324 DRY COLLODION PROCESS IN PHOTOGRAPHY. 
bromo-iodized collodion, and removing all the free nitrate, which is the element 
of instability. 
But the image produced upon a bromo-iodized silver plate, developed with 
mercury, is extremely thin and superficial, as may be proved by transferring it 
to a sheet of gelatinized paper. And similarly, the image developed by pyro- 
gallic acid upon a dry bromo-iodized collodion film is thin, and too transparent 
to yield a good printing negative. It is necessary therefore to apply to the 
film a coating of some organic substance, in order to give density to the dark 
parts of the negative. Many substances have been employed for this purpose, 
viz., gelatine, metagelatine, albumen, various syrups, gum arabic, infusion of 
malt, tannin, &c, &c.; and experimenters have, almost without exception, 
exhausted their ingenuity in varying these preservative coatings, as they are 
called, instead of seeking in the use of bromide for the true accelerating agent. 
The preservatives named have not all the same effect, and besides affecting the 
sensitiveness of the film they also determine the color of the finished negative, 
gelatine and gum giving a black, tannin a red, and albumen a yellowish color 
to the deposit in the dark parts. Much therefore depends upon the selection of 
@ proper preservative, when the most exalted sensitiveness is required. 
‘One more difficulty remained to be overcome, and it is this. When a collodion 
film has once been allowed to get dry, and is wetted a second time, itis very 
liable to split and leave the glass; or if a preservative has been applied to it, it 
is very liable to rise in blisters, which spoil the negative. But this may be pre- 
vented by giving the glass plate a preliminary coating of india-mubber dissolved 
in Kerosolene. 
The operations in the rapid dry process are therefore as follows :— 
1. Clean the glass plate, dry it thoroughly, and apply to it a solution com- 
posed of 1-gr. of india-rubber dissolved in an ounce of Kerosolene. 
2. Coat the plate thus prepared with bromo-iodized collodion sienna 
an equal number of atoms of iodine and bromine, added in combination with 
cadmium. There should be about 5-grs. of mixed iodide and bromide of cad- 
mium to the ounce of collodion. é 
3. Eseite the film in a bath composed of 30-grs. of pure recrystallized nitrate 
of silver, slightly acidified with nitric acid. 
4, Wash off all the free nitrate of silver, and pour over the film a preservative 
composed of 25-grs. of gum arabic freshly dissolved in an ounce of water. Let 
it dry spontaneously, and before putting the plate into the dark slide, dry it 
again thoroughly before a hot flat iron. 
5. Give the same exposure as for wet collodion. 
6. Develope the picture by first wetting it with distilled water, and then pour- 
ing over it a developer consisting of 1-oz. of distilled water, 2-grs. of pyrogallic 
acid, 2 scruples of glacial acetic acid, and a few drops of a weak solution of 
nitrate of silver. The image appears immediately, and very soon acquires the 
necessary intensity. 
7. Fix the negative in the usual way with a saturated solution of the hyposul- 
phite of soda or lime, and when dry variish it with spirit varnish. 3 
Negatives taken in this way are equal in every respect to those taken upon 
wet collodion plates, and the process is as simple as any of those which are now 
employed for slow dry plates. 
