1] RADIO-ACTIVE SUBSTANCES 3 
simpler forms the atoms of the elements by the action of the 
chemical or physical forces at our command. 
On this theory we are witnessing in the radio-active bodies a 
veritable transformation of matter. This process of disintegration 
was investigated, not by direct chemical methods, but by means 
of the property possessed by the radio-active bodies of giving out 
specific types of radiation. Except in the case of a very active 
element like radium, the process of disintegration takes place so 
slowly, that hundreds if not thousands of years would be required 
before the amount transformed would come within the range of 
detection of the balance or the spectroscope. In radium, however, 
the process of disintegration takes place at such a rate that it 
should be possible within a limited space of time to obtain definite 
chemical evidence on this question. The recent discovery that 
helium can be obtained from radium adds strong confirmation to 
the theory; for helium was indicated as a probable disintegration 
product of the radio-active elements before this experimental 
evidence was forthcoming. If the production of helium by radium 
is completely substantiated, the further study of radio-active bodies 
promises to open up new and important fields of chemical enquiry. 
In this book the experimental facts of radio-activity and the 
connection between them are interpreted on the disintegration 
theory. Many of the phenomena observed can be investigated in 
a quantitative manner, and prominence has been given to work of 
this character, for the agreement of any theory with the facts, 
which it attempts to explain, must ultimately depend upon the 
results of accurate measurement. 
The value of any working theory depends upon the number of 
experimental facts 1t serves to correlate, and upon its power of 
suggesting new lines of work. In these respects the disintegration 
theory, whether or not it may ultimately be proved to be correct, 
has already been justified by its results. 
2. Radio-active Substances. The term “radio-active ” is 
now generally applied to a class of substances, such as uranium, 
thorium, radium, and their compounds, which possess the property 
of spontaneously emitting radiations capable of passing through 
plates of metal and other substances opaque to ordinary light. 
1—2 
