314 RADIO-ACTIVE PROCESSES fier. 
to half value in about four days. According to the views already 
advanced, it 1s to be expected that the volume of the emanation 
itself should decrease according to an exponential law with the 
time, falling to half value in about 3:7 days. Thisisa result of the 
theory that half of the emanation at the end of 3:7 days has been 
transformed into the substance emanation X, which behaves as a 
solid and is deposited on the surface of the containing vessel. It 
seems not improbable that the rapid decrease, observed during the 
first day, may have been due to the presence, with the emanation, 
of another gas which was quickly absorbed either by the walls of 
the capillary tube or by the mercury. 
It can readily be deduced that the volume of the emanation at 
the end of the first day is equivalent to the amount derived from 
0:022 gram of pure radium in radio-active equilibrium. Taking 
the volume of the gas, 0:°027 cubic millimetre, at the end of the 
first day as the true volume of emanation, it follows that the 
volume of the emanation to be obtained from 1 gram of radium 
in radio-active equilibrium is 1:2 cubic millimetres. Taking the 
volume observed on the third day, the corresponding value is 
0-9 cubic millimetre. The calculated value of the volume of the 
emanation to be derived from 1 gram of radium is 0°3 cubic 
millimetre. The calculated value is thus of the mght order of 
magnitude. This is an indication of the general correctness of 
the different methods of calculation (see sections 104 and 162) on 
which the theoretical determination of the volume of the emanation 
has been based. | 
It will be shown later, in section 201, that the emanation of . 
radium produces helium from itself. The shrinkage of the volume 
to a very small fraction of its original value indicates that the 
helium produced was buried in the walls of the tube. This is to 
be expected if the helium consists in reality of the a particles 
expelled from the emanation and its products. The a particle is 
projected with sufficient velocity to penetrate a distance of about 
‘02 millimetre into the walls of the capillary tube. It is to be 
expected that a portion, at least, of the buried helium should be 
released when the tube is strongly heated. ] 
Of the three types of rays from the active bodies, the a and 8 
