The employment of various artificially radioactive elements as “tracers” has been reported in a considerable number of biological and physiological studies. Atoms of a radioactive isotope of any element are chemically indistinguishable and inseparable from atoms of its normal stable isotopes. If they are administered to a living organism, the result is the same as from administration of the stable isotope, unless such a large amount is given that the accompanying radiation produces an effect. Each radioactive atom, however, in disintegrating emits beta or gamma radiation (or both) which can be detected in various ways. Any desired substance can thus be “tagged” by mixing with the stable atoms a relatively very small number of radioactive ones. The tagged atoms continue to be mingled with the normal ones administered at the same time, in the same proportions throughout their whole course through the body, except for radioactive decay, which can be allowed for if necessary. Hence, by determining the activit...