SUPERSATURATED SOLUTIONS. 69 



ticularly good example of a substance apt to form supersaturated 

 solutions. If this salt be added in powder little by little to boiling 

 water contained in a clean glass flask until the water will dissolve 

 no more, and the liquid be then allowed to cool gradually and 

 without any shaking or agitation, all chance of dust falling in being 

 excluded by placing in the mouth of the flask a lump of clean 

 cotton wool, it will generally happen that no crystallisation at all 

 takes place during cooling, the contents of the flask being perfectly 

 fluid even when completely cool. If the cotton wool stopper be 

 now taken out of the neck it will often happen that a particle of 

 dust will fall in which will cause the formation of crystals round 

 the particle, which thus acts as a "nucleus " or centre round which 

 solid matter concretes. The crystallisation thus commenced 

 rapidly spreads throughout the whole of the liquid, which thus 

 becomes almost entirely solidified in the course of a minute or so, 

 so that the flask can be turned upside down without any fluid 

 escaping. If no dust fall in on removing the stopper, the 

 crystallisation can be started by throwing in a fragment of wood 

 or stone, or introducing a stick or a glass rod ; if a thermometer 

 be thus employed, it will be seen that the mercury rises very 

 perceptibly whilst the crystallisation progresses. 



This experiment in fact is the converse of producing cold by 

 freezing mixtures of snow and salt, or by dissolving saltpetre, &c., 

 in water (Expts. 21 and 24); here a liquid becomes solidified, 

 and heat is developed, so that the mass becomes warmer than at 

 first in consequence of the action ; with snow and salt, the two 

 solids become a liquid brine, and heat is absorbed, making the 

 whole colder than at first. Just in the same way, when saltpetre 

 is dissolved in water, or when sulphate of sodium is stirred up with 

 hydrochloric acid, so as to produce a chilling action, the solid 

 matter employed becomes practically liquefied, and therefore cold 

 results. 



Expt. 64. Illustration of Comparative Solubility in different 

 Solvents. It is most frequently observed that whilst a 

 given substance may be freely soluble in one kind of solvent, it 

 dissolves much less readily, or even not at all, in another kind. 

 For example, common salt dissolves readily in water ; but if added 

 to alcohol, especially if pretty strong, very little indeed is dissolved. 

 Many other substances (such as sulphate of copper) behave in the 

 same way ; so that if some alcohol be added to a cold saturated 

 solution of the substance in water and the whole well shaken 

 together, a quantity of crystallised substance will often be thrown 

 out of solution, because the weak spirit resulting from the mixture 

 of the alcohol and water present is incapable of permanently dis- 



