The threshold concentrations for sourness of nine acids have been determined with an accuracy of about 8 per cent, and the H+ ion concentration of these acids measured.

Calculations have been made of the relative concentration gradients of the undissociated acid across the cell membrane for a series of acids having equal sourness and also for a series of acids having equal penetration velocity as determined from experiments by Crozier on Chromodoris and on Allolobophora. For solutions of equal pH a high degree of sourness has been found to be associated with a high penetration velocity of the undissociated acid or of the anion. A comparison of these gradients with the results of adsorption experiments on charcoal indicates that the acids are taken into the tissues by an adsorption process.

Polar groups such as OH and Cl and Br are found to have a very marked effect in reducing the ability of organic acids to penetrate living tissues.

The important rôle of optical activity of the acids in determining their physiological action has been noted.

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