PREFACE. 
N this work, I have endeavoured to give a complete and 
connected account, from a physical standpoint, of the properties 
possessed by the naturally radio-active bodies. Although the 
subject is comparatively a new one, our knowledge of the pro- 
perties of the radio-active substances has advanced with great 
rapidity, and there is now a very large amount of information on 
the subject scattered throughout the various scientific journals. 
The phenomena exhibited by the radio-active bodies are 
extremely complicated, and some form of theory is essential in 
order to connect in an intelligible manner the mass of experi- 
mental facts that have now been accumulated. I have found the 
theory that the atoms of the radio-active bodies are undergoing 
spontaneous disintegration, extremely serviceable not only in 
correlating the known phenomena, but also in suggesting new 
lines of research. 
The interpretation of the results has, to a large extent, been 
based on the disintegration theory, and the logical deductions to 
be drawn from the application of the theory to radio-active 
phenomena have also been considered. 
The rapid advance of our knowledge of radio-activity has 
been dependent on the information already gained by research 
into the electric properties of gases. The action possessed by the 
radiations from radio-active bodies of producing charged carriers 
or ions in the gas, has formed the basis of an accurate quantitative 
method of examination of the properties of the radiations and of 
