It is not to be expected that there should be complete cor respondence between the two classes of phenomena. There is, however, sufficient indication that the effects of thermal motions in the films cannot be neglected. The analogy with liquids and gases can be further illustrated by plotting FA against F, where F is the force applied per unit length ; the resemblance to an Amagat diagram for a fluid is very striking (Schofield and Rideal, Roy. Soc. Proc. A, 109, p. 67; 11o, p. 17o). It is clear from the diagram that a film of myristic acid can be squeezed up until a molecule occupies less than 24 At the opposite end where the behaviour approaches that of a gas it is to be expected that the equation would take the form FA= RT where the value of R (allowing for only two degrees of freedom) should be 1.372 per molecule; at room temperature therefore FA should be about 40o. This has been experimentally verified as a limiting value for very low surface pressures. With the long chain fatty acids, esters and nitriles it is approached within 25%, the pressure being below o.i dyne per cm. With the dibasic ester it has been verified within io%.
The interpretation of the experimental results is still in a somewhat fluent state and it is clear that we must not press any simple gas analogy too far. A thin film upon a body of different material cannot be treated as in a similar state to the molecules in the body of a gas. Forces must exist between the film and the liquid beneath. This may account for the fact that at any rate some films do not spread indefinitely as a gas ex pands into a vacuum. Further, the most compressed state can only give an upper limit to the least cross sectional area of the molecule. The molecules are resting on the rapidly-moving mole cules beneath them and must share to some extent in their agita tion. Still it must be granted that these new investigations are throwing great light on the nature of films and on the dimensions of molecules. Where corroboration is possible, measurements made by means of X-rays are in good accord with those obtained by this method.
addition to the specific references in the text, further sources of information are (the 3rd) Lord Rayleigh's Collected Papers (several articles) ; A. W. Reinold and Sir A. W. Raker, Phil. Trans. (1886) ; Sir W. Ramsay and J. Shields, Zeitschr. Phys. Chem. I2, p. 433 (1893) ; Harkins and Brown, Am. Chem. Soc. J. (1916 and 1919) ; Richards, Speyers and Carver, Am. Chem. Soc. J. (1921 and 1924) ; Physical Phenomena at Interfaces (Faraday Society, Dec. 1925) ; N. E. Dorsey, Scientific Paper of the Bureau of Standards, Washington, D.C., No. 540 1926 (this paper contains a survey of the various methods of measuring surface tension and a very complete bibliography of that part of the subject) ; on the theoretical side J. Willard Gibbs, Collected Papers, vol. i.; R. Eotvos, (1886) ; J. D. van der Waals, G. Bakker and other writers of the Dutch school. Volume 6 of the Handbuch der Experimentalphysik Kapillariteit and Oberfidchenspannung by G. Bakker contains a very full account of the whole subject on the experimental side.
For certain properties of solutions see THERMODYNAMICS (applied to Chemistry). (A. W. Po.) a wading bird found along the Pacific coast of the Americas, from Alaska to Chile, and believed to nest on the north Alaskan tundra. Grinnell, who observed the surf-bird in the region of Kotzebue Sound, says that he was informed by the Eskimos of the district that the birds nested in the neighbour hood of some small lakes far back on the tundra and not far from the base of the mountains ; this confirms the observation of Nelson at St. Michael. The surf-bird (Aphriza virgata), which is nowhere abundant, belongs to the family Charadriidae, the plover like birds, and is placed by Knowlton between the turnstones and the oyster-catchers; it is distinguished from the turnstones by its longer tarsus, emarginate tail, and the swollen terminal por tion of the bill. The surf-bird is so in. long, dusky, with a white rump.