WILLIAM D. HARKINS 



155 



are of particular importance in biological systems, particularly in the human body 

 itself. 



If a cubic centimeter of water is sprayed into spherical droplets o.oi ix (100 A) in 

 diameter, the area of the surfaces thus formed is 600 sq.m. or approximately one- 

 eighth of an acre. The free surface energy {yy^A) for this area at ordinary tempera- 

 tures is about 2.2X10^ ergs or 10.5 calories, while the total surface energy {H) is 

 larger and equal to 16.6 calories. This is one-third as large as the total energy of heat 

 vibration of all of the molecules in the water. 



Any system in which the area of the surfaces (interfaces) becomes large enough so 

 that the surface energy is appreciable in comparison with the energy of (heat) vibra- 

 tion of the molecules of the disperse phase is considered as a "colloid." 



It is commonly observed that water drops on a hot 

 stove or on very dry dust assume a spherical shape, which 

 is the form assumed by a balloon surrounded by a uni- 

 form elastic membrane. This suggests that every liquid 

 is surrounded by an elastic film, the tension of which 

 causes the surface to contract to the smallest possible 

 area for the volume of the liquid, provided other forces 

 (such as gravitation) do not act to change the form. 



If a faucet with a narrow orifice is turned on very 

 slightly, a drop may be seen to form, and hang for some 

 time, after which it drops very suddenly. The drop is 

 supported before it falls by the vertical component of 

 the surface tension. If a capillary tube is dipped into 

 water, the liquid inside the tube rises higher than that 

 outside. Here, also, the film of liquid on the inside wall 

 of the tube exerts an upward pull. If a camel's-hair 

 brush is dipped into water, the hairs remain spread 

 apart as if they were dry and in the air, but when the 

 wet brush is pulled out of the water, the pull of the surface tension of the water 

 binds all of the hairs compactly together. 



A soap film stretched on a wire frame such as that shown in Figure 7 has two sur- 

 faces. If the distance AB is | cm., then this lower wire is in contact with ^ cm. of the 

 film or I cm. of the surface. The pull on the film as measured by the weight of the 

 wire and the weights suspended from it at W gives the surface tension of the surface 

 per unit length. This may be expressed in dynes. If the wire is pulled downward i cm., 

 then the surface increases in area by i sq.cm., so the work done is force times area 

 equals 7X1=7 ergs. This energy may again appear as work when the film contracts 

 to its original position, so it possesses the characteristics of free energy. Thus 72.8 

 dynes per centimeter is the surface tension of water at 20°, and 72.8 ergs per square 

 centimeter is the free surface energy of water at this temperature. 



Fig. 7. — Maxwell frame for 

 the determination of the surface 

 tension of a soap film. 



TENSILE STRENGTH AND TENSILE ENERGY 



There are a number of phenomena which indicate that the forces between ad- 

 jacent molecules in soUds and liquids are very high. Tensile-strength tests on bars of 



