W. F. Cooper and W. H. Nuttall 231 



up with a soap basis, however, which is the case with most cattle clipping 

 fluids and horticultural sprays, we have worked out a method which 

 affords a convenient and ready means of comparing their wetting 

 powers. The method is a slight modification of that employed by 

 Donnan (1899) for the determination of the emulsifying action of soap 

 solutions. We realise that the method is open to certain objections, 

 and that the results obtained cannot be regarded as absolute. At the 

 same time, we venture to think that it is based on sound theoretical 

 principles, and that the results obtained by its employment are suf- 

 ficiently accurate for all ■practical purposes. 

 If we consider once more the equation : 



it is evident that, if these three tensions could be determined, the 

 expression T., — (Tj + T^.,) would represent the wetting power, ino- 

 vided that the disturbing factors of solubility and surface concentration 

 did not come into play. As already stated, the surface tension of a 

 solid and the interfacial tension of a solid to a liquid are indeterminate, 

 from the point of view of a practical test. The interfacial tension 

 between a thick oil and an aqueous liquid can, however, be determined 

 without any great difficulty ; and, as the surfaces to be wetted by dips 

 or sprays are usually of a greasy or a waxy nature, there does not 

 appear to be any valid reason why in the comparison of wetting power, 

 a thick oil should not be taken to represent the solid surface. 



In our method for the determination of the wetting power of a 

 dip or spray fluid, a thick paraffin oil (liquid vaseline), free from acid, 

 was taken as a standard to represent the greasy surface (e.g. a greasy 

 hide, a waxy leaf). The surface tension of this oil was determined 

 once for all by any convenient method, e.g. capillary rise. To compare 

 the wetting power of any two preparations (soap), it was then only 

 necessary to determine their surface tension and their respective inter- 

 facial tension towards the standard oil, and to substitute these values 

 in the above equation. The values obtained in an actual comparison 

 of the wetting power of two cattle dips are tabulated below. The 

 determinations were made on the liquids diluted with water to the 

 concentration usually employed for dipping, and in this particular 

 instance castor oil was employed as the standard. As the two dips 

 under consideration were both practically neutral in reaction, the use 

 of this oil was permissible, but as a general rule, a neutral mineral oil 

 is preferable. 



Journ. of Agric, Sci. vil 16 



