(Perhaps it may be said that it depends on the dearer nf attrac- 

 tion or repulsion between the particles of the two substances. ) 



154. In the case of a solid, if there is a strong attraction between 

 the fluid and the solid, the fluid wets the solid. Will water wet 

 clean glass? Will it wet paraffin? Will mercury wet glass? 

 Try this by dipping a piece of the solid into the fluid. 



155. \Vhere there is an attraction the fluid may be lifted against 

 gravity. Try this by dipping a plate of glass into the water. Does 

 the water rise at the sides of the plate? Try with a glass tube. 

 Does the water rise in the tube? What is the form of the water 

 surface in the tube? Try water in a paraffined glass tube. 



156. This attraction may likewise pull the solid into the fluid. 

 Place a drop of water on, the edge of a glass plate. Then take a 

 very small splinter of wood, and with forceps bring one end of it 

 into the drop. Notice how it is pulled in. Observe that the water 

 rises along the splinter so that the spherical surface of the drop is 

 altered. What pulls the splinter into the drop? Show by a 

 diagram. 



157. Place a drop of chloroform on the bottom of a watch-glass 

 full of water. Bring against it a piece of hard shellac. What 

 happens ? \Vhy ? 



158. "Choice" in a drop of fluid owing to varying surface tension 

 in contact with different substances. With drops of chloroform as 

 in experiment 157, try bringing other substances into contact with 

 it. Are they accepted or rejected? The following should be tried: 

 shellac, glass, paraffin, gum arabic, chlorate of potash, resin, potas- 

 sium iodide. Is there any relation to the solubility of the substances 

 in chloroform? 



159. "Artificial Difflngia Shells." Grind up some glass finely 

 with chloroform in a mortar. Inject drops of this with a fine- 

 pointed pipette into a watch-glass of water. Notice how the glasj 

 grains come to the surface and arrange themselves in a layer, as in 

 a Difiiugia shell. 



The same experiment may be performed with linseed oil in place 

 of chloroform and jo ( / c alcohol in place of water. 



G. FORMATION OF FILMS UNDER THE INFLUENCE ( )F 

 SURFACE TENSION. 



1 60. Laws of Gibbs. Note in a warmed glass of milk the 

 gradual formation of a film at the surface. Prove that film forma- 

 tion is net due to evaporation. Substances that lower the solution 



39 



