W. F. COOPEH AND W. H. NUTTALL l'27 



The second reason why we do not consider the surface tension to 

 be a suitable criterion of the wetting power of a solution is the fact 

 that, whilst, within limits, the wetting power increases with the con- 

 centration of soap, the surface tension (static) of soap solutions is 

 practically independent of the concentration. As we have already seen, 

 this independence is due to surface concentration. It is, of course, 

 open to argument that the surface viscosity which results from this 

 surface concentration, and which itself greatly assists wetting, may 

 serve to coimteract the lack of soap in the bulk of the liquid. Although 

 not actually stated, this is possibly the idea underlying the claim of 

 Vermorel and Dantony (1910, p. 1144 et secj.), that very dilute solutions 

 of sodium oleate wet solid surfaces just as well as concentrated solutions, 

 and that, whilst the true tension, by which they evidently mean the 

 dynamic, is of no practical concern in regard to wetting power, the 

 static surface tension as determined by capillary rise or the stalag- 

 mometer affords a practical criterion of wetting power. We shall show 

 later (p. 23.3), however, that up to a concentration of about 3 %, the 

 wetting power of a soap solution does actually increase progressively 

 with the concentration. For the present, a simple experiment will 

 serve to illustrate the point. Solutions, containing respectively 10 %, 

 1 % and 0-1 % of oleic soap all possess practically the same surface 

 tension. If their wetting power be tested by their capacity to form 

 a continuous film on a piece of grease paper, it will be found that the 

 10 % and 1 % solutions give a positive result, whilst the 0-1 % solution 

 entirely fails to wet. In passing, it may be observed that the power 

 to form a continuous film does not necessarily indicate that the 10 % 

 and the 1 % solutions have the same wetting power. It is evident 

 from the diagram (Fig. 1, p. 223) that when complete wetting occurs, 

 the angle of contact between the solid and lic|uid is zero. When benzene 

 is poured on to a clean glass surface, a continuous film of lic|uid is 

 produced, or in other words, the benzene wets the glass. If ether is 

 substituted for benzene, a continuous film is also produced, and we 

 might therefore be inclined to assume that the wetting power of ether 

 is equal to that of benzene. This however is known not to be the case. 

 The angle of contact between benzene and glass is zero, which indicates 

 complete wetting. The angle of contact between ether and glass is 

 16°, which is evidence that wetting is not complete. The wetting 

 powers of benzene and ether are therefore not identical, although both 

 give a continuous film on glass. 



Sufficient evidence has now been brought forward to indicate 



