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SCIENCE 



[N. S. Vol. XXXIV. No. 871 



complete; that perhaps a few more might 

 be discovered to fill the outstanding gaps 

 in the periodic table. True, a puzzle 

 existed and still exists in the classification 

 of the "rare earths," oxides of metals oc- 

 curring in certain minerals; these metals 

 have atomic weights between 139 and 180, 

 and their properties preclude their arrange- 

 ment in the columns of the periodic table. 

 Besides these, the discovery of the inert 

 gases of the atmosphere, of the existence 

 of which Johnstone Stoney's spiral curve, 

 published in 1888, pointed a forecast, 

 joined the elements like sodium and po- 

 tassium, strongly electro-negative, to those 

 like fluorine and chlorine, highly electro- 

 positive, by a series of bodies electrically 

 as well as chemically inert, and neon, ar- 

 gon, krypton, and xenon formed links be- 

 tween fluorine and sodium, chlorine and 

 potassium, bromine and rubidium, and 

 iodine and cssium. 



Including the inactive gases, and adding 

 the more recently discovered elements of 

 the rare earths, and radium, of which I 

 shall have more to say presently, there are 

 eighty-four definite elements, all of which 

 find places in the periodic table, if merely 

 numerical values be considered. Between 

 lanthanum, with atomic weight 139, and 

 tantalum, 181, there are in the periodic 

 table seventeen 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 fourteen 

 such elements; and it is not improbable 

 that other three will be separated from 

 the complex mixture of their oxides by 

 further work. 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 ricliesse. The discovery of radioactivity 

 by Henri Becquerel, of radium by the 

 Curies, and the theory of the disintegra- 

 tion of the radioactive elements, which we 

 owe to Rutherford and Soddy, have indi- 

 cated 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 sub- 

 stances 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, car- 

 bonate, 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 ele- 

 ment. 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. Wliat are they ? 

 We can answer that question. Rutherford 

 and Soddy found that it gives a condens- 

 able gas, which they named "radium-ema- 

 nation ' ' ; and Soddy and I, in 1903, dis- 

 covered that, in addition, it evolves helium. 



