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2 RADIO-ACTIVE SUBSTANCES [CH. 
complicated phenomena observed when a discharge of electricity 
passes through a vacuum tube. At the same time, a further 
study of the cathode rays showed that they consisted of a stream 
of material particles, projected with great velocity, and possessing 
an apparent mass small compared with that of the hydrogen atom. 
The connection between the cathode and Réntgen rays and the 
nature of the latter were also elucidated. Much of this admirable 
experimental work on the nature of the electric discharge has 
been done by Professor J. J. Thomson and his students in the 
Cavendish Laboratory, Cambridge. 
An examination of natural substances, in onde to see if they 
gave out dark radiations similar to X rays, led to the discovery of 
the radio-active bodies which possess the property of spontaneously 
emitting radiations, invisible to the eye, but readily detected by 
their action on photographic plates and their power of discharging 
electrified bodies. A detailed study of the radio-active bodies has 
led to the discovery of many new and surprising phenomena which 
have thrown much lght, not only on the nature of the radiations 
themselves, but also on the processes occurring in those substances. 
Notwithstanding the complex nature of the phenomena, the know- 
ledge of the subject has advanced with great rapidity, and a large 
amount of experimental data has now been accumulated. 
In order to explain the phenomena of radio-activity, a theory 
has been put forward which regards the atoms of the radio-active 
elements as sufferig spontaneous disintegration, and giving rise 
to a series of radio-active substances which differ m chemical » 
properties from the parent elements. The radiations accompany 
the breaking-up of the atoms, and afford a comparative measure of 
the rate at which the disintegration takes place. This theory is 
found to account in a satisfactory way for all the known facts of 
radio-activity, and welds a mass of disconnected facts into one 
homogeneous whole. On this view, the continuous emission of 
energy from the active bodies is derived from the internal energy 
inherent in the atom, and does not im any way contradict the law 
of the conservation of energy. At the same time, however, it 
indicates that an enormous store of latent energy is resident in the 
radio-atoms themselves. This store of energy has previously not 
been observed, on account of the impossibility of breaking up into 
yet 
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