The quantity of chlorine necessary to produce the effect is exceedingly minute. In our early experiments we employed a quart bottle of the gas, opening it in a deep box, and leaving out the stopper while deliberately counting twenty. Replacing then the stopper, the plate was laid for half a minute over an opening in the top. After fifty experiments the gas in the bottle seemed not to have lost any of its original intensity of color. We have better arrangements at present in preparation.
Much care is necessary to avoid an excess of chlorine.
The principal cause of our early failures arose from an error of this kind. One may easily determine, with any apparatus, the time and quantity necessary, by laying a plate over the aperture and drawing it partially off at intervals. The action of the gas will then be greatest, of course, upon the part longest exposed. Too much care cannot be taken to exclude the light during the process of preparation. Posted for your enjoyment.