204 LECTURES. 



water ; 28 grains for alcohol ; and 31 grains for oil of turpentine. 

 Liquid in drops. Relative size of drops. 



(67.) The foregoing method gives us the relative cohesion, but not 

 the absolute. It is not the attraction of the whole section of the fluid 

 which produces the result, but that of the indefinitely thin film around 

 the exterior perpendicular surface, and within which the mass of fluid 

 hangs by its cohesion. [Yoiing, La Place, Poisson.'] Explanation of 

 this : See (89.) 



Connexion of the curvature of the surface with this apparent attrac- 

 tion. See Capillarity. 



(68.) The molecular attraction of water for water at 32 degrees, is 

 probably greater than ice for ice at the same temperature. The ap- 

 parent feeble attraction of water for water, is due to the perfect mobil- 

 ity of the particles which permits them to slip upon each other. 

 Explanation of this: See (89.) 



Phenomena of adhesion. 



(69.) Adhesion of solids to solids. — The solder adheres to the metals 

 which it unites. Grold leaf stamped on metals — adheres to glass. 

 Wax adheres to many solids. Bladder dried on glass_, the surface of 

 the latter torn by forcibly removing the former. 



(70.) Adhesion of liquids and solids. — The force required to separate 

 glass from mercury, shown by experiment — the cohesion is here 

 stronger than the adhesion. 



Same experiment with a clean surface of copper, the rupture is now 

 between the molecules of the liquid — adhesion stronger than cohesion. 

 The same in case of a solid, wet or infilmed with water. 



Stream of water made to follow the under side of an inclined glass 

 tube. Method of pouring a liquid into a vial with small neck. 



Explanation of the use of a lip to vessels from which liquids are to 

 be poured. The edge of the vessel touched with grease. 



Mercury poured from glass vessel, also from a tinned one. 



(71.) Adhesion of solids increased by the interposition of a liquid. 

 The adhesion increased by the solidification of the interposed substance. 

 A thin flake of tallow cooled between two discs. 



(72.) Different liquids possess difierent degrees of attraction for the 

 same solid. 



Film of water driven from surface of glass by a drop of alcohol ; the 

 attraction of the latter for the solid the stronger. 



Same effect with oil of turpentine. 



(73.) Phenomena of solution ; lead in mercury; sugar in water, 

 &c.; heterogeneous attraction stronger than the cohesion of the solid. 

 Different bodies dissolved in the same liquid. 



Effect of pulverizing in hastening solution. — Due to the increase of 

 surface. A cubic inch of matter cut into little cubes, each YiVo" of an 

 inch on the edge, will exhibit a surface of exactly 100 square feet. 

 Trituration produces a finer division than even this. 



Explanation of the cleansing effect of water. 



