290 Insecticides 



If we did, it is likely that some of them would be much the same as 

 before, since the sum total of human experience, however got, brings one 

 near the truth. Experience tells us that paraffin kills; it does not 

 tell us how ; but knowing how is not going to tell us not to use it. 

 Paraffin will still kill ; only ultimately out of knowing why, we shall 

 know whether paraffin itself is the best or whether there is something 

 better not yet tried. 



In considering the question of insecticides there are a number of 

 points and I would like to suggest two : 



1. Spreading over and wetting the plant. In many cases it is 

 necessary to spread over or wet the plant in order that the substance 

 contained in suspension or in solution should be spread over the leaf. 



If a leaf is sprayed with mercury, the mercury would not spread 

 over or wet it ; the mercury would draw into drops and run off. 



If it is sprayed with water, the water might spread and it might 

 not ' wet' ; on a cabbage leaf it would not spread or wet, on others 

 it might. 



If you sprayed it with pure paraffin it would spread over and wet 

 all the plant and the paraffin would 'creep.' AVhy this difference of 

 behaviour? It is partly due to two things: the surface tension of 

 the liquid, high in mercury, middling in water, low in paraffin; the 

 question of the inter-solubility of the liquid of some constituent of the 

 surface of the leaf, e.g. the wax on the cabbage leaf, being totally in- 

 soluble in the water and causing the w^ater to roll ofE at once. 



To spread, therefore, one must presumably have a low surface 

 tension, and to wet also have a wax solvent or a liquid that will be 

 absorbed by or dissolve the outer coat. Now leaving the question of 

 a leaf with its broad surface, take the case of a bud or a curled leaf: 

 here still more the surface tension question is apparent as the liquid 

 might need to penetrate the curled leaf and act by capillarity. I will 

 quote a case here I had need to go into carefully : we had a disease on 

 indigo in which the plant curls into a compact knot at the tip; in this 

 lives a Psylla. It was a question of finding an insecticide that would 

 wet and penetrate. We found by experiment that some insecticides 

 penetrated better than others. We found that they varied according 

 to the concentration, and we eventually found one that penetrated so 

 well as to go right in. 



Dr Leather, the Imperial Agricultural Chemist, very kindly 

 measured surface tensions for me and this last had the lowest. 

 It was an "oleic" acid soap at 1 11). in 12 gallons of water. Jt was the 



