TA _ Observations on Photozraphic 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. G 
‘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, suchas 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,t founded on his. important discovery of 
the properties of gallic acid. ‘ Ves Hae: end nacctatai nt ani 
For this the paper is washed successively with nitrate of. sil- 
ver and iodide of potassium ; then before using witha 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. modifie 
“. eation of it, nearly as sensitive, and more so than the original 
Daguerreotype plates. hah 
A piece of best glazed letter paper is fastened by means of @ 
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 acamel’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 
me _ * Report of the British Association, 1841, 
Tee 
ee OL eae 
ee 
a iz. 4 
EG Sa 
