996 ADVENTURES IN RADIOISOTOPE RESEARCH 



by directing our attention to trains of thoughts not considered previously - 

 Some of these ideas I have tried to outline in this lecture. It also induced 

 the experimental chemist to take an interest in the history of the atoms 

 and molecules with which he is dealing. 



The chemist is not a historian, he is not interested in the problem 

 whether the carbon atoms of the benzene he is experimenting with 

 were formerly stored in Welsh coal deposits, in carbon dioxide of a 

 volcanic outburst, in the carbonate of Crustacea shells, or in a mammalian 

 skeleton. In contrast to the classical chemist, the indicator chemist is 

 to some extent a historian, highly interested in the past of atoms, mole- 

 cules, and molecular aggregates. He has a great concern in the distinc- 

 tion of how far molecules present in the tissue are "old" or "new". He 

 wishes to know when the potassium atoms present in the tissue cells 

 left the circulation, when the nucleic acid molecules present in the nuclei 

 of thymus cells were formed. He is interested in questions like the former 

 abode or abodes of the carbon atoms of glycogen, the origin of faeces 

 constituents, whether they originate from undigested food or are due to 

 endogenous secretion. He may desire to know which calcium, phosphorus, 

 nitrogen, or sulphur atoms of the plant originate from the soil and 

 which from the added fertilizer, possibly even which from the fertilizing 

 pollen. 



Many of the problems attacked by the tracer chemist are such as had 

 been solved previously. The application of isotopic indicators often 

 led to a remarkable simplification of earlier methods. This is the case 

 when measuring circulation rates, water contents, blood volumes, etc. 

 On the other hand, the application of isotopic indicators opened the 

 only way to determine the rate, place, and sequence of formation of 

 many molecular constituents of the living organism. The very existence 

 of such methods was instrumental in opening new trains of thought, in 

 demonstrating the dynamicity of metabolic processes, in confirming 

 Hopkins's statement that "life is a dynamic equilibrium in a poly- 

 phasic system," in concentrating our interest on the problem of the 

 velocity of the fundamental biological processes. 



Lord Rutherford, who— as Niels Bohr, the preceding Faraday 

 lecturer, so appropriately stated— contributed more than anyone else 

 to the most remarkable development which followed Becquerel's 

 discovery, remarked, when delivering his Faraday Lecture in 1936: 

 "We can now look back with some sense of perspective and recognize 

 the extraordinary importance of the discovery of radioactivity and the 

 profound influence on a knowledge of atoms and the relation of the 

 elements which has followed from a detailed study of the radioactive 

 bodies." It was this detailed study and the following most remarkable 

 development which made possible the application of radioactive sub- 

 stances as indicators. 



