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 8 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'. 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' has a half-life of 19.5 minutes, 
too short, again, for chemical investigation; but it changes into 
radium C?, and in doing so each atom parts with a helium atom; hence 
the equation, radium C! (214.4) =helium (4) +radium C? (210.4). In 
2.5 minutes radium C? is half gone, parting with electrons, forming 
radium D. Radium D gives the chemist a chance, for its half-life is 
no less than 164 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 
