NUCLEAR PHYSICS — DUBRIDGE 237 



fairly kind in this respect in providing so many periods of suitable 

 length, and you may be sure that physicists will be busy for some 

 time to come finding new ones. 



2. Evidently a similar technique instead of being used on the body 

 as a whole may be applied to studies of metabolism within individual 

 organs, in nerves or muscles, even in individual cells. Many physi- 

 ologists have already recognized these possibilities, and some have 

 already planned programs along this line. 



3. As the chemical nature of viruses, of hormones, and of various 

 gland secretions become better known it should be possible to follow the 

 behavior of these important but elusive agents, or to gain further 

 information about their chemical composition. 



4. There are many purely chemical or biochemical problems to 

 which the tracing technique would seem applicable. Studies of chemi- 

 cal changes accompanying respiration would seem to be possible with 

 the use of radio-oxygen — though the known periods unfortunately 

 are rather short. The possibility of tracing individual carbon atoms 

 in various organic reactions might well open up a new field in organic 

 and biochemistry. 



I think these few examples will serve to illustrate the many possi- 

 bilities — some of which will lie along lines which no one can now 

 foresee. 



From what has been said you will see that the power of the radio- 

 active tracing technique results from the following fortunate facts: 



1. Kadioactive isotopes of an element are chemically identical with 

 the stable isotopes and will therefore behave in precisely the same way 

 in all chemical or biological processes. 



2. The active isotopes can be detected in extremely small quantities. 

 In some cases it is possible to detect the presence in a given sample 

 of as few as 1,000 atoms of a given isotope. The presence of a million 

 atoms would give accurately measurable results. This sensitivity 

 results from the fact that impulse counters of various types, such as 

 the Geiger-Miiller tube counter, will register each individual /3-ray 

 (+ or — electron) entering it. If the geometrical conditions are 

 properly chosen, from 0.1 to % of all the /3-rays arising in a given sample 

 may be made to enter the counter. The fundamental law of radio- 

 active decay is that the number of disintegrations occurring per 

 second is proportional to the total number of atoms present, i. e., 



dN/dt=-W, 



where X=0.693/T where T is the half-life in seconds. Thus for 

 T= 1,000 sec. = 17 min. and for N= 1,000 



dN/Jt=0.Q93 disintegrations per sec. 

 =41.5 per minute. 



