SURFACE ACTION 51 



greater than that inside a large bubble, contrary to what happens when an 

 india rubber ball is blown out. The state of affairs in a soap-bubble is due to 

 the greater curvature of the small bubble, so that the component producing 

 internal pressure is greater in the smaller one. Fig. 33 shows this in a diagram. 

 The two curved lines are of the same length. The vertical component, that is, the 

 line drawn vertically perpendicular to the chord of the arc, is obviously greater 

 in the arc with the greater curvature. 



The fact just mentioned indicates the possibility of great pressure being 

 produced in very minute spheres of liquids, such as we find in certain colloidal 

 solutions. 



SURFACE ENERGY 



If the surface of a liquid is in a state of tension, it is clear that work may 

 be done by it, when the tension is able to diminish. Surface tension, in fact, 

 is the intensity factor of a kind of energy whose capacity factor is the area 

 of surface, thus : 



Surface energy = surface tension x surface area. 



FIG. 33. DIAGRAM TO ILLUSTRATE THE EFFECT OF CURVATURE OF THE SURFACE ON THE 

 PRESSURE INSIDE A SOAP BUBBLE, A DROP OF LIQUID, OR A SOLID PARTICLE. 



The length of the arc, and therefore the total amount of surface tension, is the same in both figures. The 

 internal component of the surface tension may be roughly represented by the length of the vertical line 

 in each case. 



What is the source of this energy? There can be little doubt that it is 

 ultimately chemical. The fact that it differs according to the chemical constitu- 

 tion of the liquid is sufficient to show this. Hardy (1912, p. 621) has recently 

 made some important experiments on this question. Various liquids, insoluble 

 in water, spread out in a thin film when dropped on its surface, owing to the 

 fact that they lower the surface tension. Substances of great chemical stability, 

 such as the heavy liquid hydrocarbons, refuse to spread at all, and only very 

 slightly lower the surface tension. Esters, glycerides, for example, produce 

 great fall of surface tension and spread widely. The suggestion is made that 

 this effect is due to decomposition at the interface, causing contact difference 

 of potential between film and water. 



SURFACE TENSION AT VARIOUS INTERFACES 



Hitherto we have confined our attention to the interface between various 

 pure liquids and air. When two immiscible liquids are in contact, there is 

 also a state of tension at the interface, but less than that when either is in 

 contact with air. It can be measured by the drop method, the stalagmometer 



