The radium content of ordinary materials is determined by removing and measuring the radon which is in equilibrium with the radium. I. Generalization of the direct fusion furnace method now permits the rapid and complete deemanation of any solid by direct boiling at 2000°C. II. Improved boiling apparatus for the removal of radon from liquids avoids loss of traces of radon by absorption. III. Gaseous samples, at atmospheric pressure, may be examined directly for radon. IV. Double ionization chambers, coupled to a high sensitivity string electrometer, permit observations on very feeble sources. Statistical fluctuations in the emission of contamination alpha‐rays from the ionization chamber walls impose a natural observational limit of 7.2×10−14(n)−☒ curies of radon, when the ionization currents are observed for n hours. V. Photographic recording of the ionization current, with hourly automatic recalibration of the electrometer at three arbitrary potentials, makes possible long runs and higher precision, particularly for weak sources. VI. Induction, null‐induction and ionization methods for the measurement of the electrostatic capacity of ionization chamber systems are described. VII. Caution is given against a spurious ionization, of unknown origin, occasionally observed in connection with radon tests. VIII. The flow method for the measurement of thoron is shown to be more applicable to liquid samples than to solids. Under normal operating conditions the emanating power of liquid samples is about 0.6. IX. Mean values of the radium content of certain classes of rocks, sea waters and organisms are given.