ISOTOPES — ATEN AND HEYN 223 



ORIGIN OF THE METHOD 



The tracer method was initiated by Hevesy, who first discovered the 

 possibility of studying processes of exchange by that means and im- 

 mediately put his ideas into practice (in 1915). He used the method, 

 for instance, to test the theory of Arrhenius about the dissociation of 

 electrolytes. In essence his experiment was as follows. From a cer- 

 tain amount of normal lead a lead salt is prepared, for instance lead 

 chloride, and from a corresponding amount of the radioactive lead 

 isotope, which is formed as a disintegration product of radium, another 

 salt, for instance lead nitrate, is prepared. When the two salts are 

 dissolved in water, the solutions mixed, and then the two salts extracted 

 separately from the mixture, the two lead compounds are found to 

 have become equally radioactive. 



The lead atoms from the two salts must, therefore, have been com- 

 pletely mixed in the solution. This result agrees entirely with the 

 hypothesis that the lead compounds are dissociated in the solution, 

 i. e., that lead occurs therein in the form of free ions. 



We have just said that one salt was prepared from normal lead and 

 the other from the radioactive lead isotope. Consequently, in order 

 to carry out the experiment in this way a sample of the pure radio- 

 active lead isotope would have to be available. Actually, however, this 

 is not necessar3\ It is sufficient if one sample of lead contains only 

 a small amount of the radioactive isotope. Hevesy recognized this 

 from the very beginning, as may be seen from the curious story of 

 the way in which he came to use the isotopes in this way. He had 

 tried in vain to separate radium D from a quantity of lead containing 

 a small amount of that substance. Since radium D is an isotope of lead 

 (the radioactive isotope mentioned in the radium series; its name dates 

 from the time when there was no clear idea of the situation) , it cannot, 

 as we now know, be separated by ordinary chemical means. It was 

 just this failure that gave Hevesy the idea that he could always dis- 

 tinguish the lead of this sample "contaminated" with radium D from 

 a sample of ordinary lead : in all mixtures with ordinary lead every 

 fraction of the "contaminated" (radioactive) lead sample takes an 

 equal fraction of the original radioactivity with it and can thus be 

 determined quantitatively by measurement of the radioactivity. 



The "contaminated" lead is thus, as it were, indicated or labeled by tlie radio- 

 active isotope itself, and provided it is a homogeneous mixture the whole sample 

 can serve as a quantity of labeled atoms. Tliis is in fact obvious when it is borne 

 in mind that the radioactivity of an element only means that per unit of time 

 a certain percentage of the atoms present in a sample disintegrates spontane- 

 ously. If the sample also contains a number of isotopic atoms which are stable 

 and thus will never disintegrate, the only result, in the first instance, is that the 

 percentage of disintegrating atoms per unit of time, is smaller, thus the radio- 

 activity is "diluted." In fact, also in the first examples discussed there were 

 777488 — iS 17 



