12 president's address. 



Assuming that the metals of the rare earths fill these seventeen spaces, 

 how many still remain to be filled ? We will take for granted that the 

 atomic weight of uranium, 238.5, which is the highest known, forms 

 an upper limit not likely to be surpassed. It is easy to count the gaps ; 

 there are eleven. 



But we are confronted by an embarras de richesse. The discovery 

 of radioactivity by Henri Becquerel, of radium by the Curies, and the 

 theory of the disintegration of the radioactive elements, which we owe 

 to Rutherford and Soddy, have indicated the existence of no fewer than 

 twenty-six elements hitherto unknown. To what places in the periodic 

 table can they be assigned? 



But what proof have we that these substances are elementary ? Let 

 us take them in order. 



Beginning with radium, its salts were first studied by Madame 

 Curie; they closely resemble those of barium — sulphate, carbonate, and 

 chromate insoluble ; chloride and bromide similar in crystalline form to 

 chloride and bromide of barium ; metal, recently prepared by Madame 

 Curie, white, attacked by water, and evidently of the type of barium. 

 The atomic weight, too, falls into its place; as determined by Madame 

 Curie and by Thorpe, it is 89.5 units higher than that of barium; in 

 short, there can be no doubt that radium fits the periodic table, with 

 an atomic weight of about 226.5. It is an undoubted element. 



But it is a very curious one. For it is unstable. Now, stability 

 was believed to be the essential characteristic of an element. Radium, 

 however, disintegrates — that is, changes into other bodies, and at a 

 constant rate. If a gram of radium is kept for 1,760 years, only half 

 a gram will be left at the end of that time ; half of it will have given 

 other products. What are they? We can answer that question. 

 Rutherford and Soddy found that it gives a condensable gas, which they 

 named ' radium-emanation '; and Soddy and myself, in 1903, discovered 

 that, in addition, it evolves helium, one of the inactive series of gases, 

 like argon. Helium is an undoubted element, with a well-defined 

 spectrum; it belongs to a well-defined series. And radium-emanation, 

 which was shown by Rutherford and Soddy to be incapable of chemical 

 union, has been liquefied and solidified in the laboratory of University 

 College, London; its spectrum has been measured and its density 

 determined. From the density the atomic weight can be calculated, 

 and it corresponds to that of a congener of argon, the whole series 

 being: helium, 4; neon, 20; argon, 40; krypton, 83; xenon, 130; 

 unknown, about 178; and niton (the name proposed for the emanation 

 to recall its connection with its congeners, and its phosphorescent pro- 

 perties), about 222.4. The formation of niton from radium would there- 

 fore be represented by the equation: radium (226.4) = helium (4) + 

 niton (222.4). 



