228 Wettiiif/ I'ofvcr aial its Dvtcrininatiuii 



definitely that a determination of the static surface tension is in- 

 sutiicieiit for the purpose of an estimate of the wetting power of a 

 solution containing soap, and this therefore disposes of all the methods 

 as suggested by Hriiniiich and Smith, Vermorcl and Daiitony, and 

 Lefroy^, of comparison of wetting powers by such determinations. 



We now come to the question as to how far the weight of hquid, 

 adhering to a strip of a not easily wetted solid, which is immersed in 

 the liquid and then carefully withdrawn, can furnish a criterion of the 

 wetting power of the liquid in question. We have already seen that 

 the power to wet is dependent upon one or both of two main factors: 

 (1) a low surface tension and interfacial tension, and ("2) a liigh surface 

 viscosity. One of the results of a low surface tension and interfacial 

 tension, where surface concentration effects are not very marked, is 

 to cause the hquid to 'run' easily. AVith tree sprays, this is a most 

 desirable property, in order that the licjuid may penetrate the folds and 

 interstices of the leaves etc. In the test of wetting power under con- 

 sideration, the effect of a low surface tension and interfacial tension 

 would be to wet the surface of the immersed solid completely, from 

 which, on mthdrawal from the fluid, all excess of Hquid would 'run' 

 off very rapidly, but still a continuous li(juid film would remain adhering 

 to the surface. The effect of a high surface viscosity would be in the 

 opposite direction ; as soon as ever a thin film of liquid was produced, 

 the surface viscosity would come into play, as a result of which draining 

 would be largely hindered. The amount of li(|uid adhering to the 

 sohd after immersion would therefore be the resultant of two opposite 

 effects, and as such, we should not expect it to differ in any marked 

 degree, whatever the liquid under investigation. Our experiments 

 have proved this actually to be the case. The solid used for immersion 

 experiments was a piece of wide glass tubing, coated internally and 

 externally with a thin layer of collodion. Preliminary trials showed 

 this to be the most suitable surface. Three liquids were used for the 

 purpose of comparison: distilled water, a 1 % saponin solution, and 

 a castor soap solution containing emulsified green oiP. The licjuid 

 under investigation, contained in a large weighing bottle, in order to 

 prevent loss through evaporation, was weighed both before and after 



' It may be desirable to point out that Lefroy, after appreciating the fact that the 

 process of wetting is dependent on the relative magnitude of three surface tensions, viz. : 

 Wash/Solid, Wash/Air and Air/Solid, has confu-sed their relationship. The condition 

 for wetting is, of course, that the Air/Solid tension should exceed the sum of the other 

 two. 



- A neutral oil of lii'.'h boilintr point obtained in the distillation of coal-tar. 



