The Properties of the Surface of Liquids. 25: 



resemblance between the surface film of a liquid, whether 

 exposed to air or to the sides of the vessel, and stretched elastic 

 was shown by some experiments. A. piece of stretched elastic 

 tends to contract ; it would lift up a weight ; it would, in fact, 

 like a wound watch-spring, do work ; in scientific language it 

 possesses potential energy, energy being defined as ability to do 

 work. Now, potential energy when created always tends to 

 run down or become a minimum. The lifted weight will fall ; 

 the wound spring will unwind ; the stretched elastic will 

 contract ; and the energy of the liquid surface tends also to 

 become a minimum, this being effected, just as in the case of 

 the stretched elastic, by a reduction of superficial area. This 

 underlying principle will be found to explain many of the 

 phenomena due to surface tension. Why, for instance, is an 

 ordinary drop spherical ? Because the sphere is that figure 

 which has the smallest surface for its volume, and surface 

 contraction can proceed no farther. Mr. Barcroft went on to 

 show by experiments the application of the principles. He 

 explained why water pouring out of a tap will sometimes 

 remain as a falling column, while at other times the column 

 will break up into innumerable drops. He also explained why 

 a liquid with small surface tension, such as oil, will if poured 

 on water, rapidly spread out into a thin film, while a liquid of 

 large surface tension would under similar circumstances gather 

 itself together into a compact mass. A film of water on a glass 

 rod will gather up into a number of drops. The spider avails 

 itself of this property of liquids in forming the necessary drops 

 of sticky liquid on its web. This liquid is secreted by the 

 spider as film, covering the cobweb evenly. The lecturer then 

 proceeded to show that the influence of surface tension largely 

 regulated some very important and familiar phenomena. He 

 showed that the presence of smoke in the air greatly facilitated 

 the condensation of moisture into drops, and connected this 

 fact with the foggy atmosphere prevalent in towns. By the 

 aid of another experiment he showed the important bearing of 

 surface tension upon filters. When a stream of impure water 



