W. F. Cooper and W. H. Nuttall 221 



The wetting power manifests itself very differently, according to the 

 nature or the state of the substance to be wetted.. . .With regard to 

 plants, the wetting power of some solutions appears to depend less upon 

 the surface tension than upon the surface viscosity, as defined by 

 Plateau.' 



Lefroy (1915, pp. 291-3), in a paper dealing with the precise mode 

 of action of contact poisons in insecticides, emphasises the importance 

 of the wetting power of a spray-wash, and also touches upon the theory 

 of wetting. He points out, that the capacity of a liquid to wet a solid 

 surface is determined by the relative values of three distinct tensions: 

 viz., those of the Wash/SoHd, Wash/Air and Air/SoHd. He states 

 that the condition for the wash to wet the solid is that the tension 

 Wash/Solid must be less than the sum of the other two, viz., Wash/Air 

 and Air/Solid^. He points out that, as the tension Air/Solid remains 

 constant, and the tension Wash/Solid is indeterminate, the tension 

 Wash/Air is the only one to be considered, and the lower this tension 

 is, so the more readily will the wash wet. Thus Lefroy's method of 

 comparing the wetting power is nothing more than a determination of 

 the surface tensions of the respective washes. 



Other methods of comparing the wetting power of solutions have 

 been based upon a comparison of the amount of liquid which adheres 

 to a solid object of standard shape and size, which is dipped into the 

 liquid and then carefully withdrawn. Naturally, the nature of the 

 solid employed for the purpose is of considerable importance, and of 

 those, the use of which has been suggested, mention may be made of 

 the elytra of Blatta Peri planet a americana, strips of celluloid and of 

 grease paper. 



Most of the cattle dips in actual use contain a certain proportion 

 of emulsified oil. It has been claimed that the presence of this emulsified 

 oil increases the wetting power of the dip, and further, that the finer 

 the state of this emulsion, the more marked is this effect. It has been 

 urged, that the wetting power of such emulsified dips may be measured 

 by a comparison of the fineness of the emulsified oil particles, and 

 attempts have been made to compare the latter by observing the relative 

 capacity of the dips to give a permanent foam. 



A knowledge of the percentage of soap in any preparation gives no 

 indication whatever as to its wetting power, so that the latter cannot 

 be determined by means of a chemical analysis. Vermorel and Dantony 

 (1911, pp. 972-4) have shown that the actual mode of preparation of a 



1 See footnote, p. 228. 



