-13photons, or neutrons. These forms of energy differ in their power of penetration and ability to ionize other materials, factors that are important in determining the biological effect of tonizing radiations upon tissue. To measure the energy absorbed by the tissue other terms are used. The dose of radiation that is received by the tissue of an organism from any type of radioactive source is measured in terms of the energy absorbed per unit mass of the tissue. It is evident measure of radiation dose. that the curie is not a The unit of measurement of the radiation dose is the rad and is arbitrarily defined as the absorption of 100 ergs of energy per gram of tissue as measured in the tissue which is being irradiated. some idea of the size of this unit, To give 420,000 rads would raise the te@perature of water by one degree centigrade, assuming that all the absorbed energy is converted to heat (Platzman 1959). Another unit which has been used for many years to express the amount of radiation from X rays or gamma rays is the roentgen, which is a measure of the lonizations pro- duced in air rather than of the energy absorbed in tissue. One roentgen produces about two billion each of positive and negative ions in one cubic centimeter of air at standard