil- 

 ver and iodide of potassium ; then before using with a mixture 

 of acetic acid, nitrate of silver, and gallic acid, which must be 

 made at the time. After a short exposure in the camera, the pa- 

 per, still apparently unchanged, is washed again with the above 

 mixture, when the impression begins to grow upon the paper in 

 a very striking and beautiful manner. The objection to this pro- 

 cess is its complication. The following is a very simple modifi- 

 cation of it, nearly as sensitive, and more so than the original 

 Daguerreotype plates. 



A piece of best glazed letter paper is fastened by means of a 

 penknife point and some hard wood pegs to a piece of smooth 

 pine board. It is washed over once quickly and evenly by means 

 of a camel's hair brush, with a solution of sixty grains crystal- 

 lized nitrate of silver in one ounce of water. Let it dry sponta- 

 neously, and as soon as dry wash it for a minute with a brush 

 and solution of ten grains of iodide of potassium in one ounce 

 of water. Then instantly wash it with water by dipping it three 



* Report of the British Association, 1841. 

 t London and Ed. Phil. Mag., Aug. 1841. 



