1 6 SODDY, Evohitioti of Matter by Radio-active Elements. 



to completion in a period of time varying from a few 

 weeks to a few seconds, as in the case of the best known 

 metabolons, it follows that the energy-emission in unit 

 time would be correspondingly greater, and that a pro- 

 portionately smaller quantity would come within the 

 range of experimental detection. The quantities of 

 the recently discovered transition-forms thus brought 

 within our powers of knowledge are more appropriately 

 expressed in multiples of the number of atoms, than 

 in grams or similar measure. The energy of a single 

 a particle can be calculated from its mass and velocity,, 

 and shown to be between lO"^ and lO'^ erg, and 

 this is certainly not greatly below the limit of detection 

 by the electrical method. Indeed it is still open to- 

 question whether the scintillations discovered by Sir 

 William Crookes^ in the phosphorescence of zinc sulphide 

 exposed to the a rays of radium are not caused by the 

 several impacts of the individual particles, — that we are 

 not in fact witnessing the effects of single atoms of 

 matter. 



In the accompanying table the course of the disin- 

 tegration of the three best studied radio-elements is given 

 so far as it has yet been experimentally traced. 



It will be noticed that the resources of the nomen- 

 clature available hardly stand the strain of providing 

 names for the numerous transition-forms now known. 

 Rutherford has recently suggested the term " Emanation 

 X " for the matter causing the ' excited ' or ' induced ' 

 activity of radium and thorium. No detailed account can 

 be here given of the various new bodies, or the manner irr 

 which they have been separated from the parent-elements. 

 In these separations one relies on the ordinary material 



^ The same phenomenon was independently discovered by Elster and 

 Geitel for the radio-active matter obtained from the atmosphere. 



