ATTEMPTS to express gamma-ray dosage in terms of the roentgen have thus far yielded discordant results. Some investigators have approached the problem by endeavoring to determine the ionizing effect of the gamma rays in air. Direct measurement of this effect by means of the standard open chamber and thin-walled chambers (3, 11) has largely given place to indirect methods, such as calculation from determinations of the total gamma-ray energy per gram of radium; calculation by means of Eve's constant, and the measurement of the ionization produced in a small volume of air enclosed within an irradiated solid, such as a thick-walled thimble-type chamber. Some of these results have been collected by Sievert (17) and Friedrich (4). The numbers of roentgens corresponding to one milligram-hour of radium obtained in these various ways differ greatly among themselves. The open chamber gives values in the neighborhood of 2–3, while indirect methods give values between 8 and 11 r/mgm.-hr. The values given by open cha...