Manchester Memoirs, Vdl. xli. (1897), No. 15. 23 



with black paper or white paper ? What is the cause 

 of blackness ? The light falling upon the paper produces 

 motion in the ultimate molecules. In the case of a 

 transparent substance you have a compound vibrating 

 system going on, vibrating without change. But in the 

 case of an absorbing medium the vibrations which after 

 a time are produced in the molecules spread out into 

 adjoining molecules, by virtue of the communication of 

 the molecules with one another, and are carried away ; 

 so that in the case of an absorbing medium, there is a 

 constant beginning to set the molecules in vibration ; 

 but they never get to the permanent state, because the 

 vibration is carried away by communication from one 

 molecule to another. But in the case of the Rontgen 

 rays you have done with the pulse altogether long before 

 any harmonious vibration between the ether and the mole- 

 cules can be established; so that a state of things is not 

 brought about in which you get a, comparatively speak- 

 ing, large vibration of the molecules. Consequently, the 

 Rontgen rays do not care whether you give them black 

 paper or not. 



I must not keep you more than a minute or two 

 longer ; but I do not like to close this lecture without 

 saying a word or two regarding the Becquerel rays. 

 What takes place there ? To be brief, I must refer to 

 the most striking case of all. Take the case of metallic 

 uranium. That gives out something which, like the 

 Rontgen rays, has an influence passing through black 

 paper, and capable of affecting a photographic plate. 

 It is also capable of effecting the discharge of statically- 

 charged electrified bodies. Apparently this goes on in- 

 definitely. You do not need, apparently, to expose the 

 metal to rays of high refrangibility in order that this 

 strange thing should go on. What takes place ? My 

 conjecture is that the molecule of uranium has a struc- 



