An attempt was made to apply the assumption that a monolayer of serum exists at a certain dilution, in order to calculate the thickness of this layer or, that is to say, the mean value of one of the dimensions of the molecules of the serum proteins. The criterion taken for the existence of such a monolayer was the existence at a given concentration (1/11,000 for rabbit serum) of a maximum drop in the surface tension of serum solutions kept in watch-glasses.

A series of preliminary experiments showed:

1. That the maximum drop in 2 hours took place, for the material used, at a concentration of 1/11,000, and that it always corresponded to an absolute minimum value of the surface tension of the solution, this minimum being quite sharp and well defined.

2. That adsorption took place on the glass as well as on the free surface of the liquid, and that apparently the same part of the molecule, in both cases, was drawn toward the water.

3. That the specific gravity of the anhydrous proteins of the rabbit serum studied was 1.275, whence it followed, on the basis of 6.51 per cent protein content, that the mean thickness of the protein molecules was 35.4 x 10–8 cm. The same method applied to crystalline egg albumin, pH 6.8, in water, gave 52.8 x 10–8 cm. for the probable molecular length.

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