A study of the catalytic properties of solid surfaces has led to a belief that the distance at which molecules cease to attract one another appreciably is of the same order of magnitude as the diameter of an atom, a distance much smaller than that imagined by Laplace. On the Laplacian view the transition between liquid and vapour at a phase-boundary is gradual, but on the newer theory the non-homogeneous layer separating the homogeneous phases is little, if at all, greater than the diameter of a molecule. The work of Langmuir (1) on the influence of small quantities of certain insoluble organic substances upon the surface tension of water supports the latter view. The substances which are found to act in this way are hydrocarbons in which has been substituted an “ active ” group, such as COOH or OH. These groups tend to make the substance soluble in water, but the substitution of only one of them in a hydrocarbon containing more than twelve carbon atoms in the molecule is insufficient to cause any appreciable solution. Langmuir suggests that molecules of, say, palmitic acid placed on water orientate themselves, so that the COOH group is in the liquid phase while the hydrocarbon chain is in the vapour phase, a condition which is only realizable if the phase boundary is relatively sharp. His well-known trough experiments (2) amply support this idea, and confirm the necessary corollary : that a water surface of a given area can only accommodate a limited number of orientated molecules. N. K. Adam (3), in repeating and extending Langmuir’s experiments with more refined apparatus, has shown that the least area occupied by these molecules is in excellent agreement with that to be expected from the recent X-ray determinations of Sir William Bragg. In the paper in which his work on insoluble films is described Langmuir also deals with the lowering of the surface tension of water by soluble substances (4), such as the fatty acids and alcohols of small molecular weight. He suggests that the excess concentration of solute, which an application of Gibbs’ adsorption equation shows to exist in the neighbourhood of the phase boundary, is present as a single layer of molecules, orientated partially or completely like those of insoluble substances. In support of this he quotes a paper by Milner (5), where it is shown that soluble substances which depress the surface tension of water appear to give a surface excess reaching a constant limiting value for large depressions. This conclusion is drawn from the fact that the relation σ
0
— σ = α + β log
10
N (i) is found to represent the behaviour of solutions of acetic acid for large values of σ
0
— σ. In this equation σ is the surface tension of the solution, while σ
0
is that of pure water ; N is the molar fraction of the solute, and α and β are constants.
Free surface flow over an obstacle. Theoretical study of the fluvial case
