58 Eighteenth Report State Entomologist of Minnesota — 1920 



the surface layer so that the surface area is increased one square 

 centimeter, is the unit of measurement of surface tension. This force 

 is expressed in dynes per square centimeter. 



If the drop under consideration is an oil and it comes to rest 

 on the surface of another liquid, such as water, a slightly different 

 condition will exist. In this case, the oil may spread out over the 

 surface of the water, forming a thin film, or it may remain in the 

 form of a flattened sphere, without any spreading occurring. If such 

 a flattened drop is studied, it will be found that the surface of the 

 upper portion of the oil is in contact with air and will exhibit the 

 surface tension previously mentioned. The lower surface of the drop 

 is now in contact with the water, and a molecule in its surface is pulled 

 toward the oil by the attraction of the oil molecules and also toward 

 the water by the molecules of the water. Here again a certain tension 

 known as the surface tension exists at the oil-water interface and 

 will be referred to as the interfacial tension. The part of the surface 

 of the water not covered by the oil, is in contact with the air and will 

 exhibit a certain tension, the surface tension of the water. In such 

 a case the two liquids will arrange themselves in such a manner that 

 the total free surface energy, that is, the sums of the surface tension 

 of the oil, the surface tension of the water, and the interfacial tension 

 of the oil and water will be reduced to a minimum. This statement 

 may be made clearer by an example. 



The unit of measurement of surface tension is dynes per square 

 centimeter, hence if the surfaces considered in the illustration are 

 limited to one square centimeter or a multiple thereof, the free surface 

 energy in each case will equal the surface tension or a multiple of it. 

 Imagine a vessel of water in which the surface exoosed to the air is 

 exactly two square centimeters. Place upon its surface just enough 

 benzene to exactly cover one square centimeter of the water's surface. 

 The surfaces of the whole system will then be one square centimeter 

 of water in contact with the air. one square centimeter in contact with 

 the benzene layer, and one square centimeter of benzene in contact 

 with air. The total free surface energy would then be represented by 

 the sum of these surface tensions and interfacial tension. 



First condition — Total Surface Energy = Sur. Ten. Water + 

 Sur. Ten. Benzene -j- Inter. Ten. Benzene-\\'ater. 



If, however, the benzene is spread out so that it covers the total 

 surface of the water, only two surfaces will be exposed, namely, two 

 square centimeters of benzene exposed to air and two square centi- 



