Page:Proceedings of the Royal Society of London Vol 69.djvu/432

414 The cells were filled to the same depth with a radio-active substance chiefly containing actinium.* Over cell A was placed a piece of thick lead pipe, 28 mm. high and 25 mm. internal diameter, to insure that any emanations from the active substance in A would be confined to the inside of the hollow cylinder. The radio-active substance in B

FIG. 1. Elevation.

FIG. 1. Plan.

was freely exposed to the air, save for a pillar of lead at C, to support the sensitive film. A sensitive film was laid horizontally over the cylinder and support C. On the film was a plate of glass, and cylinder and film were pressed together by heavy weights. The whole was covered in a light-tight box and put in a dark cupboard.

At the end of 48 hours the film was removed and developed. There was a strong action shown over cell A (the one covered by the lead cylinder), but over B, the cell exposed to the air, there was no visible impression. Measured in Mr. Chapman Jones's "Opacity Meter " t the results were :

Image over cylinder opacity log.J = 0'79; opacity = 6 '17.

The experiment was repeated, using the same apparatus but a different preparation of actinium. In this case the exposure was for

since proved to be M. Debierne's Actinium.
 * The body I called Uranium X in my Eoyal Society paper, May 10, 1900, has

f ' The Photographic Journal,' vol. 20, p. 86, December 21, 1895.

J The opacity logarithm represents the density of the image, absolute density being represented by 2'00

The " opacity " is the whole number corresponding to the " opacity log." The "opacity " is directly proportional to the photographic energy acting on the sensitive surface.