138 THE ESSEX NATURALIST
number of radio-carbon atoms which existed in any particular
sample at the beginning of that period. That is to say, out
of any group of 64 radio-carbon atoms existing today, 5,700
years hence there will be 32; 11,400 years hence, 16; 17,100
years hence, 8; and 22,800 years hence, 4. The period of
5,700 years is called by physicists the "half-life" period of
radio-carbon.
But the process of radio-carbon decay is counter-balanced
by ever-renewed supplies of radio-carbon from the trans-
mutation of nitrogen in the upper atmosphere, where the
process of carbon production never ceases and may be
assumed to have been going on at the same rate certainly for
several millions of years. Therefore, a condition of equilib-
rium has been achieved between creation of new radio-carbon
and decay of the old, and the population of radio-carbon
atoms in the atmosphere remains constant. The population
density is such that the protons (atomic particles) given out
during the nitrogen-carbon transmutation amount to 2 per
second, or 120 per minute, for each square centimetre of the
earth's surface. This rate of formation must be equalled
by the rate of decay of the newly-formed radio-carbon atoms,
which are all either in the form of dissolved inorganic car-
bonates or in living matter on the earth's surface. The total
carbon in the former amounts to some 7.3 grams per square
centimetre, and the latter to about 1 gram/cm.2 of the
earth's surface, a total of 8.3 grams/cm.2 : this includes
both the normal and radioactive carbon. The radio-carbon
should be emitting electrons during its decay, such that each
square cm. is giving out 120 electrons per minute, or between
15 and 16 electrons per gram of total carbon per minute.
Now, each electron, as it is emitted, can be counted by an
elaborate technique involving Geiger counters, which I need
not describe in detail. Actual counts have now been made
of electron emission from sewer-gas, i.e., simple hydro-
carbons, largely methane, recently produced from living
creatures. The result was 14.6 electrons per minute per
gram of carbon (in fact, between 13.8 and 15.4 in various
samples), and this correspondence with the theoretical value
based on the difficult estimate of world distribution of carbon,
is amazingly close.