224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



certain dilutions of radioactive phosphorus and mercury, and this is usually the 

 case with artificial radioactive substances where the degree of "concentration" 

 of the radioactivity depends upon the preparation of the substance. 



Hevesy's applications of radioactive substances were not confined 

 to exchange experiments. He realized also the significance of radio- 

 activity measurements as a substitute for chemical analyses, owing to 

 the great ease and sensitivity of the method as illustrated by our first 

 examples, and he thus also made use of "untrue" applications of in- 

 dicators. Once, when he had reason to suspect the cleanliness of his 

 landlady, he smeared a bit of "dirt" with a radioactive substance on 

 his dinner plate and checked daily whether the plate had been prop- 

 erly washed simply by measuring the radioactivity which (literally) 

 still clung to it. He was indeed able to detect radioactivity of the 

 plate for many days. Whether or not this was to be ascribed to the 

 carelessness of the landlady or to the extreme sensitivity of the method, 

 history fails to relate. 



DENOMINATION OF THE METHOD 



Hevesy called a radioactive isotope used for the experiments de- 

 scribed, an indicator, and thus following his example one often speaks 

 of the indicator method. In recent years in English-speaking coun- 

 tries the terms "tracer method" and "tracer atoms" have become more 

 usual: The radioactive isotopes are used, as it were, for discovering 

 and following a trace. "Labeled" and "tagged" atoms are also often 

 spoken of. The terms speak for themselves. Finally, to complete the 

 list, we may mention the denoting of these atoms as "spies," as pro- 

 posed by Evans.* This name is meant to indicate that each atom of a 

 radioactive isotope can move about unrecognized in a "crowd" of even 

 similar atoms until at a certain moment it "betrays" its presence and 

 whereabouts by its disintegration. The concentration of "spies" in the 

 experiments usually lies between 1 to 10^° and 1 to 10^' normal indi- 

 viduals. Translated into terms of human society, this would be 

 equivalent to one spy among a population at least five times as large 

 as that of the whole earth. 



FURTHER EXAMPLES OF THE APPLICATION OF TRACERS 



The examples which will be discussed in the following in unrelated 

 order, will give the reader an idea of the multifarious nature of the 

 applications of indicators. In order to reduce them to some kind of 

 systematic order we have sorted out the examples into three groups 

 according to the character of the problem. In the first group the prob- 

 lem is only where something is situated (localization), in the second 



» R. D. Evans, Applied Nuclear Physics, Journ. Appl. Phys., toI. 12, p. 260, 1941. 



