208 HEAT. 



be supersaturated. On adding to the liquid a small quantity of the salt, 

 deposition at once occurs. 



This may be illustrated by a solution of sodium sulphate in water. 

 At 33, water dissolves its maximum amount of this salt. Filtering the 

 saturated solution into a clean vessel at this temperature, the liquid may 

 be cooled down without crystallisation. But on the inti'oduction of a 

 crystal of sodium sulphate, the excess of salt at once crystallises out, 

 forming a mass of crystals throughout the liquid. 



Solution resembles fusion also in requiring, in general, a supply of 

 heat to effect the change of state, though in some cases this " heat of 

 solution " is negative. 



From these general resemblances, it appears probable that solution is 

 an exchange phenomenon, a saturated solution in contact with its salt 

 taking up and depositing equal quantities of the salt. 



We shall return to the subject of solution in chap, xix., where we 

 shall discuss it by the aid of Thermodynamics. 



Evaporation from Solids. Many solids evaporate sensibly. 

 Camphor, for example, gives out a characteristic odour through evapora- 

 tion at ordinary temperatures, the solid turning at once into gas without 

 passing through the liquid stage. Ice also evaporates slowly, as may be 

 shown by the slow disappearance of snow and ice in dry winds too cold 

 to allow of melting. 



There is a maximum vapour-pressure for solids, just as for liquids, 

 definite for each temperature. Hence, we may have a process corre- 

 sponding to distillation. If a piece of camphor is heated in a test-tube, 

 the upper part of the tube being kept cool, the vapour-pressure at the 

 high temperature of the lower part of the tube is above the maximum 

 for the temperature in the upper part of the tube, and some of the 

 vapour rising there is condensed to the solid form on the side of the 

 tube. 



This process is termed " Sublimation." It is used extensively in the 

 preparation of sulphur, the product of sublimation being known as 

 flowers of sulphur. 



If a solid is supplied with heat in such a manner that evaporation can 

 take place freely, it appears that there is a definite subliming-point, at 

 which there is an arrest of temperature corresponding to the boiling-point, 

 the temperature remaining at the point at which the pressure of the 

 vapour equals the external pressure (Ramsay and Young, Phil. Trans., 

 Parti., 1884, p. 37). 



