222 Weftiuf/ Power and its Dciernnnation 



fungicide has a marked influence upon its subsequent wetting power. 

 For instance, two liijuors, identical in their content of copper sulphate 

 and sodium carbonate, but jyrepared under diverse conditions, required 

 in the one case eight kilos of soap, in the other case onl)' two kilos of 

 soap to produce an equal wetting power. 



Before we are in a position to appraise at their real value, these 

 methods for determining tlie wotting power of liquids, it will be necessary 

 to consider in some detail tlie various factors upon which the process 

 of wetting depends. Tiiough these factors were described in the main 

 Iiy Quincke (1877, p. 149) at an early date, and have since been referred 

 to in various textbooks, yet they are so little understood by the ordinary 

 chemist or biologist, that one may be excused for reiterating them 

 here. 



A drop of rain, falling freely through the air, assumes a spherical 

 form, and behaves as if it were covered with a tightly stretched elastic 

 film. The spherical form is the result of the pressure exerted by this 

 surface film, and the film pressure itself is usually termed surface tension. 



Now consider the case of a drop of oil, suspended in dilute alcohol 

 of the same density as the oil (Plateau's experiment). Tlie drop of 

 oil also assumes a spherical form, as if bounded by a tightly stretcliing 

 film. Obviously, the two cases are perfectly analogous, and the shape 

 of the drop of oil is the result of a tension exerted at the interface of 

 the oil and dilute alcohol. This tension is the surface tension of the 

 oil to the dilute alcohol, but for the sake of distinction is usually known 

 as the interfacial tension. Such a tension will exist at the interface of 

 any two immiscible li<|uids, though in some cases it is known to be 

 extremely small. 



Consider now the surface of a solid. The idea of a stretched film 

 exerting a distinct tension at the surface of a solid is not so readily 

 conceived as in the case of a liquid. Nor can the existence of a solid 

 surface tension be easily demonstrated experimentally. Nevertheless 

 certain theoretical considerations place it beyond doubt that such a 

 tension does exist at the surface of a solid*. 



Further, in the case of a liquid in contact with a solid, judging 

 from analogy, we should expect an interfacial tension to exist between 

 the liquid and the solid; and this is known actually to be the case. 



' Surface tension may be regarded as surface energy per unit area, and it follows 

 therefore that part of tlio energy of a solid body may be regarded as proportional to its 

 surface, and that in this sense the body has a surface tension, this t<'nsion being measured 

 by the energy per unit area of the surface. 



