RADIUM AND THE ELECTRON RUTHERFORD. 197 
scale of magnitude that they were difficult to explain and still more 
difficult to explain away. The systematic chemical analysis of 
uranium ores disclosed the presence of new radioactive substances 
like polonium and actinium, while the study of thorium, radium, and 
actinium disclosed the emission of radioactive emanations or gases 
and their apparently remarkable power of conferring temporary 
activity on all bodies in their neighborhood. The changes in activity 
of these substances with time and the different types of radiation 
emitted at first gave an appearance of great complexity and confu- 
sion to the rapidly accumulating mass of facts, but the whole subject 
took on an orderly and systematic development after the transforma- 
tion theor}'^ was put forward by Rutherford and Soddy in 1903 as 
an explanation of radioactivity. On this view radioactive matter is 
undergoing spontaneous transformation of its atoms with the ap- 
pearance of a succession of new radioactive bodies, each marked by 
characteristic chemical and radioactive properties. The radiations 
accompany the transformation of atoms and are a measure of the 
rate of transformation. Guided by this theory, the whole sequence 
of changes in the uranium-radium series, the thorium and actinium 
series, were investigated in detail, and in a remarkably brief space 
of time more than 30 new radioactive elements were brought to light 
and their position in the scheme of radioactive changes determined. 
Special interest attaches to the discovery by Boltwood of the sub- 
stance called ionium, which is directly transformed into radium. 
This afforded a direct experimental method of determining the aver- 
age life of radium with a result that is in close accord with the value 
calculated from the rate of emission of a-particles. The position of 
actinium in the main scheme of changes has occupied much attention. 
The constancy of the relative amount of actinium and uranium in 
uranium minerals showed that it must be derived ultimately from 
uranium, but the activity of actinium is too small to be in the direct 
line of succession. This has led to the view that actinium is a 
branch product at some point of the uranium series where about 6 
per cent is transformed into the actinium branch and 94 per cent 
into the main line of descent. The general evidence indicates that 
this branching occurs near to uranium, and possibly the branch 
product called uranium-Y by Antonoff is the first member of the 
family. Recently the intermediate parent substance of actinium 
itself has been discovered. 
While in the majority of cases the atoms of a radioactive product 
break up in a very definite fashion and in only one way, certain cases 
are known where one substance breaks into two chemically distinct 
substances. Examples of this are radium C, thorium C, and actinium 
C Usually the transformation is mainly in one direction with a 
12573°— 21 14 
