232 



Wcffiiif/ Poirrr aiid its /Jcfcniiiiinfion 



The values in the last column of Table 1 indicate that Dip A 

 should have the power of wettinj:; a surface made greasy with castor 

 oil, whilst Dip B should lack this power. This was found actually to 

 be the case. A glass sHde, smeared with a thin layer of castor oil, was 

 completely covered and wetted when the dilution of Dip A was poured 

 over it, whilst a similarly prepared slide, on being treated in tlic same 

 manner with the dilution of Dip B, was found to assert its greasy 

 nature, the dipping fluid running into drops and failing to wct^ 



TABLE 1. Welling Power of Callle Dipping Fluids 

 towards Castor Oil. 



By methods to be described later, we have determined the surface 

 tension, and the interfacial tension towards liquid vaseline, of solutions 

 at various concentrations of most of the commercial soaps usually 

 employed in the preparation of dips and sprays. There is no particular 

 point in the publication of all these results. These various determin- 

 ations were carried out to ensure that this method of comparing wetting 

 power is applicable to all kinds of soap. As the results obtained with 

 the difTerent soaps did not vary markedly from one another, those 

 given by sodium oleate are tabulated below and may be taken as 

 typical. As commercial soaps vary so greatly in composition, the 

 concentration of soap is expressed as percentage of fatty acid. The 

 determinations were all carried out at a uniform temperature of 20° C. 



Owing to the low surface tension of the liquid vaseline employed, 

 the values obtained for the expression T^ - (Tj + Tja), i.e. the wetting 

 power, are all negative. This indicates that the soap solutions used 

 are incapable of wetting the vaseline, which actual trial with a slide 



' Of course, in time both dips would wet the ghi.i.i owing to the displacement of the 

 oil from the glass surface. What we are at present interested in is the jwwer of the dips 

 to wet the surface of the oil. 



