60 Eighteenth Report State Entomologist of Minnesota — 1920 



benzene film pulls together, forming a flattened drop on the surface of 

 the water. 



These equations may be simplified and generalized by allowing 

 A to stand for the surface tension of the liquid upon which the sec- 

 ond liquid, B, is placed. The interfacial tension can then be consid- 

 ered as AB. The general formulae may be expressed as 



A + B -f AB 

 and 2B -f 2AB 



Subtracting the 2nd formula from the first, the expression A — E 

 — AB or A + (B + AB) is obtained. It is easy to see that if this 

 result is positive, spreading will occur, while if negative the liquid. 

 B, will collect in the form of a flattened drop. In short if A > B 

 -\- AB spreading will occur while if A < B + AB spreading will not 

 occur. 



Turning to the actual conditions of spraying it is found that A 

 represents the leaf surface while B is the spray, hence if the surface 

 tension of the leaf is greater than the sum of the surface tension of 

 the spray and the interfacial tension at the spray-leaf surface, spread- 

 ing will occur; while if less, the spray will collect in the form of a 

 drop and roll oft the leaf. Altho it can not be proved that solids 

 have a surface tension, all evidence tends to show that they do have 

 a high surface tension which can not be measured owing to the im- 

 mobility of its molecules. Since the surface tension of the leaf will 

 remain constant, spreading must be obtained by influencing the sur- 

 face tension of the spray or the interfacial spray-leaf tension. Lower- 

 ing both or either of these surface tensions vmtil their sum is lower 

 than the surface tension of the leaf will produce a spray which will 

 spread over the leaf surface. 



The first question arising is how can surface tension be lowered? 

 Some substances dissolved in water will tend to concentrate in the 

 interior of the liquid, so that a unit volume of the interior of the 

 liquid will possess more of substances than a unit volume of the sur- 

 face layer. Most inorganic substances, such as sodium chloride, cop- 

 per sulphate, etc., show this character. These substances, having a 

 strong tendency to move away from the surface layer, will require 

 even a larger force to bring them into a surface. The surface tension 

 of water is therefore increased by the addition of such substances. 

 The surface tension of water at 20 degrees C. is 72.8 dynes, while a 

 salt solution of a density 1.193 at the same temperature has a surface 

 tension of 85.8 dynes. 



