. Physical chemistry of vital phenomena, for students and investigators in the biological and medical sciences. Biochemistry; Chemistry, Physical and theoretical. 60 PHYSICAL CHEMISTRY that of water. If the soap solution is allowed to drop slowly from a stalagmometer it shows a lower' surface tension than if dropped fast. The rate of flow from a stalagmometer is reduced by a piece of capillary tubing forming part of the outflow tube, but the more viscous the fluid the slower it will flow. In order to reduce errors from this source, Traube uses three stalag- mometers for solutions of different
. Physical chemistry of vital phenomena, for students and investigators in the biological and medical sciences. Biochemistry; Chemistry, Physical and theoretical. 60 PHYSICAL CHEMISTRY that of water. If the soap solution is allowed to drop slowly from a stalagmometer it shows a lower' surface tension than if dropped fast. The rate of flow from a stalagmometer is reduced by a piece of capillary tubing forming part of the outflow tube, but the more viscous the fluid the slower it will flow. In order to reduce errors from this source, Traube uses three stalag- mometers for solutions of different viscosity and selects one de- livering less than one drop per second. The error in using the stalagmometer, due to lack of diffusion equilibrium, is less than the error in the methods used on old surfaces, due to the forma- tion of haptogen membranes. It should be remembered, how- ever, that the stalagmometric data are merely comparative, and only those results should be compared where the flow is slow and its rate approximately the same in each case (see Harkins and Humphrey, 1916).. Fig. 22. Scheme showing that surface tension is located in a film whose thickness equals the diameter of the sphere of molecular attraction. In order to understand the relation between surface tension and osmotic pressure certain theoretical considerations are neces- sary. Surface tension is a molecular phenomenon. The thick- ness of the surface film equals the radius of the sphere of molecular attraction. In Fig. 22, suppose m to represent a mole- cule in the interior, M a molecule on the surface of a liquid, and the circles around them to define the spheres of molecular attrac- tion. The sphere of m is entirely within the liquid, hence m attracts and is attracted equally on all sides by all molecules in this sphere. On the contrary, only the hemisphere of M is within the liquid and, hence, M is attracted laterally and downward, but not upward. This attraction which the molecules in the surface f
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