APPLICATIONS OF ISOTOPIC INDICATORS 939 



the corresponding difference between a ^^P and a ^^p atom is only 3 

 per cent. The very small difference in the chemical properties of ^^P 

 and ^^P remains at the present time within the errors of our experiments, 

 whereas between hydrogen and deuterium the difference is quite appre- 

 ciable. The same applies to HgO and T>^0. Dilute "heavy" water, how- 

 ever, contains mostly DOH molecules which exhibit in their chemical 

 behaviour a very great resemblance to HOH. 



In the study of the circulation of water in the organism, dilute heavy 

 water can therefore safely be used as an indicator. In the determination 

 of the life period of water molecules in the human organism, water 

 containing i/o per cent heavy water was used^^^\ While a small percent- 

 age (0.1) of the water was found to be excreted in so short a time as 

 26 minutes, the average life of the water molecules in the organism 

 was found to be 13.5 days. In the excreted water, molecules were thus 

 found which were taken in both a few minutes and several months 

 before the investigation. By extrapolation, however, we arrive at the 

 result that, though the number of water molecules present in an adult 

 organism amounts to as much as c. 10^', the adult organism no longer 

 contains a single water molecule taken up at birth. 



The rate of admixture of administered water with the water present in 

 the body was investigated in experiments on rabbits^^^' and guinea 

 p^gg'38) \Yhiig the water reaching the circulation was found to enter into 

 exchange equilibrium with the extracellular water (about 14 of the weight 

 of rabbit) in the course of a few minutes, the penetration of the water 

 molecules into the cells took some time. As is seen in Fig. 2, about 30 

 min passes before the exchange equilibrium is reached between the 

 water administered and extracellular and intracellular water present in 

 the tissues of the rabbit. In the guinea pig, 73 per cent of the water 

 in the blood is exchanged for extracellular water every minute. 



Time does not permit me to discuss the extended application of heavy 

 hydrogen, heavy carbon and heavy nitrogen, and to treat the numerous 

 important results obtained by the use of these isotopes as indicators. 



APPLICATION OF ARTIFICIALLY RADIOACTIVE ISOTOPES 



During the lengthy operations preceding the early experiments of 

 self- diffusion in lead, we often discussed the great progress which might 

 be expected if radioactive indicators of the common elements were 

 made available to chemical and biological research. This wish, which 

 seemed Utopian in those remote days, was fulfilled by Frederic Joliot 

 and Irene Joliot-Curie's^^^^ fundamental discovery of artificial 

 radioactivity, followed by Fermi's^^^^ work leading to the discovery 

 of many more artificially radioactive isotopes. Soon after the announce- 



