1. The equilibrium of the serum corresponding to its normal minimal surface tension is as stable and difficult to break, under ordinary conditions, as the osmotic tension equilibrium. The addition of a strong surface-active substance (sodium oleate, glycocholate, or taurocholate), will not lower it definitely, unless the substance is present in large amounts and in solution. After the first rapid drop has occurred, a process of recovery takes place, which brings back the normal surface tension in a short time (from 2 to 6 minutes in the case of pure serum). As a drop in the surface tension of the serum of animals may be very injurious to the red cells, this process of recovery is a normal one of defense in all cases in which surface-active substances (bile) are set free in the blood.
2. When diluted, the serum shows the same phenomenon to a smaller extent; the time of recovery is very much longer and the final surface tension is always lower than the original value. At a dilution of 1/10,000, no recovery takes place, the dilution being too high to overcome the lowering action of 1/10,000 of sodium oleate.
3. The recovery is stronger when the surface-active substance is added powdered or in a highly concentrated solution, and not stirred.
4. The recovery does not seem to be inversely proportional to the concentration of sodium oleate, when added superficially. Doubling the concentration at 1/2,000, for example, gives the same curve of recovery. This happens under certain conditions; namely, when the liquid is not stirred after the addition of sodium oleate.
5. This recovery is due to a purely physical phenomenon, namely adsorption, and is not specific for the serum. Other colloidal solutions, such as gum arabic, egg albumin, gelatin, and silver and gold sols, show it, only to a smaller degree. The process of recovery follows a logarithmic law in all cases, expressed by an equation of the form
See PDF for Equation
6. Temperature affects this phenomenon. At first it enhances it, but finally decreases it. This would seem to connect the loss of the property of the serum known as complement in a serum with a modification of the physical properties of this serum. This phenomenon is being investigated further.