226 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



by preparing a sample of the compound with a strongly radioactive 

 thorium isotope, saturating a given volume of nitrogen with the vapor 

 of the compound and passing the gas through acidified alcohol, in 

 which the compound is absorbed. The thorium concentration can 

 then be calculated from the radioactivity of the liquid and from that 

 the amount of vapor in the given volume of nitrogen. 



In general it may be said that radioactive isotopes render valuable 

 service in microchemistry, i. e., the chemical investigation of extremely 

 small quantities of a substance, as for instance in adsorption phenom- 

 ena, in very dilute solutions, etc. Another interesting fact is that 

 radioactivity has made possible the investigation of the chemical prop- 

 erties of the elements "43" and "85" and of several "transuraniums," 

 elements which could not be found in nature but from which radio- 

 active isotopes could be prepared artificially in imponderably small 

 quantities. 



Important perspectives are opened by the application of radioactive 

 indicators in chemical analysis, so often constituting daily routine 

 work in technology. One example out of many is the following. It 

 is desired to determine the bromine content in a mixture of a bromide 

 and a chloride. A complete separation of the two compounds is very 

 difficult and takes up a great deal of time. If, however, a little radio- 

 active bromine ( in the form of the compound in question ) is added to 

 the mixture, only a partial separation of the bromine is sufficient. Due 

 to the homogeneous mixing it is known that the ratio between the bro- 

 mine separated and the total amount of bromine is equal to the ratio 

 between the radioactivity separated out and the original radioactivity. 

 Since the latter ratio is easily determined, it is possible to calculate 

 the desired total content of bromine directly from the amount of 

 bromine separated out. 



An application in biology somewhat resembling the above is the 

 determination of the total amount of blood in an animal. After a 

 certain amount of blood has been taken death inevitably sets in and it 

 is then impossible to draw off the rest of the blood. If, however, a 

 solution of some substance or other containing a known quantity of 

 radioactive atoms is injected into the test animal intravenously and 

 given time to distribute itself homogeneously throughout the whole 

 circulatory system, a small sample of the blood suffices, for the total 

 amount of blood can then be calculated from the percentage of injected 

 radioactive atoms recovered in the sample. (Of course, no appreciable 

 part of the injected substance must have been transferred from the 

 blood to other parts of the body, ) 



The fact that it is practically unnecessary to interfere with the 

 normal life of the test animal is of importance for many investiga- 

 tions, especially those of a pharmacodynamic nature. We may men- 



