# Page:Popular Science Monthly Volume 22.djvu/413

399
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${\displaystyle =}$ Ag2Cl ${\displaystyle +}$ 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