184 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1915. 



radioactive substance. Radium differs from an ordinary element 

 in its power of spontaneously expelling alpha particles with very 

 great speed. This property is ascribed to an inherent instability 

 which is not manifest in the atoms of ordinary elements. A small 

 fraction of the radium atoms — about 1 in 100,000,000,000 — break 

 up each second with explosive violence, expelling a fragment of the 

 atom (the alpha particle) with very great speed. The residue of 

 the atom is lighter than before and becomes the atom of an entirely 

 new substance, which is called the radium emanation. The atoms 

 of the latter are far more unstable than those of radiimi, for half 

 of them break up in 3.85 days, while half of the radium atoms break 

 up in about 2,000 years. After the loss of an alpha particle an atom 

 of the emanation changes into an atom of a new substance (radium 

 A), which behaves as a solid. 



Radium A is very unstable, half of it brealdng up in 3 minutes 

 with the emission of an alpha particle, and gives rise to radium B. 

 The latter differs from the substances already mentioned in the nature 

 of its radiation, for it emits beta rays but no alpha raj^s. Notwith- 

 standing this fact, it is transformed according to the same law as 

 an alpha ray's substance, and gives rise to an entirely distinct ele- 

 ment, radium C. In the transformation of the latter, not only are 

 swift alpha rays emitted but also beta rays of great speed. There is 

 some evidence, however, that the substance called radium C is com- 

 plex, and that the alpha and beta rays arise from two distinct sub- 

 stances. 



The successive substances arising from radium C are radium D, 

 radium E, and radium F. The two former, like radium B, emit 

 only beta rays; the latter, known generally as polonium, emits only 

 alpha rays. It is believed that the sequence of changes ends with 

 the transformation of radium F, which is supposed to change into 

 the well known nonradioactive element lead. 



According to the transformation theory, radium, like all other 

 radioactive products, must be regarded as a changing element, but 

 one whose rate of transformation is very slow compared with its 

 successive products. Boltwood showed experimentally that radium 

 is half transformed in about 2,000 years, and a quantity of radium 

 would practically have disappeared as such in 100,000 years. In 

 order to account for the continued existence of radium in the earth, 

 it is necessary to suppose that it is steadily produced from some 

 other element. Boltwood showed that the parent substance is a 

 radioactive element called ionium, which is itself derived from th^i 

 transformation of uranium. A quantity of ionium, entirely freed 

 from radium, will grow radium at a slow but constant rate. The 

 primary element of the ionium-radium series is uranium, which we 

 can calculate should be half transformed in 5,000,000,000 years — a 



