576 The Evolution of Matter 
curve. Thus the activity of a rod exposed to radium emanation for 
1 minute decays in accordance with the curve of Fig. 2, which 
represents the activity as measured by the a-rays. If the electro- 
scope be screened from the a-rays, it is found that the activity of the 
rod in 8- and y-rays increases for some 35 minutes and then diminishes. 
(Fig. 3.) 
These complicated relations have been explained satisfactorily 
and completely by Rutherford on the hypothesis of successive changes 
of the radio-active matter into one new body after another’. The 
experimental curve represents the resultant activity of all the matter 
present at a given moment, and the process of disentangling the 
component effects covsists in finding a number of curves, which 
express the rise and fall of activity of each kind of matter as it is 
produced and decays, and, fitted together, give the curve of the 
experiments. 
Other methods of investigation also are open. They have enabled 
Rutherford to complete the life-history of radium and its products, 
and to clear up doubtful points left by the analysis of the curves. 
By the removal of the emanation, the activity of radium itself has 
been shown to consist solely of a-rays. This removal can be 
effected by passing air through the solution of a radium salt. The 
emanation comes away, and the activity of the deposit which it 
leaves behind decays rapidly to a small fraction of its initial 
value. Again, some of the active deposits of the emanation are 
more volatile than others, and can be separated from them by the 
agency of heat. 
From such evidence Rutherford has traced a long series of dis- 
integration products of radium, all but the first of which exist in 
much too minute quantities to be detected otherwise than by their 
radio-activities. Moreover, two of these products are not them- 
selves appreciably radio-active, though they are born from radio- 
active parents, and give rise to a series of radio-active descendants. 
Their presence is inferred from such evidence as the rise of 8 and y 
radio-activity in the solid newly deposited by the emanation ; this 
rise measuring the growth of the first radio-active offspring of one of 
the non-active bodies. Some of the radium products give out a-Yrays 
only, one 8- and y-rays, while one yields all three types of radiation. 
The pedigree of the radium family may be expressed in the following 
table, the time noted in the second column being the time re- 
quired for a given quantity to be half transformed into its next 
derivative. 
1 Rutherford, Radio-activity (2nd edit.), Cambridge, 1905, p. 379. 
