524 Journal of Agricultural Research voi. xiii. no. h 



Nuttall (2) have studied the physical principles governing the wetting 

 and spreading of contact insecticides and have endeavored to devise 

 means by which these important properties may be easily measured. 

 In all of these papers the authors have failed to distinguish between 

 wetting and spreading. In this paper a distinction will be drawn be- 

 tween these two terms. If a liquid is placed upon a solid, and there is a 

 specific attraction between the two, they will come into actual contact. 

 The slight chemical affinity exhibited between the two substances is 

 what is denoted as wetting, or adhesion between the liquid and the solid. 

 For example, if a drop of mercury is placed upon glass, there is no specific 

 attraction between the two, owing to the film of moisture and air on the 

 surface of the glass (j, p. 176). Hence, the mercury is said not to wet 

 the glass, and there is no adhesion between the two. By boiling the 

 mercury in a glass tube and thus expelling the moisture and air between 

 the mercury and the glass, it is found that there is an actual wetting of 

 the glass by the mercury, as is indicated by adhesion. 



If a liquid is brought into contact with a solid and wetting takes place, 

 the spreading of the drop into a thin film may or may not occur. The 

 law governing spreading has been carefully explained by Cooper and 

 Nuttall {2) . They find that if the surface tension of the substance upon 

 which the spray is placed is greater than the surface tension of the spray 

 plus the surface tension at their interface (interfacial tension), the 

 liquid will spread. Otherwise there will be no spreading. Bigelow and 

 Hunter (/) have given a very much simpler explanation of the whole 

 matter. They consider {p. 377-387) that if a liquid is in contact with a 

 solid — that is, actually wetting the solid — two forces are at work : 



First, the cohesion between the like particles of the liquid which, in the surface 

 layer is denoted by the phrase "surface tension," and second, the adhesion between 

 the liquid and the walls [solid] . 



Thus, if adhesion to the solid is stronger than the cohesion of the 

 liquid, the liquid will spread over the solid. The same law applies when 

 two liquids are in contact. The following experiment will serve as an 

 example. If a filter paper is soaked in water and spread out flat on a 

 glass plate, a drop of kerosene on this wet paper will quickly spread into 

 a thin film. In this case the adhesion between the water and the kero- 

 sene is greater than the cohesion of the kerosene. On the other hand, 

 if the filter paper is soaked in kerosene and a drop of water placed upon 

 it, the water does not spread out into a thin layer. In this case the 

 cohesion of the water is greater than the adhesion between the water 

 and the kerosene, which is the same in both experiments. Thus, it is 

 clear that there must be wetting before there can be spreading, but it 

 does not necessarily follow that when there is wetting there must be 

 spreading, for otherwise the water would have spread over the kerosene. 



One other factor has a considerable influence on the spreading of an 

 insecticide — namely, viscosity. Viscosity may be defined as "the inter- 



