44 ILLINOIS ACADEMY OF SCIENCE. 



(ii) Does it exhibit elasticity of both bulk and shape? Or 

 does it exhibit only one of these ? 



(iii) Is it, like ordinary matter, a vehicle of energy; and how 

 is its radiant energy distributed? 



(iv) How does it behave as regards heat conduction? 



(v) What are its electrical properties? 



(vi) What are its magnetic properties? 



(vii) What are its optical properties? 



(viii) Has it a characteristic spectrum? 



Instead of attempting the impossible by considering all these 

 various questions seriatim, many of which, in the case of radium, 

 have not been answered at all, I shall ask you to consider with 

 me, first, those three properties which serve to define a radio- 

 active substance and which are perhaps quite as fundamental 

 as any, and later, to consider some three or four properties which 

 are of interest, not so much because they surprise us, as because 

 they are the fundamental phenomena of radio-activity, the phe- 

 nomena upon which Rutherford has established his disintegration 

 theory. 



DEFINITION OF RADIO-ACTIVITY. 



The three defining properties which apply not only to the four 

 radio-active elements, but also to their various products, are: 



(i) The ability to blacken a photographic plate in the dark. 



(ii) The ability to produce phosphorescence and fluorescence 

 in any appropriate body placed near the radio-active substance, 



(iii) The ability to transform our most important insulators 

 of electricity — gases — into conductors of electricity. 



The first of these properties is so familiar as to need no illus- 

 tration. The second is simply and clearly shown by the spinthari- 

 scope of Sir William Crookes. 



The third of these defining properties is perhaps the most 

 simply demonstrated in a manner suggested by Soddy. This silk 

 tassel is really nothing more than a multiple electroscope, and as 

 such is at least as early as the time of Benjamin Franklin. You 

 observe that in ordinary air it remains charged for a long time ; 

 during an interval of one minute the amount of discharge is not 

 perceptible. 



When, however, a small bit of radium — certainly not as much 

 as a 1/10 milligram — is brought underneath the tassel, you 

 observe that it at once begins to collapse — just as if it were sud- 

 denly immersed in a conducting medium. 



