THE CHEMICAL ELEMENTS — BAMSAY. 187 



chemically inert, and neon, argon, krypton, and xenon formed links 

 between fluorine and sodium, chlorine and potassium, bromine and 

 rubidium, and iodine and caesium. 



Including the inactive gases, and adding the more recently dis- 

 covered elements of the rare earths, and radium, of which I shall 

 have more to say presently, there are 84 definite elements, all of 

 which find places in the periodic table, if merely numerical values be 

 considered. Between lanthanum, with atomic weight 139, and tan- 

 talum, 181, there are in the periodic table 17 spaces; and although 

 it is impossible to admit, on account of their properties, that the 

 elements of the rare earths can be distributed in successive columns 

 (for they all resemble lanthanum in properties), yet there are now 

 14 such elements; and it is not improbable that other 3 will be sepa- 

 rated from the complex mixture of their oxides by further work. 

 Assuming that the metals of the rare earths fill these 17 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 1 1 . 



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 26 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 deter- 

 mined 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 1 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 I in 1903 discovered 



