THEORY OF SOLUTION AND CRYSTALLISATION. 2^1 



cases to be given further on is also applicable to others which at 

 first sight appear to be of a different character {e.g., lime). 



It is a question which has not yet been satisfactorily determined 

 hy experiment, whether the solubility of all salts does not attain at 

 some temperature a maximum beyond which it diminishes. In the 

 case of sulphate of sodium, acetate of barium, and other well-known 

 salts, this maximum is readied below the boiling point of water. 

 In other cases the temperature required is much higher. But in 

 no well-established case has it been shown that the solubility 

 diminishes thereafter to zero, the salt becoming insoluble. 

 Common salt which is sometimes referred to as an exception 

 is in reality nothing of the kind, as its soluoility does increase 

 with rise of temperature, though very slowly below the boiling 

 point of water, as shewn in the table. 



3. There is usually a limit to the solubility of a given solid in 

 a liquid. This seems to be universally true of crystallisable 

 substances, but among uncrystallisable solids, such as resin or 

 glue, there appears to be no limit either way to the proportions in 

 which the solid and liquid will combine. 



The quantity of a salt taken up by a liquid at a given temper- 

 ature is a definite quantity which never varies if an excess of the 

 solid is added to the liquid and the temperature gradually raised to 

 the required degree. We have examples in the table just given. 

 But the result is different in certain cases if the liquid is first 

 saturated at a higher temperature and then filtered, or poured off 

 quite clear from the solid remaining undissolved, and allowed 

 quietly to cool. A much larger quantity of solid may thus be 

 retained by the liquid which is then called a "" super -saturated 

 solution." The phenomena of super-saturation are well shewn 

 by solutions of sulphate, acetate, and carbonate of sodium, by the 

 sulphates of magnesium, zinc, aluminium, and many others, and 

 have been very frequently described by numerous observers. The 

 various circumstances which affect the crystallisation of such a 

 solution will be described further on. 



