336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



have been shown experimentally to be constant in all living matter. 

 The radiocarbon constantly disintegrates, but it is continually replaced 

 through the life processes which are in exchange with the atmosphere. 

 When an animal or a plant dies there is no further replacement of 

 carbon 14. That remaining, however, continues to disintegrate at a 

 rate that is the same everywhere. For that reason the amount of 

 radiocarbon still present is in direct proportion to the time that has 

 elapsed since death occurred, and by measuring the constantly 

 diminishing rate of disintegration it is possible to calculate the age of 

 an organic sample. 



It is not possible in an article of this nature to explain and discuss 

 in detail the scientific processes involved in radiocarbon dating, but 

 the main features in the development of the method should be men- 

 tioned. In 1934, not long after the discovery of artificial radioactivity, 

 Dr. A. V. Grosse suggested that the existence of radioactive elements 

 produced by cosmic rays would be established (Grosse, 1934). Some 

 10 years later Dr. W. F. Libby, of the Institute of Nuclear Studies of 

 the University of Chicago, predicted that living matter would be 

 found to contain natural or "cosmic" carbon 14 (Libby, 1946). The 

 next year he and Dr. Grosse tested methane gas derived from sewage 

 and found the expected amount of carbon 14 (Grosse and Libby, 

 1947). Dr. Libby and his associates then proceeded to demonstrate 

 experimentally that carbon 14 occurs in the same concentration in all 

 living matter. In doing this they tested living material from many 

 parts of the world — from different latitudes, altitudes, and different 

 geographical situations (Libby, Anderson, and Arnold, 1949). The 

 materials consisted of wood from Chicago, Mount Wilson, New Mex- 

 ico, Bolivia, Ceylon, Tierra del Fuego, Panama, Palestine, Sweden, 

 New South Wales, and North Africa ; sea shells from Florida ; and seal 

 oil from the Antarctic. 



In the course of the various studies it had been determined that 

 the half -life of carbon 14 is about 5,568 ± 30 years, which means that 

 one ounce of the material is reduced by decay to half an ounce during 

 the 5,568-year period and that half of the remainder decays during 

 the next 5,568 years leaving a quarter of an ounce, etc. (Engelkemeir 

 and Libby, 1950; Jones, 1948; Miller et al., 1950). Because of this. 

 Dr. Libby was convinced that the amount of carbon 14 present in any 

 particular object would be an indication of the age of that object and 

 proceeded to develop a method for measuring it. This involved the 

 perfection of a specially constructed and extremely sensitive screen 

 wall counter, a form of Geiger counter, which would measure the rate 

 of atomic disintegration of natural carbon 14 without the use of a ther- 

 mal diffusion column. The problem also included the development of 

 a complicated chemical separation unit to reduce the carbon 14 to its 



