CHAPTER III 



INTERFACIAL PHENOMENA 



There are a number of familiar observations which indicate that the 

 surface layer of water possesses certain distinctive properties which are not 

 exhibited within the body of the liquid. For example, if a perfectly dry, 

 clean, steel needle be carefully laid on the surface of some still water it will 

 float, in spite of the fact that steel is heavier than water (Fig. 2). Careful 

 observation shows that the water surface beneath the needle is depressed, 

 forming a tiny liquid cradle, within which the needle is supported. This 

 phenomenon is clearly due to the properties of the surface film of the water, 

 because if the needle is laid on the water at a slight angle, so that this film 

 is punctured, it will immediately sink to the bottom of the vessel. A number 



Fig. 2. Needle floating on water. 



of other familiar phenomena are possible principally because of the distinctive 

 properties of the surface film of water. The ability of many kinds of insects 

 to "walk" on water, the formation of drops by water under certain conditions, 

 the rise of water in capillary tubes, soil, blotting paper, etc., the upward 

 bulging of the water surface in a vessel when it is filled to the very last 

 drop, and many other aspects of the behavior of water, are due largely or 

 entirely to the properties of its surface layer. The surface layers of other 

 liquids also exhibit distinctive properties, but in no common liquid except 

 mercury are such properties as well marked as in water. 



Surface Tension. — Every molecule within the body of a liquid, although 

 in rapid motion as a result of its kinetic energy, attracts and is attracted by 

 other molecules. The attractive forces which any molecule exerts upon its 

 neighbors may be considered as acting along lines of force which radiate from 



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