NATURE OF LIFE AND LIVING MATERIAL 23 



WATER -^ 



AIR 





; * 

 '■•..>■' '• 



: • ! /I 

 ••...X ■•...x 



Vii^ 



water the water molecules attract each other, the property being 

 termed cohesion. In the deeper portions of the water, away from 

 the sides and surface, each molecule is attracted by adjacent mole- 

 cules in all directions with equal force (Fig. 3). But at the surface, 

 the interface between the water and the air, the water molecules 

 there located are attracted downward by the underlying water 

 molecules and upward by the molecules of 

 the gases in the air. The attraction of the 

 gas molecules for the water molecules is not 

 so great as that of the water molecules to- 

 ward each other. The pull on the surface 

 water molecules is therefore unequal. 

 Equilibrium is attained when the number 

 of the water molecules at the surface is the 

 least possible, that is, when the surface is 

 as small as possible. So surfaces always tend 

 to contract; hence a drop of a Hquid always 

 tends to assume a spherical form. The ten- 

 sion that results is termed surface tension 

 and differs widely between different sub- 

 stances. The surface tension of water is 

 given as 72 dynes per centimeter; that of 

 ether is 16 dynes per centimeter. 



The surface of a liquid substance, whether 

 in contact with air or with a suspending medium, being in a state 

 of tension, is a seat of potential energy. Consequently, if the area 

 of the surface is reduced, energy is thereby released; since free 

 energy always tends to become bound it is transformed into some 

 other type. 



It has been established that substances which reduce surface ten- 

 sion accumulate at surfaces. This means that when some suitable 

 substance, such as alcohol or ether, is added to water, the molecules 

 of the added substance accumulate in greater proportion at the sur- 



FiG. 3. — Scheme illus- 

 trating the theory of sur- 

 face tension. Water mole- 

 cules at the boundary be- 

 tween the water in the 

 beaker and the air above 

 are subjected to unequal 

 pull. They are more 

 strongly attracted by the 

 water molecules below 

 than by the molecules of 

 the gases of which the air 

 above is composed. Be- 

 cause the liquid is incom- 

 pressible, its surface is 

 therefore in a state of 

 tension. 



