=23. Explanation of the Surface Film.=--Beneath the surface of a liquid each molecule is attracted by all the other molecules around it. It is attracted equally in all directions. Consequently the interior molecules move very easily over each other in any direction. A molecule at the surface, as at _A_, Fig. 14, is not attracted _upward_ by other liquid molecules. Its freedom of motion is thereby hindered with the result that a molecule at the surface behaves differently from one beneath the surface. The surface molecules act as if they form an elastic skin or membrane upon the liquid surface. [Illustration: FIG. 15.--Capillary attraction in tubes.] =24. Capillarity.=--A striking action of the surface film of a liquid is seen in the rise of liquids in tubes of small bore when the liquid _wets_ them. If the liquid _does not wet_ the tube, as when mercury is placed in glass, the liquid is depressed. It is found in general that: _Liquids rise in capillary tubes when they wet them and are depressed in tubes which they do not wet; the smaller the diameter of the tube the greater the change of level._ (See Fig. 15.) This action is explained as follows: The molecules of a liquid have an attraction for each other and also for the sides of a tube. The former is called "cohesion for itself," the latter is called "adhesion for the sides of the containing vessel." If the cohesion for itself is greater than the adhesion for the side of the containing vessel, the liquid is pulled away from the side and is depressed. If the adhesion is greater, the liquid is elevated. This action is called "capillary action" from the Latin word (_capillus_) signifying hair, since it shows best in fine hairlike tubes. There are many common illustrations of capillary action: oil rising in a wick; water rising in a towel or through clothes; ink in a blotter, etc. The minute spaces between the fibers composing these objects act as fine tubes. If cloth is treated with a preparation which prevents water from adhering to its fibers, the material will not be wet when water is poured upon it, because the water will not run in between the fibers; a surface film spreads over the cloth so that no water enters it. _Cravenette cloth_ has been treated in this way and hence is waterproof. The action of this film may be shown by the following experiment. Dip a sieve of fine copper gauze in melted paraffin, thus coating each wire so that water will not adhere to it. Water may now be poured into the sieve, if a piece of paper is first laid in it to break the force of the water. On carefully removing the paper the surface film of the water will prevent the passage of the water through the sieve.