CHAPTER IX 

 EFFECT OF HEAT ON VOLUME 



61. Cohesion. The force which attracts the molecules of a 

 substance to one another is called cohesion. It is this force 

 which we must overcome in splitting or pulling a thing apart, 

 and which gives hardness, definite shape, and solidity to differ- 

 ent substances. Heat weakens cohesion by increasing the 

 speed of the molecules and causing them to move farther 

 apart. Liquids when heated expand or increase in volume 

 more readily than solids, and gases more readily than either. 

 When heat is withdrawn from a body, cohesion draws the 

 molecules together again and causes it to contract. 



52. Melting Point. If sufficient heat is added to a solid, a 

 point is finally reached when cohesion is overbalanced. The 

 substance then ceases to have a definite shape and, breaking 

 down, becomes a liquid, or as we say, it melts. It is not pos- 

 sible to say in advance at what temperature an unknown 

 substance will melt, but when this point is once determined, we 

 may be sure that the substance will always melt at this tem- 

 perature under similar conditions. Different substances, as 

 might be assumed, have different melting points, but these 

 points for each substance do not vary unless the conditions 

 are changed. Crystalline substances, that is, those which form 

 crystals on solidifying, show most sharply the changes from the 

 solid to the liquid state. A few substances such as glass, pitch 

 and wax, do not have definite melting points but slowly soften 

 under the influence of heat and form a pasty mass. On the 

 other hand, certain other substances, such as iodine and cam- 

 phor, do not melt at all under ordinary conditions but appear 



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