SUPPLEMENTS FOR COPPER FUNGICIDES lOi 



sion in:."icative and later (59) a desirable attribute, although not a definite 

 criterion. Verniorel and Dantony (90) state that spreading occurs when 

 tJie cohesion of the molecules of the liquid is less than double their ad- 

 hesion for the solid and correlate a low "static" surface tension with 

 spreading, but subsequently decided (91) that it is not an index and that 

 a high surface viscosity has greater influence. Lefroy (56) states there 

 are three surface tensions — spray, leaf and interface — involved in spread- 

 ing and that the first should be low but the others cannot be determined. 

 Willows and Hatschek (96) claim that the adsorption or concentration of 

 a solute at the liquid-solid interface reduces interfacial tension. Cooper 

 and Nuttall (17) accept the Quincke (79) theory, — that if the surface 

 tension of the solid is greater than the sum of the surface tension of the 

 spray and of the interface, spreading will occur; but believe the solvent 

 action of the spray on the solid and the surface concentration of the solute, 

 with a high surface viscosity, may vitiate the equation. They consider that 

 interfacial tension alone will suffice for comparative purposes, and with 

 soap solutions is directly proportional to the drop number (stalagmometer 

 reading), since spreading varies inversely with interfacial tension. The 

 general formula for the wetting of a solid by a liquid, as given by Nuttall 

 (66, p. 38), is as follows: 



T, must be 2^>. T^H-,. 



Tj surface tension of liquid/air. 



Tj surface tension of solid/air. 



Tj2 interfacial tension of liquid/solid. 



Thus, spreading can be increased by reducing the surface tension of the spray or of 



the leaf-spray interface and T„ and T,^ are indeterminate. 



Freundlich (26, cited 66, p. 39) points out that a liquid may not wet 

 the surface of some solids owing to its inability to dissolve the surface 

 layer of moist air adsorbed on them, and sometimes the wetting power of 

 a liquid (saponin, proteids) is higher than would be expected, due to an 

 exceptional power of surface concentration (capillary adsorption) mani- 

 fested by a superficial viscosity first observed by Plateau (76) and later 

 by Vermorel and Dantony (91). Donnan (21) and Hillyer (43, p. 524) 

 have shown that a soap solution acts as an emulsifier by virtue of its low 

 interfacial tension. Ruth and Kelly (82) claim that direct impact and 

 drenching favor the formation of a film. Harkins, Brown and Davies (37), 

 Harkins, Davies and Clark (38) and Langmuir (51) treat various phases 

 of interfacial tension, influence of the size and shape of the molecule, active 

 and inactive portions, orientation, etc. Moore (64), Nuttall (66) and 

 Taylor (88) present a review of the subject and its present status. 



Subsfcmces Emplof/ed. 



The substances that have been used by different investigators to in- 

 crease wetting and spreading include substantially all those that were em- 

 ployed in this Station for increasing suspension (Sec. 2) and in addition 

 emulsions, creosote, some phenol derivatives and several of the higher 

 alcohols. There is apparently a clear line of demarcation between wetting, 

 spreading and distribution and adhesiveness and yet the two issues are 

 sometimes confused. Furthermore, the results are generally based on the 

 character of the wetting and the visible deposit on the foliage — a matter 



