41 



a limit at which no more of the substance dissolves, unless we 

 warm the water. If we now poured off the solution of Alum 

 and weighed it, and repeated the experiment with more Alum 

 and water at the same temperature as in the first experiment, 

 we should find the same weight of water at the same tem- 

 perature would always dissolve the same weight of Alum. 



Experimenting on the various substances which dissolve 

 in water in this manner, using throughout the same weight 

 of water at the same temperature, we are able to arrange the 

 substances in the order of their solubility. 



A cold saturated solution of any substance is obtained by 

 stirring the solid with cold water until further solution ceases, 

 i.e., the limit of solubility is reached for that temperature. 



A hot saturated solution is obtained by treating the solid 

 with boiling water until again solution ceases. 



If a hot saturated solution be allowed to cool gradually, 

 the converse process takes place, the water as it cools is 

 able to dissolve less and less, and the excess is deposited on 

 the sides of the vessel, becoming finally a cold saturated 

 solution. 



This deposition is very prettily seen when a hot 

 saturated solution of Lead Iodide in water is cooled by run- 

 ning cold water over the outside of the containing flask. 

 Almost immediately, crystalline plates, resem^bling fragments 

 of gold leaf, separate out from the liquid, for the water, as it 

 cools, being unable to hold so large an amount of the solid 

 in solution, deposits it gradually and continuously until just 

 so much is left in solution as it can dissolve when cold. 



In the previous experiment, it was seen that a hot 

 saturated solution began to deposit the excess of dissolved 

 matter immediately its temperature fell, but it has been 

 found that under certain circumstances a hot saturated solu- 

 tion may be cooled without such deposition taking place. 



If distilled water be boiled in a flask, and Alum added 

 so long as it dissolves, the water boiling steadily the while, 

 a true hot saturated solution is obtained, close the neck of 

 the flask with a plug of clean cotton wool whilst the liquid 



