25a THEORY OF SOLUTION AND CRYSTALLISATION. 



4. Heat is generated in the act of solution. This fact is 

 recognised in such cases as the dissolution of caustic potash or 

 soda, calcium chloride, anhydrous sulphate of copper, and many 

 other compounds in water, chlorides of calcium or zinc'in alcohol, 

 and sodium in mercury. And further it may often be observed 

 that a marked rise of temperature occurs on diluting a concentrated 

 solution, as when water is added to a strong solution of calcium 

 chloride or ferric chloride. On the other hand the production of 

 cold by mixing solids with liquids in which they can dissolve is a 

 very familiar phenomenon. When water is poured upon nitre for 

 example, or sal-ammoniac or nitrate of ammonium, a considerable 

 reduction of temperature is at once observed, but this is, of course, 

 to be ascribed to the fact that much heat is absorbed in conse- 

 quence of the change from solid to liquid experienced by the salt. 

 The reduction of temperature observed is merely the resultant 

 of the heat evolved ( + ) by combination of the water and the salt, 

 the heat ( + ) attending the contraction of volume which occurs 

 and the heat absorbed ( — ) by reason of the liquefaction of the 

 salt. Sufficient experimental data are, however, still wanting 

 concerning the latent heat of liquefaction of salts, the elevation or 

 reduction of temperature which occurs during solution, the 

 amount of contraction which occurs, etc., so that we do not yet 

 know what proportion of the thermal change is^actually due to the 

 combination of water and salt. 



5. When a solid dissolves in a liquid, the volume of the 

 resulting solution is not represented by the sum of the volumes of 

 the solid and liquid, but is always less. Indeed it has been found 

 that some salts, such as sulphate of copper, when added in the 

 anhydrous state and in small quantity to water, produce a solution 

 which occupies a smaller space than the water alone, so that the 

 addition of the solid actually causes the liquid to contract. (Favre 

 and Valson, Comptes Rendus, and Ewing and MacGregor, 

 Trans. R Soc. Edin., 1873.) After the addition of a certain 

 quantity of the salt expansion occurs, and proceeds thereafter 



