1023 



Comment to paper 98, 99, 100 



The History of the application of radioactive tracers covers 5 epochs From 

 1913 to 1932 only radioactive isotopes found in nature were applied in tracer 

 studies. From 1933 onwards first diluted, later concentrated deuterium and also 

 concentrated i^N and ibq were available for indicator studies mainly due to the 

 discoveries of Urey. The fundamental discovery of artificial radioactivity by 

 Frederic Johot and Irene Curie made it possible to label almost every element 

 From 1934 to 1937 artificial isotopes produced under the action of neutrons emitted 

 by radium-beryUium sources were applied in the production of radioactive tracers 

 The year 1937 witnessed the opening of a new epoch due to the availability of 

 strongly active samples produced by the cyclotron. Ernest Lawrence's ingenuity 

 and generosity made these radiactive bodies available. 



After the second world war the last epoch of the application of radioactive 

 tracers started. Pile produced radioactive isotopes of ahnost unHmited activity 

 became available, among others "C and 3H. Samples of mmute activity of these 

 radioactive bodies were available prior to that date, "c discovered in 1940 by 

 Ruben and Kamen became the most frequently applied radioactive isotope in 

 hte sciences, instead of the formerly most extensively used ^2?. 



The investigations described in paper 98 were aH carried out prior to the advent 

 ol the last epoch of tracer studies, in contrast to those discussed in paper 99 

 In paper 100 some outstanding studies covering aU 5 periods are described. It is 

 emphasized that while tracer methods made it possible to measure the rate of 

 renewal of body constituents, the knowledge that such a renewal takes place 

 IS iar from bemg a novel one. It goes back to the first part of the thirteenth 

 century and the first to arrive at this knowledge was St. Thomas Aquinas. When 

 pondering about what would happen at the day of resurrection to a man, who 

 had been a cannibal through all his Hfe, and whose ancestors might have been 

 canmbals as well, he arrived at the conclusion that "the identity of the body 

 IS not dependent on the persistence of the same material particles", and that 

 durmg life, by the process of eating and digesting the body undergoes perpetual 



