842 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



varying his postulates and points to the lack of evidence of high tempera- 

 ture gradients in Paleozoic and Proterozoic times. 



In contemplating this elaborate and ingenious argument as compared 

 with the uncertain character of the data and the lack of agreement with 

 significant facts, one is reminded of Huxley's well known saying, that — 



"Mathematics may be compared to a mill of exquisite workmanship, which 

 grinds you stuff of any degree of fineness ; but nevertheless what you get out 

 depends upon what you put in; and as the finest mill in the world will not 

 extract wheat-flour from peascods, so pages of formulae will not get a definite 

 result out of loose data." 



Part IV. — Measurements of Time based on Eadioactivity 



OUTLINE OF THE THEORY 



The detection in 1896 of the Becquerel rays given out by uranium 

 minerals led up to the epoch-making discovery of radium. This was the 

 opening which led to the revelation of the whole series of radioactive ele- 

 ments whose parents, uranium and thorium, have the highest atomic 

 weights of all known elements. Each of these parental elements slowly 

 and regularly breaks down into a descendent series. In so doing they 

 liberate a, jS, and y-rays which enable the process of disintegration to be 

 detected and constitute the radiations discovered by Becquerel. The a-rays 

 are positively charged atoms of helium, shot out of the parent atoms 

 with enormous velocities. The ^-rays are negatively charged electrons, 

 having a mass of about 1/1,250 that of a hydrogen atom, and expelled 

 even more swiftly than the a-particles. y-rays are not material particles 

 in rapid motion, but appear to be of the same nature as X- or Eontgen- 

 rays. 



Confining attention to the uranium series, the evidence indicates that 

 eight successive ejections of helium atoms, separated by other and inter- 

 mediate ^ and y changes, result in the making of lead out of uranium. 

 The atomic weight of helium being 4.0 and uranium 238.2, it follows that 

 the atomic weight of lead derived from uranium should be 206.2. 



In the enormous number of atoms which exist in even a chemical traice 

 of a radioactive substance, a few pass each second into a condition of 

 instability and, by ejecting either an a or ^ particle, pass into another 

 atomic state. The number of atoms is so great that, as in the kinetic 

 theory of gases, the law of averages strictly applies. In the successive 

 products from uranium down to lead various degrees of stability are 

 shown, the more stable atom enduring on the average a longer time before 

 it passes into a configuration of iiitornal instability and as a result breaks 



