1908] Radio-active Changes in the Earth. 147 



WEEKLY EVENING MEETING, 



Eriday, March 27, 1908. 



The Right Hon. Loed RayleiCxH, O.M. P.O. M.A. D.C.L. 

 LL.D. Sc.D. Pres.R.S., in the Chair. 



The Hon. Robekt John Strutt, M.A. F.R.S. 



Radio-active Changes in tJie Earth. 



You will have already heard during the present Lecture Session from 

 Professor Rutherford, of the recent developments of the new science 

 ■of radio-activity. I venture to hope, however, that there may be 

 room to say something of aspects of the subject not treated by him. 

 I wish particularly to refer to manifestations of radio-activity which 

 are observed not in artiiicially prepared materials like radium, but in 

 the rocks and minerals of the earth's crust, as we find them in nature. 

 Let us consider in the first place the most conspicuous cases of this 

 kind. The source from which radium is obtained is the mineral 

 pitch-blende. This mineral occurs in veins, like the majority of the 

 useful metals ; I may refer particularly to the mineral veins of 

 Cornwall, so long famous as a source of tin. These veins are of the 

 nature of cracks, running through the granite and through the slate 

 which adjoins it. The cracks have been filled up by the various 

 metallic ores which have been introduced by precipitation or sub- 

 limation, the exact nature of the process being somewhat obscure. 



I will now show you an experiment, due to Sir W. Crookes, which 

 illustrates the radio-activity of pitch-blende in a very beautiful 

 manner. A flat polished slab of pitch-blende intergrown with a 

 variety of other material which is not radio-active was laid face to 

 face with a photographic plate which was developed, after the lapse 

 of about a week of contact. The radium and other radio-active 

 substances contained in the pitch-blende have acted photographically 

 upon the plate, while, of course, those portions of the material whicii 

 are not radio-active have exerted no such action. Thus pitch-blende 

 has, as it were, taken its own portrait, which I now show you on the 

 screen. 



Pitch-blende, the principal radium ore, contains, as you know, 

 only an infinitesimal percentage of radium, the bulk of the substance 

 being made up of oxide of uranium. Uranium is commonly spoken 

 of as a rare metal ; but terms of this kind are comparative only, and 

 in contrast with radium, which is more than a million times scarcer 



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