43 



constant, and then add powdered Nitre (Potassic Nitrate) to 

 the water, stirring to promote solution, we find that the 

 coloured water rises rapidly in the narrow tube, indicating 

 the production of cold, or, to state it in scientific language, 

 there is an absorption of heat. And experimenting on various 

 soluble substances in succession, we find that heat is 

 absorbed — or rendered latent — during solution, and that 

 that amount of heat is evolved, or given back to us on 

 crystallisation. 



The general law may be stated that a solid, on melting 

 to a liquid, or a solid dissolving in a liquid, absorbs heat, 

 which is given out again when the molten liquid solidifies, 

 or crystals are deposited from solution. 



Having considered the evolution of heat on crystallising 

 a supersaturated solution, we will now notice some special 

 properties pertaining to the state of supersaturation. 



In the first place, the existence of a supersaturated 

 solution evidently depends upon the plugging of the neck of 

 the flask, and as the early experiments were made with 

 flasks corked or sealed whilst boiling was in progress, it was 

 supposed that a partial vacuum being created, crystallisation 

 could not take place. 



Now we take a flask containing a supersaturated solu- 

 tion of Sulphate of Soda (Glauber's Salt), which has been 

 plugged with an india-rubber cork whilst the liquid boiled ; 

 and we have some difficulty in removing the cork, a partial 

 vacuum being created by the cooling and consequent con- 

 traction of the air in the flask, but immediately we succeed, 

 the air rushes in with considerable violence, and crystal- 

 lisation immediately follows. 



Apparently, then, something in the air itself causes 

 crystallisation, and we see that this active principle is 

 removed by allowing the air to filter through a plug of cotton 

 wool (for this latter may be substituted for a cork in the 

 flask) the liquid still remaining supersaturated. 



It sometimes happens that if we remove the wool air- 

 filter with very great care, that the solution does not 



