414 



Sir W. Crookes. 



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 



* The body I called Uranium X in my Eoyal Society paper, May 10, 1900, has 

 since proved to be M. Debierne's Actinium. 



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. 



