RADIOACTIVITY 



By Professor W. H. BRAGG, F.R.S. 



The facts and Itheory of radioactivity have been with us for 

 several years and we have acquired more or less familiarity 

 with the ideas which were at first so startling. In the case of 

 those who themselves handle the radioactive substances and 

 investigate their properties the first sense of strangeness quickly 

 disappears, for they soon learn to have no doubt of the reality of 

 the things with which they have to deal. To those, however, 

 whose knowledge is indirect or who have had little time to 

 follow a subject which has developed with sensational rapidity, 

 the theory can still seem a provisional hypothesis and to such 

 a brief restatement of the whole position may be of some use. 

 Not that it is possible to give a complete historical and logical 

 account in a short paper such as the present must be. The 

 story is now too long. But perhaps something much less 

 ambitious may be attempted which may serve a purpose. I 

 propose therefore to state what seem to be the fundamental 

 ideas of radioactivity, on the novelty and importance of which 

 rest the claims of the new subject to be recognised as a science 

 in its own right. And then, as it would not be possible to state 

 all the arguments in their favour or to work out their conse- 

 quences, I shall merely attempt their illustration by following 

 up the investigations which have been made on the passage of 

 the new rays through matter. Adhering to this limited inquiry 

 it is possible, I think, to drive a line through the subject 

 from various points on which may be seen the most of what 

 there is to be seen ; and I shall not be trying to do the 

 impossible. 



First, then, the fundamental ideas. These seem to me to 

 be (a) the disintegration of the atom, (b) the existence of the 

 electron, (c) the penetrability of matter to an extent not realised 

 previously. 



The first of these is due to Rutherford. Radioactivity itself 

 is supposed by him to be due to a radical convulsion in the 

 atom of the radioactive substance. The result is the expulsion 



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