COHESION. EXAMPLES. 27 



COHESIOX. 



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 smoothly-shaven surfaces 

 w^ith a knife, pressing them firmly together with a tw^ist- 

 ino- motion (fig. 14). The asperities of the surfaces are 

 thus pushed down, and tho F'g 14. 



particles are brought into f^T^^^^W^^'^^^^ 

 close contact, so that cohc- 'ijpi^j^^ ^Jfe 



Sion immediately takes place cohesive attraction m two lead balls. 



between them, and some force will be required to draw 

 them asunder.* Two pieces of melted wax adhere to- 

 gether 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, w^hen cold, a weight of half a ton to draw 

 them apart. Plates of brass of the same size, cemented 

 by means of pitch, required 1400 pounds. On this prin- 

 ciple depends the efficacy of those substances which are 

 used for cementing broken vessels. 



The most perfect artificial polish which can be given to 

 hard metals is still so rough as to prevent the faces from 



* That this is not atmospheric pressure, like that -whicli 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 exhaust- 

 ed receiver of an air-pump. Besides this, atmospheric pressure is much 

 weaker than this force, with so small a surface. 



