1. a. For adsorbents for use in respirators, retentivity may be defined as the weight of adsorbed gas or vapor per unit volume of adsorbent in equilibrium in air with a concentration of the gas or vapor just below that of physiological significance. There is a considerable variation in the retentivity, thus defined, for substances of the type of ethyl chloride among gas-adsorbent activated charcoals of different manufacture. 2. b. Within the limits studied, changes in the flow rate of desorbing air, or changes in the volume of charcoal and the shape and dimensions of the bed, do not affect the relationship between the amounts of ethyl chloride retained and desorbed. 3. c. No correlation is indicated between the heats of wetting of various activated charcoals in benzene and the retentivities of the charcoals for ethyl chloride. This is attributed to the smallness of the heat effects and small proportions of adsorbent surface concerned with retentivities in this case as compared with the total quantities of heat evolved on the immersion of the adsorbent in a liquid.