ATOMIC NUCLEI — HAENWELL 201 



that atoms can be endowed with this radioactive property without 

 losing their ordinary chemical properties makes the work of much 

 greater importance than the discovery of any amount of radium 

 would be. As a single instance the thyroid gland is known to have 

 the property of assimilating iodine. It is possible to render iodine 

 radioactive in such a way that it decays with a rriean life of about 

 25 minutes with the emission of ordinary electrons. Thus radioactive 

 iodine injected into the body will tend to be localized in the thyroid 

 gland and affect it specifically in a way not possible with ordinary 

 radium. Also the atom resulting from the emission of the electron 

 is no longer radioactive and peculiarly inert chemically, which re- 

 moves any possible hazard of the treatment. In a similar way phos- 

 phorus and calcium which are bone constituents may be rendered 

 radioactive, localizing radioactive material in the bony structure. 



The applications in the closely associated field of physiology are 

 of equally great interest. As it is easy to detect the presence of a 

 very minute number of radioactive atoms among a great number of 

 the ordinary variety characteristic of the element, it is possible to 

 prepare samples of elements that retain their identity and may be 

 followed through physiological reactions. It has been reported, 

 though not as yet verified, that long-lived radioactive carbon atoms 

 can be prepared. These may form molecules of organic compounds 

 which can then be followed through the body, opening up a new 

 and highly important field of biological research. The physiologist 

 can investigate the details of processes for which no technique was 

 previously available, and hitherto unsuspected phenomena may be 

 uncovered. As an instance it is known that the thyroid assimilates 

 iodine, but if other glands assimilate selectively specific elements they 

 do it in such small quantities as to be undetectable at present. How- 

 ever, if the elements were introduced in a radioactive form, simple 

 and powerful techniques of detection could be employed which would 

 bring any selective assimilation to light. 



I can only refer even more briefly to the application of this 

 technique in other fields. Radioactive "tracer atoms" can be em- 

 ployed in chemistry to follow chemical elements through exchange 

 reactions. They may also be used to investigate the baffling prob- 

 lems of catalyst poisoning and surface adsorption. In metallurgy 

 and the physics of solids they may be used to study the diffusion of 

 atoms through single crystals and across and along crystal bound- 

 aries. Studies of this sort under various conditions of temperature 

 and strain should clarify many of the problems that are now pre- 

 sented by the solid state and lead to alloys with more desirable 

 properties for all the various applications. Applications to many 



