14 THE PRINCIPLES OF AGKICULTURE. 



solved in the water; iind carbonic acid, which escapes in 

 the form of a gas, causing the effervescence. 



Cohesion and Adhesion. — As atoms have attractions 

 wliich cause them to unite in molecuk'S, so the molecules 

 themselves have similai* attractions for one anothei". 



When the attraction is between molecules of the same 

 kind, it is called cohesion ; when between molecules of 

 different kinds, it is called adhesion. 



Molecules of water have an attraction of cohesion for 

 one another, but the adhesion between water and glass is 

 sufficient to overcome this, and to cause a piece of glass 

 to be moistened when di})ped in water. On the other 

 hand, an oily stick will not be moistened by water, be- 

 cause the cohesion in water is stronger than the adhesion 

 between water and oil. 



The adhesive attraction of water for gases causes the 

 moisture of the atmosphere to absorb impurities and 

 bring them down with the rain. 



The attraction of charcoal for various substances 

 renders it useful as a filter for cleansing water, refining 

 sugar, etc. 



In order that the forces of cohesion and adhesion may 

 act, it is necessary to bring the molecules very near to 

 one another. In l)reaking a piece of iron, we exert a 

 force ui)on it sufficient to ))ull its molecules so far a])art 

 that the force of cohesion no longer acts. 



In order to weld the sej)arated parts it is necessary to 

 heat them until their molecules will move more easily, 

 and then beat them together by hammering until the 

 molecules are again brought within the range of cohesive 

 force. 



in stretching a jjiecc of i-ul)ber, we draw the molecules 

 farther and fartlier aj)art, until finally the force ai)plied 



