DUANE— THE RADIO-ACTIVE POINT OF VIEW. 287 



Among the radio-active changes appear processes in which the 

 metal uranium transforms itself through successions of intermediate 

 stages into the metal lead and into the gas helium. It does not seem 

 necessary to describe in detail these series of transformations at this 

 time. Descriptions of them may be found in the literature on radio- 

 activity. It sufifices for our purposes to say that the rate of trans- 

 formation is such that 5 per cent, of a quantity of uranium changes 

 into lead and helium in about 370 millions of years. 



We find uranium, lead and helium associated together in a great 

 many minerals and it is natural to suppose that the helium and the 

 lead were produced by the disintegration of the uranium during the 

 past ages. Further, if we determine the relative amounts of uran- 

 ium, lead and helium in a mineral we can form an estimate as to how 

 long these chemical elements have been in contact with each other. 

 Estimates of this kind that have been made from the quantities of 

 helium in uranium ores vary between 8 and 700 millions of years, 

 according to the locality from which the ore came. Since some of 

 the helium (it being a gas) may have leaked out of the ores these 

 intervals of time must be regarded as minimum estimates. The 

 uranium and helium must have been in contact with each other for 

 at least as long as the periods mentioned, but they may have been 

 together for much longer intervals of time. 



Calculations based on the quantity of lead in uranium ores vary 

 from 340 millions to 1.700 millions of years, according to the locality 

 from which the ore is obtained. In this case another complication 

 appears. We have learned to distinguish several different kinds of 

 lead from each other. The various kinds of lead have similar chem- 

 ical properties but differ from each other in their atomic weights. 

 All the different kinds of lead do not come from uranium ; only lead of 

 atomic weight about 206 may be regarded as produced from uranium. 

 Until, therefore, we have determined exactly what the atomic weights 

 of the lead in the various ores really are, we cannot be sure that the 

 lead came from the uranium. We can assert, however, that there 

 is no more uranium lead in a given uranium ore than the amount 

 of lead actually found. Unless, therefore, the atomic weight of the 

 lead in an ore has been actually determined and found to be about 

 206, we must consider the estimate of the age of the ore as a maxi- 



