Page:Popular Science Monthly Volume 22.djvu/413

Rh which has not been acted upon by the light. It has been usually said that alkaline development is only available for bromide of silver, but my experience has taught me that iodide of silver is as amenable to alkaline development as the bromide, although not so rapidly, and that chloride is very amenable to it, and will give most beautiful pictures.

Another mode of development, now very much in vogue, is that with ferrous oxalate. In this case we have an organic salt of iron in the ferrous state, which is capable of reducing silver bromide, iodide, and chloride to the metallic state, while itself is reduced to the ferric state. This process also requires a restrainer.

I have found a kindred developer, the use of which I consider one of the most recent advances in photography. It is an iron developer, which is capable of being used without any restrainer whatever. I call it ferrous citro-oxalate. It is made by adding to a solution of citrate of potash ferrous oxalate till no more will dissolve; the resulting compound is probably citrate of iron, but in a stronger form than is usually found.

Mr. Berkeley has lately introduced an improvement in the ordinary alkaline developer, in which he mixes with the pyrogallic-acid solution four times the weight of sulphite of soda. The action, apparently, is that the sulphite of soda absorbs the oxygen with greater avidity than does the pyrogallic acid, thus leaving that agent to do its work; consequently, we have a developer which remains uncolored for a very long period.

Another developer, which is competent to work also without a restrainer, but has not been used to a very great extent on account of its high price, is hydro-kinone. It is a much more powerful absorber of oxygen than pyrogallic acid, to such a degree that one grain of it, is as active as two grains of that substance. Not requiring any restrainer, even when so troublesome a salt as silver chloride is used, it is able to give a better detail and allow a shorter exposure in the camera than when the ordinary alkaline developer is used. It is applicable to any plate with which you can work.

The next point to which I wish to call attention is the action of sensitizers. It may be proper first to explain what a sensitizer is. When you have chloride of silver exposed to light, you have a new compound formed, which is called subchloride, or argentous chloride (Ag2Cl2$$=$$ Ag2Cl $$+$$ Cl), and chlorine is liberated. This chlorine is very difficult to eliminate, if you do not give it something that can take it up; for instance, if you place perfectly pure chloride of silver in vacuo, without any trace of organic matter present, you will find that you get no darkening action, even if it is exposed to brilliant sunlight for months. If a white powder of the kind was submitted to you, to determine its character, you would say at once that it was not chloride of silver, because it was not darkened, since one of the