320 RADIO-ACTIVE PROCESSES [CH. 
radio-active elements possess the property of abstracting energy 
from the gas. If the moving molecules, impinging more swiftly 
on the substance, were released from the active substance at a 
much lower velocity, the energy released from the radio-elements 
might be derived from the atmosphere. This theory was advanced 
again later to account for the large heat emission of radium, 
discovered by P. Curie and Laborde. 
Fillipo Re? recently advanced a very general theory of matter 
with a special application to radio-active bodies. He supposes 
that the parts of the atom were originally free, constituting a 
nebula of extreme tenuity. These parts have gradually become 
united round centres of condensation, and have thus formed the 
atoms of the elements. On this view an atom may be likened 
to an extinct sun. The radio-active atoms occupy a transition 
stage between the original nebula and the more stable chemical 
atoms, and in the course of their contraction give rise to the 
heat emission observed. 
Lord Kelvin in a paper to the British Association, 1903, has 
suggested that radium may obtain its energy from external sources. 
If a piece of white paper is put into one vessel and a piece of black 
paper into an exactly similar vessel, on exposure of both vessels to 
the light the vessel containing the black paper is found to be at a 
higher temperature. He suggests that radium ina similar manner 
may keep its temperature above the surrounding air by its power 
of absorption of unknown radiations. 
199. Discussion of Theories. From the survey of the 
general hypotheses advanced as possible explanations of radio- 
activity, it is seen that they may be broadly divided into two 
classes, one of which assumes that the energy emitted from the 
radio-elements is derived at the expense of the internal energy of 
the atom, and the other that the energy is derived from external 
sources, but that the radio-elements act as mechanisms capable of 
transforming this borrowed energy into the special forms manifested 
in the phenomena of radio-activity. Of these two sets of hypo- 
theses the first appears to be the most probable, and to be best 
supported by the experimental evidence. Up to the present not 
1 C. R. p. 136, p. 1393, 1903. 
