276 SALTS. 



described. The second is illustrated in the production 

 of alum crystals, by cooling a hot solution. The third 

 consists in dissolving a solid in some liquid, and then 

 boiling away the latter. The experiment may be tried 

 with a saturated solution of salt and water. As fast as 

 the water is boiled away, the portion which has lost its 

 solvent, re-assumes the solid form. 



691. CHANGE OF SUBSTANCE DISSOLVED. 



Describe two , . , , . 



cases by The change in the substance dissolved, is 



Stance' ^ mb ' effected ? in some cases > by addition, and in 

 others by subtraction. Carbonic acid, 

 blown through lime water, precipitates it, by addition. 

 The precipitate is chalk, or carbonate of lime. Pot- 

 ash, added to a solution of sulphate of copper, precip- 

 itates it by subtraction ; the precipitate is oxide of 

 copper, deprived of its acid by the potash. 



692. EXPLANATION. The above cases 



State the cause f . . , ~ 



of predpita- of precipitation, demand some further ex- 

 hon. m the planation. As fast as carbonic acid is blown 



above cases. 



into the lime water, in the first case, the 

 new substance, chalk, or carbonate of lime, is produced 

 throughout the liquid. We may suppose that innume- 

 rable particles are first formed, before they unite to 

 form a precipitate. But the cohesive attraction put 

 forth by the particles of this new compound is so great 

 that the opposing attraction of the water is overcome, 

 they rush together, and assume the solid form of a pre- 

 cipitate. This did not happen in the case of lime alone, 

 because the cohesive attraction between its particles is 

 inferior to the opposing attraction of the water. The 

 second case is to be similarly explained. 



