292 Insecticides 



If the reverse, the Wash spreads over the whole surface and will 

 wet the Solid. 



Assuming then that the surface tension of the air and the solid 

 (the leaf) is always the same, and that the surface tensions of Wash- 

 Solid and Wash- Air vary proportionately according to the nature of 

 the wash, then clearly the measure of the surface tension of Wash-Air 

 is an indication to us of the wetting power, and the lower this is the 

 greater the chance of wetting. 



The equation is 



w u o Tj /Wash- Air 

 Wash-Sohd < , ,. a ^■ ■> 

 \ -f Air-Sohd, 



clearly the lower the two Wash surface tensions, the more likely are we 

 to get the one we want. 



, /lir- fVash per sq. cm. 



Leaf -Air 

 \ per sq. cm. 



Leaf '.^ash-Leaf 



per sq. cm. 



The Air and Water surface tensions of these liquids are: 



Air Water 



Water 8-253 — 



Petroleum 3-233 2-834 



Mercury 55-03 42-58 



I have not the surface tensions of any of these with a leaf surface 

 so I cannot work out an equation, but the above figures show why, for 

 instance, mercury and petroleum and water behave differently as regards 

 leaf surfaces. 



It will be noticed that it is not only the wash-air surface tension that 

 counts, but the wash-solid and the solid-air. The difference between 

 the wetting power of water on a cabbage and on an apple leaf is due to 

 the difference in surface tension to air and wash between the wax on 

 the cabbage leaf and the substance on the apple leaf. In the first case 



Wash-Wax ^^''^^^■^ 

 / Wax- Air 



and no wetting occurs by the wash. 



