x] RADIO-ACTIVE PROCESSES 325 
must ipso facto be undergoing change, it follows that none of the 
active products, for example, the emanations and Th X, can consist 
of any known kind of matter; for there is no evidence to show that 
inactive matter can be made radio-active, or that two forms of the 
same element can exist, one radio-active and the other not. For 
example, half of the matter constituting the radium emanation 
has undergone change after an interval of four days. After the 
lapse of about one month the emanation as such has nearly 
disappeared, having been transformed through several stages into 
other and more stable types of matter, which are in consequence 
difficult to detect by their radio-activity. 
The following table gives the list of the active products or 
metabolons known to result from the disintegration of the three 
radio-elements. In the second column is given the value of the 
radio-active constant X for each active product, v.e. the proportion 
of the active matter undergoing change per second; in the third 
column, the time 7 required for the activity to fall to one-half, ve. 
the time taken for half the active product to undergo change; in 
the third column, the nature of the rays from each active product, 
not including the rays from the products which result from it; in 
the fourth column, a few of the more marked physical and chemical 
properties of each metabolon. 
There are two well-marked changes in uranium, five in 
thorium, and six in radium. It is quite probable that a closer 
examination of the active products may lead to the discovery of 
still further changes. For example, the evidence obtained by 
von Lerch (section 179) from the electrolysis of a solution of 
emanation X of thorium points to the conclusion that there is an 
additional change occurring in emanation X, for which the value 
of tis 3 or 4 hours. The experiments of Pegram (section 179) 
also suggest that another radio-active product, of which the value 
of 7 is about 3 minutes, is present in thorium. The change of 
thorium X into the emanation would have been difficult to detect 
if the product of the change had not been gaseous in character. 
Besides the changes mentioned above, it is thus quite possible 
that other and more rapid changes may be taking place which 
have not yet been detected. 
It has been pointed out in section 188 that the fourth change 
