ISOTOPES — ATEN AND HEYN 221 



WHY THE TRACER METHOD IS SO IMPORTANT 



From these few examples we can already deduce the most important 

 aspects which have lent such great significance to the tracer method. 



We first call attention to the last example discussed. The transfer 

 of material can also be measured when the two surfaces sliding over 

 each other contain the same metals or even when they are exactly 

 identical. It must be realized that this would not be possible by any 

 other known method, since the transfer takes place in both directions : 

 there is an exchange of identical particles. It would not be possible 

 by any chemical or physical method to ascertain the origin of the 

 metal present on one of the surfaces after the experiment, whereas 

 the radioactive isotope immediately gives the answer. 



Processes in which there is an exchange of identical particles are 

 very common in nature, not only in chemistry and metallurgy, but 

 especially in the physiology of plants and animals. The tracer method, 

 which offers the only method of approach, has been used on a large 

 scale for the investigation of such processes, and the publications in 

 that field are innumerable. In a survey of such investigations for 

 the year 1940 ^ in physiology alone more than 300 publications are 

 cited. But also in the field of technology, for routine tests, and like- 

 wise in agriculture and chemistry, the method is being more and more 

 widely applied.'^ 



Although the last example discussed illustrates the possibility of 

 studying the exchange of identical particles with the help of an isotope, 

 the employment of the tracer method in that case is not motivated 

 only by the possibility mentioned. When two different metals slide 

 over each other the transfer of material could in principle be studied 

 also by other methods. The fact that a radioactive tracer is, never- 

 theless, used, is due to the fact that a much greater sensitivity can be 

 attained with the radioactivity measurements, i. e., much smaller 

 amounts of a substance can be detected than with other methods so far 

 available. The same applies to the case of the determination of 

 mercury. In the determination of phosphorus, which was discussed as 

 the first example, the indicator method is not necessary in principle 

 either, but it was applied there because of the greater ease with whidi 

 the quantity of phosphorus could be determined, compared with a 

 chemical or other analysis. Finally, an important point in the em- 

 ployment of a radioactrve isotope is that it can be localized so easily 

 when mixed with a different or a chemically identical substance, while 

 also its distribution can be determined. (See the radiograms of pi. 1, 

 figs. 1 and 2.) 



« J. R. Loofbonrow, Rev. Mod. Phys., vol. 12, p. 267, 1940. 



T See, for example, the survey of chemical applications by G. Seaborp, Chem. Rev., vol. 27, 

 p. 199, 1940, where more than 500 publications are mentioned, most of them different from 

 those in the article by Loofbourow. 



