166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



CP* is more nearly identical in properties to the natural isotope 

 CI" than is the other natural isotope CP^. And again I would say 

 that the radioactive characteristic of CP* becomes evident only at the 

 moment it blows up to turn into the neighbor element sulfur. 



RADIOACTIVE TRACER ATOMS 



In these radioactive transformations of the artificial radioactive 

 isotopes, the radiations given off are so energetic that the radiations 

 from individual atoms can be detected with rugged and reliable 

 instruments, called Geiger counters. Thus, radioactive isotopes can 

 be admixed with ordinary chemicals to serve as tracer elements in 

 complicated chemical or biological processes. 



As an illustration of the power of this new technique of labeling 

 and tracing atoms, let us consider iodine in relation to the thyroid 

 gland. It is well known that the thyroid takes up and stores iodine, 

 and this fact can be demonstrated strikingly by feeding an individual 

 iodine including a small quantity of radioactive iodine. Before the 

 feeding, the radioactivity of the food can be measured by placing it 

 near a Geiger counter, thereby giving a measure of the iodine con- 

 tent. Later the progress of the iodine through the body can be 

 observed by placing the Geiger counter next to various parts of the 

 body. Likewise, the proportion of the fed iodine in the various 

 body fluids at any time can be determined quickly by taking small 

 samples of the fluids and measuring their radioactivity. After some 

 hours it is found that a large part of the iodine taken in has col- 

 lected in the thyroid, a fact that is readily established by placing a 

 Geiger counter next to the gland (pi. 1, fig. 1) and observing the 

 activity while finding no appreciable activity elsewhere. This tech- 

 nique makes it possible to study the behavoir of the thyroid in 

 health and in disease, and much interesting work along this line has 

 been carried out recently. 



RADIO-AUTOGRAPHY 



Although the tracer elements are readily detected with the Geiger 

 counter, there is a photographic method which for many purposes 

 has obvious advantages. This method is sometimes called radio- 

 autography and is illustrated by plate 1, figure 2. Here a minute 

 amount of radioactive phosphorus in the form of sodium phosphate 

 was added to the nutrient solution of a tomato plant, and after a 

 day or so leaves were placed against a photographic film enclosed in a 

 light tight paper envelope. The penetrating rays from the radio- 

 active phosphorus produced the developed contact image shown in 

 the figure, which gives an accurate and detailed picture of the uptake 

 of phosphate by the plant. Now, indeed, the same method works 



