Establishing the relative age of various materials with the use of carbon-14. This involves measuring the amount of 14C and of 12C and comparing the measured ratio to the one established by the production of 14C in the upper atmosphere by cosmic rays. When an organism is alive the 14C/ 12C ratio in its biomass is constant (because of constant atmospheric 14C production, diffusion to the lower atmosphere, absorption by organisms, and because of constant 14C radioactive decay) but when the organism dies the ratio begins to change--at a very predictable rate. Therefore knowing the carbon-14/carbon-12 ratio now in some artifact gives a very good measure of how long the carbon has been "dead." The object however must obviously have organic material either in it or on it for this method to work. Also the length of time one can "look back" is limited because the amount of carbon-14 must be detected with some certainty for the age to be known with confidence, and the longer the sample has been dead the less 14C is present because it is continually decaying. [New Scientist; v274; 18; 1996.] [Scientific American; v150; 24; 1996.] [Bowman, Sheridan, Radiocarbon dating--Interpreting the past. British Museum Press; 1995.]