THE MYSTERY OF MATTER. 189 



of some other eleinent, and that it is itself as constantly 

 being transformed into some more stable elementary siibstance. 

 Rutherford is strongly of opinion that ui-aniiim is the parent 

 substance from which radivun is derived. Uranium itself emits 

 only a rays ; it is transformed into uranimn X, which emits only 

 yS and 7 rays ; and uranium X is transformed into what seems to 

 be a rayless product. Rutherford thinks that uranium X probably 

 undergoes one or more further changes of a long period, possibly 

 rayless in character, and is finally transformed into radium. 

 Rutherford also considers that the final product into which 

 radium changes is probably lead. He mentions in corroboration 

 of his views that uranium and raditun are always associated 

 together in the minerals which contain them in certain definite 

 and unvarying proportions, and that lead is always associated 

 with radio-active minerals, especially such as are rich in uranium. 

 It is important to note that some of the radium transformation- 

 products are rayless ; that is to say, they are transformed into 

 other substances without any evidence of the emission of 

 a or fi particles. This may be explained in two different ways. 

 First we may consider that the transformation consists in a 

 re-arrangement of the component parts of the atom to form 

 a new temporarily stable system. On the other hypothesis we 

 may suppose the transformation to be similar in character to the 

 change which is accompanied by the emission of a rays ; the 

 only difference being that the a particles, although emitted with 

 great speed, do not travel with a velocity sufficient to cause 

 appreciable ionisation of the gases through which they pass, and 

 so escape observation. Rutherford seems to consider the latt«r 

 explanation to be the more probable. 



Each of the other radio-active elements, thorium and actinium, 

 passes through a series of somewhat similar changes. Rutherford, 

 however, considers that actinium is not in itself radio-active, 

 but that the activity which it manifests is entirely due to its 

 decomposition-products, and he also thinks that the radio- 

 activity of piire thorium when freed from its transformation- 

 products is very doubtful. 



Thorium is one of the rarer elements, and is found in thorite 

 (a sihcate of thoriiun) and various other minerals. Its oxide 

 thorin is the principal constituent of the Welsbach gas-mantles 

 used for lighting piu-poses. The activity of thorium is much 

 the same as that of uranium in amount. An interesting 

 experiment may be made by spUtting up a Welsbach mantle and 

 placing it on a prepared photographic plate, wrapping up the 



VOL. XIII. — PAKT III. 14 



