¹ From the Department of Physiology, The School of Medicine and Dentistry, of The University of Rochester, Rochester, N. Y.

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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