H. M. Lkkkoy 291 



best soap we got and it was better than rosin compound, oil emulsions, 

 soda solutions and other things. An oil emulsion in -8 per cent, soap 

 solution has a higher tension than the soap solution. We tested 

 'penetration' by dipping curled heads of the plants, then cutting them 

 open and seeing what had happened. 



Now in this case it was purely one of getting a commercial product 

 of low enough surface tension to wet and penetrate the curled plant. 

 We had to think afterwards of what we could put in to kill the insect. 

 A similar case exactly will be described to you in the paper by Mr P. R. 

 Awati. 



Apart from the mere surface tension, what of the different behaviour 

 of a liquid on an ordinary leaf or on a cabbage leaf : is there a reaction 

 between the liquid and the surface depending on the latter ? I am not 

 a physicist and cannot say, but I believe there is and, so far as I under- 

 stand it, I explain it below. But you can get round this in some cases 

 by adding to the liquid a solvent of the wax (if any) in the plant. For 

 instance, a cabbage leaf behaves very differently to a liquid containing 

 a solvent of its wax than it does to the liquid without ; for the solvent 

 dissolving the wax eliminates the wetting question since the wax is 

 there already, and it becomes therefore a question of 'spreading.' 

 Spreading power, even more than wetting, depends on low surface 

 tension and we may, I think, distinguish "spreading of a liquid over a 

 surface it never wets" and wetting due to the liquid actually entering into 

 solution with a substance on the leaf surface. You may attain 'spread- 

 ing' of the liquid by either method but not wetting. 



I am indebted to Dr Vaughan-Cornish for a reference to Clark- 

 Maxwell's Theory of Heat, in which surface tension is discussed. 

 I have endeavoured to extract the gist of the physics of surface tension 

 so far as it refers to this problem. 



The spreading power of a liquid on a solid depends in this way on 

 surface tension : 



W^hen a solid body (the leaf) is in contact with two fluids (air, wash) 

 then if the tension of the surface separating the solid from the second 

 fluid (wash) exceeds the sum of the tensions of the other two surfaces, 

 the second fluid will gather itself into a drop and the first fluid 

 spread over the surface, i.e. 



i( Q T f A^^ 1 w r^ \ '"^-T- of Wash and Air 



It . b.i. of W ash and bolid > , .. i ^ tj 



/ -\- Air and Sohd, 



then the Wash goes into a drop. 



Ann. Biul. i 20 



