252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



Further, it is well established that a gram of uranium as found 

 along with its products in rocks and minerals is now changing at a 

 rate represented by the production of 1.88 XlO" 11 grams of helium and 

 1.22X10 -10 grams of lead isotope per annum. We have not time this 

 morning to consider the methods by which these figures have been 

 reached. It must suffice to say that in the case of helium it amounts 

 practically to direct observation, while in the case of lead isotope the 

 evidence, though less direct, is very strong, and, so far as I am aware, 

 is not contested by any student of the subject. I have said that this 

 is the rate at which one gram of uranium as found in the earth is pro- 

 ducing helium and lead isotope at present. It is important to inquire 

 whether one gram of uranium did the same in the past. This we can 

 not, of course, determine directly. It is certain that nothing we can do 

 in a laboratory in the way of change of temperature and pressure can 

 alter the rate sensibly, and enough has been done in this way to make 

 it unlikely that any pressures and temperatures encountered in the 

 superficial parts of the earth could have such an effect. It has been 

 suggested by Professor Joly that the absolute age of a gram of 

 uranium may affect its rate of disintegration. All possibilities should 

 be considered, but this suggestion derives no support from the be- 

 havior of the short-lived radioactive substances the behavior of which 

 we can watch. 



Upon the whole, therefore, it would seem that in the disintegra- 

 tion of a gram of uranium we have a process the rate of which can be 

 relied upon to have been the same in the past as we now observe it 



to be. 



The application is either to individual uranium minerals or to the 

 earth's crust as a whole. Taking first the minerals containing ura- 

 nium, these are found in all cases to contain helium and lead. The 

 helium in them, which appears to be retained mechanically, may safely 

 be treated as wholly a radioactive product. The lead in some cases 

 conforms closely to the expected atomic weight of 206, about one unit 

 lower than common lead, and in such cases we may safely regard the 

 whole of it as a product of uranium disintegration. 



Thus take the broggerite found in the pre-Cambrian rocks at 

 Moss, Norway. The lead in this mineral has an atomic weight of 

 206.06 as determined by Honigschmied and Fraulein St. Horovitz. 

 The ratio of lead to uranium is 1 : 3. Taking the lead as all produced 

 by uranium at the rate above given, we get an age of 925,000,000 years. 

 Some minerals from other archsean rocks in Norway give a rather 

 longer age. 



In other cases there is some complication, owing to the fact that 

 thorium is associated with uranium in the mineral and that it, too, 

 produces helium and an isotope of lead of atomic weight probably 

 208 exactly, about one unit higher than common lead. 



