president's address. 13 



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 pro- 

 perties were being determined. Its product of change was named by 

 Eutherford ' 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 three 

 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 twenty- 

 seven minutes it has half changed into radium C 1 . In this case, how- 

 ever, 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. Badium 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. 



Eadium D gives the chemist a chance, for its half-life is no less 

 than sixteen and a half years. Without parting with anything detect- 

 able, radium ~D passes into radium E, of which the half-life period 

 is five 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 

 atomic weight of lead is 207.1, and not 206.4; however, it is possible 

 that the atomic weight of radium is 227.1, and not 226.4. 



We have another method of approaching the same subject. It is 

 practically certain that the progenitor of radium is uranium ; and that 

 the transformation of uranium into radium involves the loss of three 

 alpha particles ; that is, of three atoms of helium. The atomic weight 

 of helium may be taken as one of the most certain; it is 3.994, as 

 determined by Mr. Watson, in my laboratories. Three atoms would 

 therefore weigh 11.98, practically 12. There is, however, still some 

 uncertainty in the atomic weight of uranium; Eichards and Merigold 

 make it 239.4; but the general mean, calculated by Clarke, is 239.0. 

 Subtracting 12 from these numbers, we have the values 227.0, and 



