TRACER CHEMISTRY 291 the same token, while the amount of scientific knowledge which is at our disposal concerning the phyco- colloids appears fairly considerable, the complexity of the subject is such that many features and many avenues remain to be explored and it may be confidently expected that fields of application for products so far developed and for products still to be developed will be con- stantly widened. Indeed, the in- terest created by these products and the advantages inherent in their use are such that the research laboratories of many manufacturers which are either actual or potential users of these products are today furnishing substantial aid in these scientific investigations. TRACER CHEMISTRY* By ALEX MESHBANE Tracerlab, Inc., New York, N.Y. 'i•HE PRESENT status of tracer chemistry is undoubtedly due to the tremendous efforts made during the last war to produce fissionable material in adequate quantity so that the war might be brought to a rapid end through the use of atom bombs. The first uranium "pile" was an attempt to produce a con- trollable nuclear reaction, where neutrons would be evolved in a chain reacting process. Today we see "piles" springing up all over the country and plans being formulated in foreign countries as well to take advantage of the ever growing de- mand for unstable isotopes and develop means for harnessing the tremendous energy available from controlled nuclear reactions. The discovery of radioactivity by Becquerel in 1895 was rapidly * Presented at the May 15, 1952, Meeting, New York City. taken advantage of by the medical profession who used the ionizing property of radium to effect the course of certain common diseases. The penetrating abilities of the rays from radium were soon em- ployed to study the internal incon- sistencies of various metals by a technique that we commonly refer to as radiography. These two broad applications were in use within a relatively short time after the discovery of radium. The employment of unstable, radio- active elements as tracers did not come into its own until around 1920 mainly because adequate, precise, radiation instruments had not yet been devised to measure these elusive and mysterious rays. In 1920 Hevesey and Zechmeister (1) using Thorium B, which is an iso- tope of lead, showed that there was no exchange between lead ions and

TRACER CHEMISTRY 291 the same token, while the amount of scientific knowledge which is at our disposal concerning the phyco- colloids appears fairly considerable, the complexity of the subject is such that many features and many avenues remain to be explored and it may be confidently expected that fields of application for products so far developed and for products still to be developed will be con- stantly widened. Indeed, the in- terest created by these products and the advantages inherent in their use are such that the research laboratories of many manufacturers which are either actual or potential users of these products are today furnishing substantial aid in these scientific investigations. TRACER CHEMISTRY* By ALEX MESHBANE Tracerlab, Inc., New York, N.Y. 'i•HE PRESENT status of tracer chemistry is undoubtedly due to the tremendous efforts made during the last war to produce fissionable material in adequate quantity so that the war might be brought to a rapid end through the use of atom bombs. The first uranium "pile" was an attempt to produce a con- trollable nuclear reaction, where neutrons would be evolved in a chain reacting process. Today we see "piles" springing up all over the country and plans being formulated in foreign countries as well to take advantage of the ever growing de- mand for unstable isotopes and develop means for harnessing the tremendous energy available from controlled nuclear reactions. The discovery of radioactivity by Becquerel in 1895 was rapidly * Presented at the May 15, 1952, Meeting, New York City. taken advantage of by the medical profession who used the ionizing property of radium to effect the course of certain common diseases. The penetrating abilities of the rays from radium were soon em- ployed to study the internal incon- sistencies of various metals by a technique that we commonly refer to as radiography. These two broad applications were in use within a relatively short time after the discovery of radium. The employment of unstable, radio- active elements as tracers did not come into its own until around 1920 mainly because adequate, precise, radiation instruments had not yet been devised to measure these elusive and mysterious rays. In 1920 Hevesey and Zechmeister (1) using Thorium B, which is an iso- tope of lead, showed that there was no exchange between lead ions and