1. Two methods are given for measuring the rate of diffusion of CO2 in tissue membranes. Methods are also given for the determination of tissue thickness and the absorption coefficient for CO2 in tissues.

2. The values obtained for the permeability constant (P x 104) at 22°C. for CO2 in the following tissues are:—frog skin, 3.05; connective tissue (dog), 2.65; smooth muscle (cat), 5.00; frog muscle, 5.29; striated muscle (dog), 4.70. P is expressed as cc. per cm.2 per minute under a pressure gradient of one atmosphere per cm.

3. Evidence is presented to show that in a "steady state" bicarbonate contributes a negligible amount to the diffusion of CO2.

4. The absorption coefficient for CO2 in frog skin is 0.73 cc. per cc. and for frog muscle 0.78 cc. per cc.

5. In all of the tissues studied the diffusion of CO2 is slower than in water. The diffusion coefficients (K x 104 in cm.2/minute) at 22°C. for tissues as compared with water are:—water (16°C.), 9.5 (Hüfner, 1897); frog skin, 4.1; connective tissue, 3.7; frog muscle, 6.8; striated muscle (dog), 6.0; smooth muscle (cat), 6.4.

6. The time course of saturation of a tissue with CO2 is altered in the presence of available base. Non-acidified tissues saturate more slowly than acidified tissues and the rate of saturation is dependent on the CO2 tension.

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