CONCERNING THE AGE OF THINGS 137
All living bodies, plants or animals, exchange carbon with
their surroundings continuously while they live; after death,
their decaying bodies are normally isolated from further
absorption of carbon. Some non-living deposits also carry
within them, locked away from further change, the carbon
which they contained at their formation, but such material
is less useful or reliable for archaeological dating than the
once-living material which we shall now consider. When I
refer to carbon, I mean not only molecular carbon, as in
charcoal, but also carbon in combination with other
elements, in such forms as carbohydrates, proteins and
simpler combinations as calcium carbonate and carbon
dioxide.
The significance of carbon exchange is this : owing to
continuous bombardment by neutrons due ultimately to
cosmic radiation, some of the atoms of the nitrogen, which
forms four-fifths of our atmosphere, are transmuted into
carbon, indistinguishable chemically from common carbon,
but, in fact, a radioactive isotope of atomic weight 14. I
omit an intermediate stage of very brief existence for sim-
plicity. This transformation is particularly liable to occur
at great heights in our atmosphere, although it does occur
at all levels, and the carbon produced in this way from
atmospheric nitrogen combines with oxygen to form radio-
active carbon dioxide gas. This special CO2 distributes
itself throughout the atmosphere and in due course is taken
into the structure of green plants, where, by the usual
metabolic processes, it becomes incorporated into the plant's
tissues and within a short time is absorbed by such animals
as may eat the plant, or falls to the ground and is buried in
the decaying remains of the plant.
Thus, all living things, plants and animals, including
ourselves, contain carbon, most of which is ordinary carbon,
while a minute proportion (a few parts in a million) is the
radioactive isotope. Now, this radio-carbon, as the C14
isotope is called, does not remain for ever, but its atoms
break down, with the emission of electrons, at a steady rate
quite uninfluenced by any changes of climate, temperature
or pressure to which they can be subjected.
The rate of decay back to ordinary nitrogen is such, that at
the end of every period of 5,700 years there remain half the