If bubbles are blown on the expanded ends of two funnels (the stems being stopped by the fingers) they can be made to coalesce by carefully bringing them into contact. (A slightly electrified rod brought not too near will assist the coalescence.) On draw ing the funnels slowly apart a cylindrical bubble is obtained. The excess pressure is now 20/R where R is the radius of the opening of the funnel. Further separation causes the cylinder to contract in the middle; one of the two curvatures is now negative; the surface is an anticlastic one. With further extension of length the surface becomes unstable and the film collapses. It has been shown by Clerk*Maxwell that a cylindrical film becomes unstable when its length is greater than its circumference.
Films on Liquids.—In the second group of cases a liquid or solid is on one side and a gas on the other. To this group belong all cases of the spreading of oil or other substance on water or other liquid or solid. This old familiar subject has recently acquired very great importance in its bearing upon molecular structures. The first to experiment in detail on such films was F. H. R. Liidtge (Pogg. Ann. cxxxix., p. 62o), who showed how a film of high surface tension is replaced by one of lower surface tension. Akin to these are experiments made on the erratic movements that are observed when fragments of camphor are sprinkled on a clean water surface. A trace of grease such as may be communicated to the surface by dipping a finger in the water may be sufficient practically to stop this motion. The thickness of oil which is required may be spoken of as the "camphor-point." The first to determine it was (the 3rd) Lord Rayleigh who in 1890 (Roy. Soc. Proc.) showed that the thickness of the film of olive oil, calculated as if continuous (i.e., non-molecular) which corresponds to the camphor-point is about 2X mm. ; i.e., it is only a moderate multiple of the supposed diameter of a gaseous molecule and perhaps scarcely exceeds at all the diameter to be attributed to a molecule of oil; and he ultimately realized that this phenomenon was therefore entirely outside the scope of a theory of surface action such as Laplace's in which matter is regarded as continuous and that an explanation required a direct consideration of molecules.
In 1891 Miss Pockels (Nature, XLIII., 437 [1891]) showed by means of a movable slider on water (the surface tension being measured by means of the attraction on a small disc) that the contaminating material on the surface could be squeezed up and concluded from her experiments that the water-surface can exist in two sharply-contrasted conditions ; the normal condition, in which the displacement of the partition makes no impression on (the value of) the tension; and the anomalous condition in which every increase or decrease (of the surface) alters the tension.
The question was taken up again by Lord Rayleigh in 1899 (Phil. Mag. xLvni., 1899) with apparatus designed on the lines of that of Miss Pockels (but employing a different way of measuring the surface tension) and he concluded that the first drop in tension corresponded to a complete layer one molecule thick and that the diameter of a molecule of oil is about i.ox mm.
Later investigations have been made by I. Langmuir (in America : 1. Amer. Chem. Soc., 1915 to 1918) and N. K. Adam (in England) and their coworkers. By measuring the surface area that can be completely covered by a weighed quantity of material Langmuir determined both the cross-sectional area and the length of a molecule and proved that the length for organic molecules like those of palmitic, stearic, cerotic acids, etc., was nearly five times the breadth, while in cetyl palmitate it is nearly ten times. Adam has developed the technique and has very much extended the theoretical interpretation (Roy. Soc.
Proc. A. 1921 to present time). Both Langmuir and he measured the amount of "oil" and measured directly the force required to compress the surface to any given area by means of a floating barrier. The actual force measured is the length of the barrier multiplied into the difference of the surface tension on the two sides of it. On the outside the surface is that of pure water— on the Inside that of water on which a thin film has been formed. It is found that when the molecules in the film are so sparse as to be widely separated from one another the force due to the film is an expansive one.
This cannot be explained on Laplace's theory : it is necessary to modify it by allowing for the effects of thermal motion ; the film in such a case is analogous to a two-dimensional gas. If the barrier is now moved so as gradually to compress the film various changes take place successively which are depicted in fig. I I for the case of a film of myristic acid on weak HC1 (N/ioo). The unit of area adopted is a square each side of which is an Angstrom unit (i.e., metre), and the area specified is that occupied by one molecule of the acid. The ordinates of the curves are the difference of the tensions on the two sides of the barrier. The curves are isothermals extending from 2.5° to C. They exhibit some of the characteristics of the p, v, curves for a con densible substance. At high temperatures the curves appear to be approximately rectangular hyperbolas. For small values of the area they approach the form for liquids. At intermediate stages, however, there is no constant pressure isothermal as in the analogous case of the vapour pressure of a liquid below the critical point (the form is more nearly that for the vapour pressure of a mixture of two liquids; this fact may indicate that the under lying water takes a part in the changes that occur).