Surface Forces in Fluids. 3.') 9 



menon as the spread of one liquid over another, for example 

 of a drop of alcohol over mercury; for in the latter case the 

 spreading is aided by the tension-forces within the mercury, 

 which cause a motion of its superficial parts, while in the 

 solid these forces are, as we have seen, rendered inoperative 

 by the internal friction. Moreover, we cannot decide in the 

 same way whether the observed value of the tension at the 

 common surface of the two liquids is a sum or a difference. 

 In the case of the solid it is evident that the liquid cannot 

 wet it unless it is condensed at the surface of the solid, i. e. 

 unless the attraction of the solid for the liquid is stronger than 

 that of the liquid for itself. If, besides, the mutual attraction 

 of liquid and solid is stronger than the self-attraction of the 

 solid, then the surface would be unstable but for the rigidity 

 of the solid. If this rigidity gradually gave way, the surface 

 of contact would extend, and the solid would mix with the 

 liquid. 



We thus obtain an explanation of the fact that a liquid 

 which dissolves or acts chemically on a solid always "wets " 

 it, and are led to a view of the nature of the beginning of 

 solution much resembling that put forward by Mr. W. W. J. 

 Nicol (Phil. Mag. Feb. 1883). 



When, on the other hand, the liquid-angle of contact with 

 the solid is obtuse, as for instance with mercury and glass in 

 air, it is not possible to decide in the same way whether the 

 surface-force within the liquid in the neighbourhood of the 

 solid is a tension or a pressure. If it is a pressure, then it 

 follows that the surface-pressure of air on glass is even greater 

 than the experimental surface-tension of mercury, which, of 

 substances that are liquid at ordinary temperatures, exhibits 

 the greatest of all the surface forces that have yet been 

 measured. This result would seem to find some independent 

 confirmation in the fact that glass is hygroscopic, and slightly 

 soluble in water, which is generally present in the air in which 

 the contact-angle is observed. Experiments on perfectly dry 

 surfaces — such, for instance, as those of Quincke with freshly 

 split laminae of talc before they have had time to condense 

 aqueous vapour upon themselves — throw some light on this 

 point, the further discussion of which is, however, beyond 

 the object of the present paper. 



19. I have aimed in this paper at proving directly, from 

 consideration of the elastic and thermal properties of solids, 

 liquids, and gases, the reality of the surface-tensions and 

 pressure, since, from the principle of the surface-tension, if 

 once admitted, when used in combination with the experi- 

 mental fact of hydrostatics, that in a liquid acted on by gravity 



