Pace Ten 



EVOLUTION 



February, 1928 



How Old Is the World? 



THE EVIDENCE FROM RADIO-ACTIVITY 

 By Allan Strong Broms 



TN the last few years a very reliable test 

 ■*■ of the age of the earth has been found 

 in the breakdown of the radioactive ele- 

 ments uranium and thorium into ordinary 

 lead. We can measure the rates of break- 

 down in our laboratories by counting the 

 number of helium atoms released in the 

 process. The rates prove to be so very 

 slow that one-half of a particle of thorium 

 needs five billion years to "transmute" 



into lead, while one-half of a particle of 

 uranium takes even longer, about thirteen 

 billion years. To transmute one-half of 

 the remaining particle takes another five 

 or thirteen billion years, and so on in- 

 definitely. These rates are unchanging, 

 for heat, cold, pressure, electricity, any- 

 thing we can do, seem to have no effect. 

 The elements uranium and thorium are 

 found in small quantities in rocks of 

 various geologic ages and if we can only 

 determine how long they have been break- 

 ing down since they were included in those 

 rocks, we will know just how old the rocks 

 are. This very thing has been done by a 

 number of scientists, among whom Pro- 

 fessors Soddy and Joly are conspicu- 

 ous. It has been done in three quite 

 independent ways, with results fairly 

 in agreement. 



Uranium is the heaviest of the 

 elements, its atom weighing about 

 238 times as much as that of hydro- 

 gen, its atomic weight being there- 

 fore 238. It breaks down by a series 

 of steps, at certain of which it gives 

 off helium atoms (atomic weight 4) 

 and thus loses weight until it winds 

 up as a kind of lead having an 

 atomic weight of 206. Thorium, 

 which starts with an atomic weight 

 of 232, also breaks down by steps, 

 loses helium atoms and weight and 

 ends as another kind of lead of 

 atomic weight 208. Our ordinary 

 lead, which weighs about 207, is a 

 mixture of these two pure kinds. 



The Ubanium-Lead Ratio 

 If a mineral is found to contain uran- 

 ium (but no thorium) and lead of atomic 

 weight 206, we need only measure their 

 relative amounts, apply a simple mathe- 

 matical formula and learn how long the 

 uranium must have been breaking down to 

 produce that certain proportion of lead. 

 The longer the time, the more lead there 

 will be and the less uranium. Many in- 

 vestigators, analysing rocks from widely 

 separated parts of the earth's surface, have 

 independently reached results remarkably 

 close, the ages for the oldest known rocks 

 ranging from one to one and a half bil- 

 lion years. The overlying, hence younger, 

 rocks always show a lower proportion of 

 lead, a very convincing test of the method. 

 Of course, the thorium-lead ratio can 

 be used instead. The ages indicated are 

 generally less, but the fundamentalist will 

 find no consolation in this, for they still 

 run into the hundreds of millions. 



The Uranium-Helium Ratio 

 In breaking down to lead, the uranium 

 atom gives off eight helium atoms and the 

 thorium atom gives off six. If we meas- 

 ure the relative amounts of uranium (or 

 thorium) and helium, we ought again to 

 be able to fix roughly the ages of the 

 rocks. But as helium is a gas and likely 

 to be lost from any but the most solid 

 rocks, the results should be somewhat 

 smaller and so we find they are. 



Discoloration Haloes 

 Professor Joly has made much use of 

 a third method depending on the fact that 

 a particle of uranium or thorium embedded 

 in certain colored micas will slowly dis- 

 color the nearby mineral matter, produc- 

 ing minute spherical shells of discolora- 

 tion. When viewed in cross-section, these 



appear as concentric rings or haloes. 

 They are produced by the helium atoms 

 shot out at the several steps of radioactive 

 breakdown. As they are shot out at dif- 

 fering (yet definite) speeds at each of the 

 several steps, they travel to different dis- 

 tances and thus produce clearly defined 

 rings. 



In the diagrams, these various penetra- 

 tion distances are shown for the helium 

 atoms from both uranium and thorium. 

 Each step in the breakdown is given its 

 consecutive number, each disintegration 

 product is named and its atomic weight 

 given. Obviously, the high speed helium 

 atoms fropi Radium C and Thorium C, as 

 they get no help from their brother atoms, 

 can discolor the outer rim of the haloes 

 but faintly. In the microphotograph of 

 the thorium haloes, this outer shell is clear. 



Note also in the diagrams that the heli- 

 um atoms from thorium penetrate further 



and produce larger haloes. The largest 

 uranium halo is only one four-hundredth 

 of an inch in diameter, while that from 

 thorium is one three-hundredth. The sizes 

 therefore identify the parent radioactive 

 substances, and the degrees of discolora- 

 tion measure the ages of the surrounding 

 mineral. Some of the oldest show very 

 clearly the effects of "over-exposure," just 

 like a photographic plate. Joly found 

 that the younger a rock was geologically, 

 the less discoloration there was. His es- 

 timates of rock ages based on this method 

 involve hundreds of millions of years. 



The three methods of fixing geologic 

 ages from radio-activity agree too closely 

 in their results to leave any serious doubts 

 as to the enormous age of the earth crust. 

 And before the earth crust — who knows 

 how many billions of years must have 

 passed? Certainly the fundamentalists' 

 six thousand years are as impossible as 

 the rest of their absurd notions. 



NEXT; Some more on radioactivity 

 and its part in the evolution of the earth. 



