soil in an unnatural state. Today a sort of double-barreled radiation method can be used to measure these two soil characteristics.

Neutrons are readily scattered by water but not by soil; gamma rays are absorbed by both soil and water. In practice the experimenter drills two holes in the soil a few feet apart. Into one he puts a gamma-ray source; into the other, a radiation detector. The reading on his detector dial tells him the amount of gamma rays absorbed by both soil and water. Replacing the gamma-ray source with a neutron source, he obtains a reading on absorption by water only. The difference between the two readings is ascribed to the density, or degree of compaction, of that soil in its native state.

Conclusion

Radioactive tracers and radiation sources have become indispensable to all phases of agricultural research. They have helped answer questions that seemed unanswerable. But there will always be more questions to put to Nature. The physicists-philosophers of 1890 were confident that they had obtained all significant knowledge of the physical universe. Discoveries of the next twenty years revealed the immaturity of that conviction.

The modern poet Archibald MacLeish has dramatized the meagerness of knowledge:[3]