To account for the behavior of a solid particle in the interface between two fluids it is necessary to consider, as indicated by Clark Maxwell, three surface tensions: Tso, the tension in the interface between the solid particle and the organic phase; Tsw, the tension in the interface between solid particle and aqueous phase; and Tow, the organic phase-water interfacial tension.

If Tso > Tsw + Tow, (2), the stronger solid-organic phase tension should pull the line of intersection of the three phases around the periphery of the solid particle until the particle is completely enveloped in the water phase.

If Tsw > Tow + Tow (3), the solid-water tension should pull the line of meeting of the phases about the particle until it is enveloped in the organic phase.

If

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the particle should be stable in the interface, only leaving it when mechanical work overcomes the equilibrium due to the balance of interfacial tensions.

The ordinary bacteria used have been stable in the interfaces between water or aqueous solutions and all organic liquids tested; i.e., condition (4) obtains. In preparations in which Tow is large, stability has been found by experiment to be greater than when Tow is small, as follows from condition (4).

The force, dependent upon condition (4), which holds bacteria in the liquid-liquid interface, and the force, dependent upon unequal distribution of tension in the liquid-liquid interface, which causes bacteria to glide along the interface, prove to be of the same order of magnitude as the force due to bacterial flagella. Interfacial tensions or its own motility may dominate the movement of the bacterium, according to circumstances. When bacteria thresh their way out of the interface, escape is into the aqueous phase.

Acid-fast bacteria possess very low or, in some cases, no stability in the interface, passing easily or even spontaneously into the organic phase.

Good evidence has been advanced by other workers to indicate that the surfaces of ordinary bacteria contain many polar radicals; on the other hand, the acid-fast microorganisms are coated with predominantly non-polar substances. It follows from known principles, therefore, that Tsw should be greater than Tsw with ordinary bacteria, and Tsw should be greater than Tso with acid-fast bacteria. Consideration of relations (2) and (3) above will show that these conditions should result in the differences in behavior of acid-fast and ordinary bacteria actually found by experiment.

The theoretical and experimental data here developed contradict the theoretical formulations of the surface tension factor in phagocytosis advanced by Rhumbler and by Tait and substantiate those of Fenn.

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