CHAPTER LI. 
RADIO-ACTIVE SUBSTANCES. 
1. Introduction. The close of the old and the beginning 
of the new century have been marked by a very rapid increase of 
our knowledge of that most important but comparatively little 
known subject—the connection between electricity and matter. 
No study has been more fruitful in surprises to the investigator, 
both from the remarkable nature of the phenomena exhibited and 
“from the laws controlling them. The more the subject has been 
examined, the more complex does the constitution of matter appear 
which can give rise to the remarkable effects observed. While 
the experimental results have led to the view that the constitution 
of the atom itself is very complex, at the same time they have 
strongly confirmed the old theory of the discontinuous or atomic 
structure of matter. The study of the radio-active substances and 
of the discharge of electricity through gases has supplied very 
strong experimental evidence in support of the fundamental ideas 
of the existing atomic theory. It has also indicated that the 
atom itself is not the smallest unit of matter, but 1s a complicated 
structure made up of a number of smaller bodies. 
A great impetus to the study of this subject was initially 
given by the experiments of Lenard on the cathode rays, and 
by Rontgen’s discovery of the X rays. An examination of the 
conductivity imparted to a gas by the X rays led to a clear view 
of the mechanism of the transport of electricity through gases 
by means of charged ions. This ionization theory of gases has 
been shown to afford a satisfactory explanation not only of the 
passage of electricity through flames and vapours, but also of the 
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