21 4 SOLUBILITY. 



solid molecules : in that case the molecules of the solid, 

 if the latter is placed in contact with the liquid, will 

 obey the preponderant force, their cohesion will be over- 

 come by adhesion, and they will be dispersed among the 

 molecules of, the liquid. The solubility of solids in 

 liquids is extremely variable : water dissolves sugar 

 a considerable amount, until the solution becomes 

 thick liquid like syrup ; of common salt about one part 

 may be dissolved in three parts of water. Many other 

 bodies are very soluble; of others, again, only small 

 quantities are dissolved ; of gypsum only one part 

 soluble in 400 parts of water. 



Most bodies are more soluble in hot water than in 

 cold, but this rule is not without exception : of commoi 

 salt and gypsum nearly equal quantities are dissolv< 

 whether the water is hot or cold. 



Many substances, if their solution is evaporated, 01 

 when a saturated hot solution is allowed to cool, ai 

 separated again from the liquid as ''crystals' that is, th< 

 molecules arrange themselves so as to form bodies 

 having regular geometrical outlines bounded by plan< 

 surfaces, and being mostly transparent. Common salt 

 petre (potassic nitrate, usually called nitre) is soluble ii 

 less than four times its weight of cold water, and ii 

 less than half its weight of hot water. 100 or 200^ 

 of nitre may be covered with an equal weight of 

 water, and the whole heated until all the nitre is dis- 

 solved. The solution is allowed to cool slowly, without 

 being stirred or shaken ; beautiful crystals in the shape 

 of six-sided columnar prisms will then crystallise from it. 



The solution should be heated in a small china saucer, of the kind 

 usually used in chemical laboratories, and called an ' evaporating 



