6 SODDY, Evolution of Matter by Radio-active Elements. 



which it exists in the mineral is probably less than one part 

 in a million, and many tons of the ore had to be worked" 

 up before enough of the new element was obtained for 

 investigation. The radio-activity on the other hand is 

 enormous, being at least a million times greater than that 

 of either uranium or thorium. Mme. Curie also obtained 

 evidence of the existence of another new element, which' 

 she named polonium, and Debierne of a third, actinium,, 

 both present in pitchblende in such minute quantity that 

 their isolation as specific substances has not yet been 

 effected. Radium however has been obtained in sufficient 

 quantity for its atomic weight and spectrum reaction to 

 be determined, and these leave no doubt whatever of its 

 elementary nature. Its atomic weight, as determined by 

 Mme. Curie, is 225, while that of thorium is 232, and of 

 uranium 238, and these three are the highest known.. 

 Radio-activity is thus the property of the three heaviest 

 types of atoms at present recognised. 



The reason whyit is necessary to consider that polonium 

 and actinium are new elements, although neither has yet 

 been isolated, is a point of some fundamental importance. 

 Radio-activity is a property of the atom of the element in 

 question which cannot be modified in extent or altered in. 

 quality by any known means. It can be detected, by the 

 aid of the electrical method, with quantities of the element 

 far smaller than can be directly recognised. In the case 

 of radium, although its spectrum reaction is one of the 

 most characteristic and easily detected, a quantity of the 

 order of a million times less than the spectroscope would 

 recognise, can be readily distinguished by its radio-activity- 

 The nature of the radio-activity of any one element is 

 not only an unalterable property of the element, but can 

 readily be employed to distinguish and differentiate it 

 from other elements possessing the property. It is not 



