42 ■MECHANICS, 



SECTioiT n. 



The attraction of gravitation, as we have just seen, 

 takes place between bodies at a greater or less distance 

 from each other. There is another kind of attraction, 

 acting only when the parts of substances are in actual 

 contact; this is called cohesion. It is this which 

 holds the parts of a body together and prevents it from 

 falling to pieces. It may be shown by taking two 

 pieces of lead, and, after having made upon them two 

 smootlily-shaven surfaces with a knife, pressing them 

 Fig. 14. firmly together with a 



twisting motion {Fig. 14). 

 The asperities of the sur- 



Cohesive attraction m two lead balls. facCS arC tllUS pUshcd 



down, and the particles are brought into close contact, 

 so that cohesion immediately takes place between them, 

 and some force will be required to draw them asunder.* 

 Two pieces of melted wax adhere together in the same 

 way. Melted pitch or other similar substance, smeared 

 thinly over the polished surfaces of metal or glass, also 

 causes cohesion to take place between them. Smooth 

 iron plates, two inches in diameter, have been made to 

 stick together so firmly with hot grease as to require, 

 when cold, a weight of half a ton to draw them apart. 

 Plates of brass of the same size, cemented by means 

 * That this is not atmospheric pressure, like that which holds two 

 panes of wet glass together, is shown by the fact that it requires 

 nearly as great a force to separate them when they are placed under 

 the exhausted receiver of an air-pump. Besides this, atmospheric 

 pressure is much weaker than this force, with so small a surface. 



