184 THEORY OF COHESION. 



or any sudden force, they are removed in the 

 slightest degree from their place, the cohesion is 

 broken, and they fly asunder. In the state of 

 liquids, their particles glide freely over each 

 other, without losing their hold, as one magnet 

 glides over another. This is the case with all 

 the maleable metals, and with the tenacious oils, 

 gums, glue, &c. until reduced to very low tem- 

 peratures, when they all become more or less 

 brittle like ice. It is owing to the augmented 

 cohesion and hardness of very cold iron that it 

 will not bend, but snaps asunder, that is, its 

 particles refuse to glide over each other ; while, 

 at higher temperatures, the greater extent of the 

 igneous atmospheres that surround them, holds 

 on to them at greater distances, though with less 

 force, causing toughness or tenacity. 



When two pieces of lead, tin, zinc, or any 

 other soft metal, are pressed together with suf- 

 ficient firmness, their atoms are brought within 

 the range of this attractive medium. The same 

 is true of the hardest substances, when softened 

 by heat. Even polished plates of glass, marble, 

 wood, and many other bodies, when brought into 

 close apposition, cohere, but still better when 

 fused together by heat. 



It was before stated that the aethers, alcohol, 

 and many other liquids, have no cohesion what- 

 ever at ordinary temperatures ; and when released 

 from the pressure of the atmosphere, are ex- 



