One of the most striking manifestations of the underlying unity of all knowledge is the way in which a discovery in one branch of science often finds fruitful application in another. A recent example of this is the development by Professor Libby in America of a new method of dating archaeological samples using the techniques of nuclear physics. The basic fact underlying the method is that all archaeological specimens containing carbon also contain a small amount of radioactive carbon, or radiocarbon, which becomes progressively less in quantity as time goes on. The fraction of radiocarbon present can be measured, and hence the age of the sample deduced. To explain this in more detail, it is necessary to explain what is known as the carbon cycle and then to discuss some of the radioactive transformations which take place in the earth’s atmosphere.

The chemical element carbon, which plays an essential part in all vital processes, continually takes part in a cyclic series of interchanges between the earth and its atmosphere. Plants obtain the carbon they need to grow by absorbing carbon dioxide from the air and, with the aid of sunlight, build up complex chemical compounds containing carbon by the process known as photosynthesis. Some carbon is also absorbed by roots from the soil, where it is present as a weak solution of carbonic acid. When a plant dies it is often burned, and the complex compounds are broken up and the carbon returned to the atmosphere as carbon dioxide.