288 DUANE— THE RADIO-ACTIVE POINT OF VIEW. 



mum estimate only. The lead and uranium cannot have been in 

 contact with each other for a period of time longer than that calcu- 

 lated from the known rate of transformation of uranium into lead. 



The atomic weight of the lead in a few ores has been found to 

 be very close to 206. In one of these the age of the mineral has 

 been estimated at a little over 900 millions of years. 



The calculation of the age of uranium deposits by means of radio- 

 active data rests upon the laws of nature as we now believe them to 

 be. It would be a waste of time to speculate on future discoveries 

 (new radio-active elements, for instance, or alterations in the rates 

 of radio-active processes) or on a possible evolution of natural law. 



The ages calculated from radio-active data represent the length 

 of time during which we may suppose the chemical elements to have 

 been in more or less mechanical contact with each other. They do 

 not represent the time that has elapsed since the earth may have 

 reached a state capable of supporting organic life as we now know it. 



Harvard University Medical School^ 

 Boston, Mass. 



