H. M. Lefkoy 293 



In the yecond case 



Wash-Apple leaf 



y AVash-Air 



Apple leaf- Air 

 and wetting occurs. 



Therefore, the nature of the actual substance on the surface of the 

 leaf determines whether the wash will wet or not. 

 If then we knew the surface tensions of 



plant-air, 

 plant-wash, 

 wash-air, 

 we could always tell if the wash would roll off or not. So for the insect : 

 if we knew the surface tension of 



aphis-air, 



aphis- wash, 



wash-air, 

 we should know if our wash would wet the aphis or not. As we do not 

 know these, and as our plant-air, or aphis-air, remains a constant, we 

 strive to make the wash a practical success by getting one in which 

 wash-air (measurable) and therefore wash-plant is as low as possible. 

 One imagines that, had insecticides received attention from chemists 

 and physicists, the constants surface tension of plant-air, plant-wash, 

 wash-air could be looked up, as also the more complex surface tensions 

 of emulsions. When drops of oil float in an emulsion they affect the 

 surface tension of the emulsion in relation to air and plant. 



2. The next point is this: how do these insecticides act; must the 

 liquid spread over the insect, must they 'wet' it, and, if so, then how 

 do they act ? 



There are, I think, three points : 



(1) Mere spreading over mechanically. 



(2) Wetting with spreading. 



(3) Toxic action after wetting. 



(1) In the first case I take such actions as lime wash, which spread 

 over mechanically and never wet ; the action may be due to mechanical 

 causes solely, probably suffocation. 



(2) Wetting, including spreading, means the insecticide is in 

 physical contact with the body. What does this really mean? Does 

 it mean that the head, the segments, the antennae and legs are wetted, 

 or that only parts are, or that there is a film of the liquid lying over the 

 whole insect but not actually in contact, say, with the under surface? 



2U— 2 



