222 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 7 



Where radioactive isotopes are applied for the sake of the advan- 

 tages of greater sensitivity and easier working, while in principle other 

 methods would also provide an answer to the questions raised, one 

 might speak of "untrue" applications of the indicator method. In 

 such cases the nature of the isotope is sometimes a matter of indiffer- 

 ence; in order to obtain a radiogram of the grain boundaries in a 

 polycrystalline material it would also be possible to use a radioactive 

 isotope of some metal other than lead, provided it does not dissolve in 

 the base metal. In the "true" applications of the tracer method it is 

 quite different, for there it is essential that an isotope can be detected 

 in identical surroundings. 



In this connection attention should be called to the fact that the 

 tracer method can also be applied with nonradioactive isotopes. The 

 atoms of such isotopes are recognizable (labeled) by their different 

 atomic weight and the properties connected therewith, such as specific 

 weight, velocity of diffusion, heat conductivity, etc. The most im- 

 portant atoms to be considered are "heavy hydrogen" (deuterium) of 

 atomic weight 2 (approximate), the oxygen isotope of atomic weight 

 18, and the nitrogen isotope of atomic weight 15. With such stable 

 isotopes the measurement of the radioactivity of mixtures of isotopes 

 is replaced by measurements of density or the like. 



These measurements are generally much less easy than the measure- 

 ment of radioactivity and also not so sensitive by far. With stable 

 isotopes there is, therefore, no question of "untrue" applications of the 

 tracer method. The reason for using these is solely the possibility of 

 studying processes of exchange, where no suitable radioactive isotopes 

 can be found. 



Owing to the very large number of applications of the indicator 

 method (true and untrue) it has become impossible, as well as purpose- 

 less, to give a survey of these applications, even if one confined oneself 

 to a definite field. We shall not, therefore, attempt to do so, but in the 

 following we shall say something about the origin of the method and 

 follow this up with a number of suitably chosen examples, with the 

 intention of showing the possibilities of the method from different 

 angles. In a subsequent article we shall go more deeply into the 

 practical performance of investigations with radioactive and also 

 with stable isotopes. As to this practical performance we can only 

 point out here that it is not necessary to prepare the radioactive (or 

 stable) isotopes oneself, for they can be obtained from certain suitably 

 equipped laboratories. In Europe the Philips Laboratory in Eind- 

 hoven, among others, has already supplied suitable radioactive sub- 

 stances for a number of applications. 



