1. The experiments show that suspensions of finely divided particles of insoluble proteins incapable of swelling in acid (denatured egg albumin, casein trichloroacetate, sulfate, etc.) raise the viscosity of the suspension but little and that the influence of acid on the viscosity is negligible.

2. The same is true for solutions of certain genuine proteins such as genuine crystalline egg albumin.

3. In contrast with these are proteins which swell in acid. It can be shown that where acid swelling of particles occurs, the viscosity is of a higher order of magnitude than where the swelling of particles is impossible and that the influence of acid on viscosity runs in these cases parallel to the influence of acid on swelling. This is shown to be the case for casein in HCl.

4. It is shown that the swelling of the powdered particles which determines the high order of viscosity varies according to the theory of membrane equilibria.

5. These results are used to ascertain with the aid of viscosity measurements whether the ultimate units of genuine protein in solutions are aggregates large enough to give rise to a Donnan equilibrium; or whether they consist of particles below this limit; and in what porportion the two kinds of units are contained in the solution.

6. It is found on the basis of viscosity measurements that when 1 gm. of isoelectric casein is dissolved in HCl so that the solution has a pH of 2.45, more than one-half of 1 gm. of casein must exist as units too small to give rise to a Donnan equilibrium, while the rest must exist in units still capable of undergoing swelling in acid.

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