Manchester Memoirs, Vol. xlviii. (1904), No. 8. 7 



possible, for example, that the activity of uranium can be 

 due to a trace of admixed radium. Polonium is dis- 

 tinguished from all the other types by the fact that its 

 radiation is entirely of the a or non-penetrating kind. 

 Actinium is distinguished by a characteristic emanation, 

 which loses its activity much more rapidly than those 

 produced by either thorium or radium. The cases con- 

 sidered constitute at least five specific examples of radio- 

 activity, and these require the existence of a similar number 

 of radio-elements. It is interesting to note that this con- 

 clusion is arrived at without appeal to a single chemical 

 property of the matter in question, A new radio-element 

 may be recognised by the means indicated, even when 

 enough of the substance has never been obtained either 

 for its atomic weight, spectrum reaction, or any chemical 

 property to be determined. Knowledge with respect to 

 these properties will follow, as in the case of radium, when 

 a sufficient quantity is accumulated. But this sufficiency 

 may be of the order of a billion times greater than that 

 required for a complete examination of its radio-active 

 properties. 



The five elements considered differ enormously in the 

 extent of their radio-activity. Uranium and thorium are 

 so feebly active that they had been studied in the past 

 for something like a century before their radio-activity was 

 discovered. Radium was suspected on account of its 

 powerful activity, and for a long time in the early stages 

 of the search this was the sole evidence of its existence, 

 while with actinium and polonium our knowledge has not 

 yet advanced beyond this stage. But the general character 

 of the a radiations emitted is similar for each case, and all 

 but polonium emit j3 and probably 7 rays also. Although 

 the characteristics of any one type of radiation vary 

 somewhat in the different elements, the differences are not 



