APPLICATIONS OF ISOTOPIC INDICATORS 935 



Very different results were obtained in all cases in which the lead 

 atoms were joined to carbon. Between lead chloride and lead tetraphenyl 

 in pyridine, between lead nitrate and lead tetraphenyl in amyl alcohol, 

 and between lead nitrate in aqueous ethyl alcohol, no change in the 

 places of lead atoms could be detected, although in every combination 

 investigated one of the molecular types was capable of electrolytic dis- 

 sociation. 



The lack of interchange of atoms present in organic binding [hydrogen 

 atoms bound to oxygen or nitrogen being an exception, as shown by 

 BoNHOEFFER^^^'], such as that of carbon atoms in glycogen or phos- 

 phorus atoms in lecithin with other carbon and phosphorus atoms re- 

 spectively, was found to be of great significance for the application of 

 isotopic indicators in biochemical research. Owing to the absence of 

 such an interchange, the presence of labelled carbon atoms in glycogen 

 molecules, or of labelled phosphorus atoms in lecithin molecules, ex- 

 tracted from the organs, proved that a synthesis of these molecules 

 took place after the labelled atoms were administered. This principle 

 enables us to distinguish between "old" and "new" molecules and to 

 determine the rates at which molecules of the different compounds are 

 built up and carried to the different organs. 



A prompt interchange of the electrical charges between Pb"*'+ and 

 Pb"'"'''"^"^ ions was found to take place in experiments where plumbeous 

 acetate and labelled plumbic acetate (or vice versa) were dissolved in 

 glacial acetic acid and then separated by crystallization^^^^ The same 

 holds for T1+ and T1+ + + ions*^^'. An interchange of lead atoms takes 

 place between fused lead and fused lead chloride, lead oxide or lead 

 sulphide^"°\ 



After artificially radioactive isotopes became available as indicators, 

 interchange processes were studied in numerous cases. A rapid inter- 

 change of charges was found to take place between Fe"*"^ and Fe "*'"'"'''. 

 Cu+ and Cu++, etc.^'^). 



ANALYTICAL CHEMISTRY 



Analytical chemistry proved to be a fruitful field for the application 

 of isotopic indicators. A knowledge of the total lead content of the 

 earth's crust, for example, is of considerable chemical interest. In view 

 of the small lead content of the average rock sample, the quantitative 

 determination of its lead content involves some difficulties. These have 

 been eliminated by making use of an isotopic indicator^"^\ An amount 

 of radium D, known in relative radioactive units, is added to the solution 

 of the rock sample ; the radium D is then recovered by electrolysis as 

 peroxyde. If 100 per cent of the added radium D is recovered, we may 



