312 RADIO-ACTIVE PROCESSES [CH. 
elements by the process of diffusion, These emanations have been 
shown to possess the properties of gases. The radium emanation 
diffuses and distributes itself between two reservoirs kept at 
different temperatures according to the laws of gases. Both the 
emanations can be condensed by cold, and by that means can be 
removed from the other gases with which they are mixed. The 
emanations possess the property of being occluded in some bodies, 
including the radio-elements themselves, and can be liberated by 
heating or solution. They diffuse through porous partitions, and 
in general behave like chemically inert gases of high molecular 
weight. Other radio-active products, for example the emanations 
X of radium and of thorium, are not gaseous, but attach themselves 
to the surface of bodies and can be removed from them by solution 
or heating. The emanation X of thorium, for example, possesses 
some chemical properties which distinguish it not only from the 
emanation from which it 1s derived, but also from the other active 
product Th X. It is soluble in some acids and not in others. If 
the emanation X of thorium is removed from an active wire by 
solution in hydrochloric acid, the active matter attaches itself to 
some metals dipped in the acid but not to others, and in general 
possesses the properties of matter of definite chemical behaviour. 
The differences in the chemical and physical properties of the 
different products of a radio-element can be well illustrated in the 
case of thorium. Thorium X is soluble in ammonia, while thorium 
is not. Thorium X produces the emanation which is a gas, in- 
soluble in acids but condensed by cold. This, in turn, produces 
the matter emanation X, which is insoluble in ammonia but soluble 
in hydrochloric and sulphuric acids. There can be little doubt 
that these active products are material in nature. They differ 
from ordinary matter in their power of emitting rays of a special 
character, and by the fact that they exist in the radio-elements in 
minute quantities which are, in most cases, too small to be detected 
by the balance or the spectroscope. Approximate estimates (sec- 
tion 162) can be made of the amount of these active products that 
are present in a radio-element when in radio-active equilibrium, 
and it has been shown that, except in the case of a very active 
body like radium, the amount is too small to be detected by 
ordinary chemical means. 
