Spread of Liquids on Solid Bodies. 421 



is so thin that it can only be perceived in solitary cases with 

 difficulty and by optical aids) may itself consist of the liquid 

 whose drops have been placed upon the surface of the solid 

 body. 



As a drop of oil, placed upon a clean surface of water, 

 divides itself when sufficiently large into two parts, of which 

 one part is drawn in a very thin film over the clean water- 

 surface, whose capillary constant or surface-tension is dimi- 

 nished, thereby rendering it possible that the rest of the oil 

 may remain lying in a lenticular globule* upon the modi- 

 fied surface of the water, so when water or aqueous saline 

 solutions are placed upon the clean surface of a solid body, a 

 portion of the liquid spreads out in an excessively thin film 

 upon the solid surface. The original surface-tension (if we 

 retain the expression on account of the analogy with clean 

 surfaces of liquid) of the clean solid surface is thereby dimi- 

 nished ; the remainder of the liquid stays upon the modified 

 solid surface in a lenticular form, and with an edge-angle 

 >0°. 



This film of foreign liquid forming over the surface of the 

 solid body at the first moment with very great rapidity, is of 

 a thickness varying with the temperature and purity of the 

 surface, and is usually thinner than the double radius 21 of 

 the sphere of sensible action, or greater than 0*000050 millim. 

 But according to the rapidity of the spread or of the forma- 

 tion this film has different thicknesses and different properties, 

 and modifies the tension of the previously clean surface of 

 the solid substance in different manners also. The change of 

 density which the liquid in the immediate neighbourhood of 

 the solid surface experiences must depend upon the time 

 during which the molecular forces of adhesion act upon these 

 liquid particles. 



Upon a liquid particle at a distance < I from the solid wall 

 there acts the difference of the molecular forces which are 

 exerted by the substance of the solid wall in one direction, 

 and by the substance of the liquid and air lying on the other 

 side of it in the opposite direction. 



The action of the air is negligibly small; and one may 

 therefore say that upon the liquid particles in the immediate 

 neighbourhood of the solid wall there is exerted a greater 

 molecular force, and that the change of density thereby pro- 

 duced is greater, in proportion as the film of liquid which lies 

 over it is thinner. The change of density will be different 

 at different distances from the solid wall, and at given points 



* Pogg. Ann. cxxxix. p. 76 (1870). 



