322 RADIO-ACTIVE PROCESSES [CH. 
emit to any greater extent than would be expected from their 
density. It has been shown (section 79) that this is true in the 
case of uranium. Even if it were supposed that the radio-elements 
possess the property of absorbing the energy of some unknown 
type of radiation, which is able to pass through ordinary matter 
with little absorption, there still remains the fundamental difficulty 
of accounting for the peculiar radiations from the radio-elements, 
and the series of changes that occur in them. It is not sufficient 
to account for the heat emission only, forit has been shown (section 
106) that the emission of heat is directly connected with the radio- 
activity. 
In addition, the distribution of the heat emission of radium 
amongst the radio-active products which arise from it is extremely 
difficult to explain on the hypothesis that the energy emitted 
is borrowed from external sources. It has been shown that more 
than two-thirds of the heat emitted by radium is due to the 
emanation together with the matter emanation X, which is pro- 
duced by the emanation. When the emanation is separated from 
the radium, its power of emitting heat, after reaching a maximum, 
decreases with the time according to an exponential law. It 
would thus be necessary on the absorption hypothesis to postulate 
that most of the heat emission of radium, observed under ordinary 
conditions, is not due to the radium itself but to something pro- 
duced by the radium, the power of which of absorbing energy from 
external sources diminishes with time. 
The strongest evidence against the hypothesis of absorption of. 
external energy is that such a theory ignores the fact, that, when- 
ever radio-activity is observed, it is always accompanied by some 
change which can be detected by the appearance of new products 
having chemical properties distinct from those of the original 
substances. This leads to some form of “chemical” theory, and 
other results show that the change is atomic and not molecular. 
200. Theory of radio-active change. The processes occur- 
ring in the radio-elements are of a character quite distinct from any 
previously observed in chemistry. Although it has been shown 
that the radio-activity is due to the spontaneous and continuous 
production of new types of active matter, the laws which control 
