RADIOACTIVITY— ARTIFICIAL AND NATURAL 295 



Uranium, then, is a radioactive element. But it is not the only one; 

 there were numerous others, even in the days before the physicists 

 had started making new ones. This is where the elder Curies enter the 

 story: Pierre and Marie Curie, in 1898. First they measured the 

 activity of uranium, pretty carefully. Then they started measuring 

 the activity of various minerals containing uranium, and they found too 

 much : these minerals were more active than by virtue of their uranium 

 content alone, they should be. The Curies suspected that some other 

 radioactive element was lurking in the depths, and they undertook to 

 get it out. This was of course a chemical problem primarily, and as a 

 matter of fact, their Nobel prize was the chemistry prize, and quite 

 rightly. Eventually they isolated their new element, or rather, two 

 of them, which they named "polonium" and "radium." Having at 

 last got their radium and weighed it, they found that it amounted to 

 only two parts in a hundred million (by weight) of the rock from which 

 they had extracted it. They required three tons of the rock, in order 

 to get one one-hundredth of an ounce of the radium. Two parts in a 

 hundred million! and yet, while it was still dispersed in that almost 

 incredible scantiness through the rock, they had already been able to 

 detect it! This is the point which I most wish to bring out, at this 

 stage. A radioactive substance is far more easy to detect than any 

 which is not. It is like salt in food, and the radioactivity is like the 

 flavor of the salt, which shows the presence of a dash so insignificant 

 that anything else in the food would go untasted if it were equally 

 rare. Fortunately for us the instruments which are used to detect a 

 radioactive body are not our tongues, but apparatus of a much less 

 vulnerable kind. 



Now I mention only one name before reaching the recent develop- 

 ments, but this the greatest name of all: RUTHERFORD. Ruther- 

 ford was the first to understand radioactivity — the first to prove the 

 contemporary atom-model — -and the first to achieve transmutation. 

 Any one of these achievements by itself would have secured undying 

 glory to its author, but this great man made three. As lately as two 

 years ago I wrote in a book that every leading figure in the history of 

 transmutation was still living and still ardently at work. As lately as 

 last October I could have repeated that, but now the Master is gone — 

 quite suddenly gone in the fullness of his powers. This lecture is a 

 memorial to Rutherford, not because it was expressly so designed, but 

 because any lecture on the new radioactivity or on the old involves 

 so much of his thought and so much of his work that it would be reduced 

 to a few incoherent bits if everything not traceable to Rutherford should 

 be left out. 



