DRY COLLODION.

EVERY photographer knows how difficult it is to find a collodion suitable for the collodio-albumen, gelatine, or meta-gelatine processes, i. e., a collodion giving a film neither tenacious nor contractible, but very porous, friable, and adherent. Old collodions are recommended as possessing such properties, bat, besides that they are often too much iodised for that purpose, it is to be remarked that, if the collodions prepared with alkaline iodisers answer very well, those iodised with metallic salts (cadmium generally) do not always work as well, for although the collodion becomes coloured, the film keeps for a long time all the characters of the collodion one or two weeks old.

Believing, from observation, that the alkaline and metallic bases reacted on pyroxyline in different manners, desirous to explain that fact, and also to have a formula by which I can surely prepare good collodion for the dry process, I made the following experiment to study the action of alkaline bases in collodion

To a plain collodion giving a thick strong film, very contractible, and easily lifted up in long rays, I added liquid ammonia; immediately it was troubled, and, after a few hours, gave a thinner film, very porous, rotten, and opaque. it took twenty-four hours to clear up, became a fine amber colour, and left a white precipitate of decomposed pyroxyline (cellulose). Caustic potash and caustic baryta in small quantity acted nearly in the same way. Hence, alkaline bases react powerfully on pyroxyline, it is disorganised, and a part is decomposed.

This is very important. It explains--1st. The great fluidity of collodions prepared with alkaline iodisers (particularly when iodide of ammonium is used), and, partly, their instability. 2nd. Why those collodions give a film with less and less body, and the causes of the want of success resulting from it. 3rd. The advantage of alkaline collodion for Taupenot's and Norris's processes, and, generally, for all dry preparations on collodion. It, will be observed that the amount of ammonia added to the plain collodion ought to be proportioned according to the kind of pyroxyline, that is, to the more or less tenacity or contractibility of the collodion, and that ammoniacal collodions cannot support as much iodiser as other ones--4 grains to the ounce is a good proportion for the collodio-gelatine process, and 2 grains are enough for the collodio-albumen.

But since it is easy to prepare a collodion with all the proper qualities for dry preparations, I have adopted a truly dry collodion process (without any kind of preservative' coat) which is very sensitive, and has the advantage of great simplicity. The preparations are--

After sensitising, immerse the plate in water for three or four minutes, and having washed it with distilled water, let it dry in the dark. To develop--first wet the collodion film with water, and then spread upon it a mixture of 1 drachm of the developer, and 3 or 4 ounces of water, afterwards add a small quantity of a solution of nitrate of silver, at 3 per cent, to strengthen the negative. Fix with hyposulphite of soda.

As it has been said, this process is very rapid, and the failures often arise from an excess of exposure; if it is too long, the sky does not blacken, the picture is too equal, and does not take enough intensity in the high lights.