274 MODERN SCIENCE READER 



per hour, and that without the state of the substance being 

 appreciably altered during many years. 



This extremely remarkable fact establishes a fundamental 

 distinction between radium and ordinary elements, and is 

 in accord with the actual conception which attributes 

 radioactivity to a transformation of the atom. 



The radioactive substances may possess a constant activ- 

 ity, at least, apparently, within the limits of our observa- 

 tions: such are uranium, thorium, radium, actinium. In 

 other substances, e. g., in polonium, a slow diminution of 

 activity in the lapse of time has been observed. 



Lastly, radioactive phenomena of much shorter duration 

 still, have been observed. 



. Thus, radium, thorium, and actinium disengage contin- 

 uously radioactive gases called emanations, whose activity 

 in time disappears; quite slowly in the case of radium, 

 very rapidly in the case of thorium and actinium. 



These emanations themselves produce on the exposed 

 surfaces active deposits which also disappear in the course 

 of a few hours or days. This is the phenomena of induced 

 radioactivity. 



We can, also, by means of suitable chemical reactions, 

 separate from uranium or thorium radioactive substances 

 which are continuously produced by these bodies, and whose 

 activity disappears progressively in a few months. 



All these phenomena can be explained satisfactorily by 

 admitting the production and destruction of radioactive 

 matter according to precisely determined laws. 



The radioactive properties are in fact very varied; the 

 diverse forms of ephemeral radioactivity are distinguished 

 from each other by the nature of the rays emitted, and by 

 the rapidity of the disappearance. 



It may be admitted that the production or the destruction 

 of a distinct form of radioactivity corresponds to the pro- 

 duction or destruction of a chemically distinct substance, 

 and since radioactivity is an atomic phenomenon, it con- 

 cerns the production and destruction of atoms. 



