DISCUSSION ON BACTERIAL RESPIRATION 257 



The chief feature in favor of the stable isotopes is, of course, their 

 stabihty. The extreme speed necessary in handhng radioactive iso- 

 topes of short half-hfe, such as C" and W^, has led to some un- 

 fortunate errors, as has been pointed out by Dr. Wood. With the 

 stable isotopes time is not a factor, and compounds can be separated 

 and analyzed at leisure. When isotopes become available for general 

 distribution, the stable forms may be stocked vi^ithout decomposition. 



The objection that radiations from radioactive tracers may injure 

 tissues is scarcely a serious one, since in most cases there is a wide 

 margin between the level of radioactivity necessary for measurement 

 and the level that will be injurious. With the stable tracers the ques- 

 tion of radiation does not arise. Only in the case of deuterium, with 

 its marked differences in properties from its analogue, have injury 

 eflFects been noted when high concentrations of a stable isotope were 

 present. 



Although the stable isotopes have not been prepared in as great 

 variety as the radioactive isotopes, the elements of chief interest to 

 the biologist have been concentrated. H- (deuterium), C^^, N^^, O^^, 

 and S^* have been concentrated by Urey. Deuterium is an item of 

 commerce, and N^^ and C" may be on the market soon. 



For quantitative experimentation the stable isotopes are far su- 

 perior to the radioactive isotopes. Rittenberg has found that in the 

 analysis of N^^ with the mass spectrometer he can expect a precision 

 of ± .003 atom per cent N^^. To obtain comparable precision in meas- 

 uring a radioactive substance with a Geiger counter it would be 

 necessary to count in the order of a million impulses. If a Geiger 

 counter capable of handling a thousand impulses a minute were 

 available, and the sample under examination were suflBciently con- 

 centrated to give this output of charged particles, sixteen and two- 

 thirds hours would be required to register a million counts. It is 

 obvious that radioactive nitrogen with a half -life of 10.5 minutes or 

 radioactive carbon with a half-life of 20.5 minutes could not be 

 measured with precision. Nor is short half -life the only factor that 

 interferes with the quantitative measurement of radioactive isotopes; 

 many of the radioactive isotopes with a long half -life (C", S^^, Fe^^, 

 etc.) emit such soft radiations that measurement is extremely diffi- 

 cult, and background counts constitute a considerable percentage of 

 the total counts. Radioactive phosphorus is an ideal tracer with a 

 long half -life and an intense radiation, and in this case precise quan- 

 titative measurements can be obtained. But with the elements of 

 chief interest to the biologist— carbon, hydrogen, and nitrogen— the 



