188 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1911. 



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-emana- 

 tion, 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 with 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 properties), about 222.4. The formation of niton 

 from radium would therefore be represented by the equation, radium 

 (226.4)= helium (4)+ niton (222.4). 



Niton, in its turn, disintegrates, or decomposes, and at a rate much 

 more rapid than the rate of radium; half of it has changed in about 

 four days. Its investigation, therefore, had to be carried out very 

 rapidly, in order that its decomposition might not be appreciable 

 while its properties were being determined. Its product of change 

 was named by Rutherford radium A, and it is undoubtedly deposited 

 from niton as a metal, with simultaneous evolution of helium; the 

 equation would therefore be, niton (222.4)= helium (4) + radium A 

 (218.4). But it is impossible to investigate radium A chemically, for 

 in 3 minutes it has half changed into another solid substance, radium 

 B, again giving off helium. This change would be represented by 

 the equation, radium A (218.4)= helium (4) + radium B (214.4). 

 Radium B, again, can hardly be examined chemically, for in 27 

 minutes it has half changed into radium C 1 . In this case, however, 

 no helium is evolved, only atoms of negative electricity, to which the 

 name electrons has been given by Dr. Stoney, and these have minute 

 weight which, although approximately ascertainable, at present has 

 defied direct measurement. Radium C 1 has a half-life of 19.5 minutes, 

 too short, again, for chemical investigation; but it changes into 

 radium C 2 , and in doing so each atom parts with a helium atom; hence 

 the equation, radium C 1 (214.4) =helium (4)+radium C 2 (210.4). In 

 2.5 minutes radium C 2 is half gone, parting with electrons, forming 

 radium D. Radium D gives the chemist a chance, for its half-life is 

 no less than 16J years. Without parting with anything detectable, 

 radium D passes into radium E, of which the half-life period is 5 

 days; and, lastly, radium E changes spontaneously into radium F, 

 the substance to which Madame Curie gave the name polonium in 

 allusion to her native country, Poland. Polonium, in its turn, is half 

 changed in 140 days with loss of an atom of helium into an unknown 

 metal, supposed to be possibly lead. If that be the case, the equation 

 would run, polonium (210.4) =helium (4)+lead (206.4). But the 



