When we find a piece of charcoal in a cave or a piece of wood in some ancient structure, for example, we can measure the amount of carbon in it, determine how much of it is ¹⁴C, and then calculate back to the time when the radioactivity from the ¹⁴C was the same as we now find in living wood. In other words, if we assume that we know from the observed secular equilibrium how much ¹⁴C originally was present in living material, then we can calculate the time of death of any similar but ancient material. That is the basis of the ¹⁴C method of age determination. [Illustration: _Dinosaur tracks imprinted in rock in Navajo Canyon, Arizona, arouse the professional interest of this scientist. Fossil traces of extinct prehistoric creatures were for a long time the best clues to the age of rock formations._] [Illustration: _A scientist using liquid nitrogen to freeze carbon dioxide gas made from a sample of ancient material that he is preparing for age determination by the carbon-14 technique._] For example, a bit of a rafter from a prehistoric cliff-dwelling or a remnant of charcoal from an ancient fire may be analyzed for its remaining ¹⁴C content, and its age determined accurately within the margin of a few hundred years. This fixes the time at which the wood for the rafter or the firewood was broken or cut from the living tree, and hence the period in which the men lived who used the wood. [Illustration: _The most useful samples for carbon-14 age determination are charcoal, wood, and shells._] Carbon-14 Counting Carbon-14 measurements are made by taking a known amount of carbon, reducing it to a gas, and then counting the ¹⁴C disintegrations in the gas. This may sound simple, but in reality the measurement process is a formidable undertaking, because the amount of the ¹⁴C isotope in the carbon is so extremely small. (The remainder of the carbon, of course, consists of other isotopes—¹²C or ¹³C, which are stable.) There are two basic techniques. The carbon can be: 1. Burned with oxygen to form carbon dioxide, or 2. Reduced chemically to methane or ethane, or to a carbide from which acetylene can be evolved by adding water. (See booklet cover and description on page 59.) The first technique is the simpler, but carbon dioxide (CO₂) contains only one atom of carbon per molecule, whereas acetylene (C₂H₂) and ethane (C₂H₆) each contain two. Consequently, the SPECIFIC ACTIVITY of acetylene or ethane is twice that of carbon dioxide, other things being equal. For that reason acetylene or ethane are the preferred gases in some laboratories. On the other hand, they are explosive, and that cautions other scientists into using the carbon dioxide method.