52 CHEMICAL PHYSICS. 



points, which fact is utilized in chemistry extensively for determining 

 the identity of substances and their purity. A small amount of 

 impurity in a substance often causes a notable change in its melting- 

 point and the temperature rises between the beginning and the end of 

 melting, instead of remaining constant. 



Some solids, like wax and paraffin, which are mixtures, do not remain 

 at the same temperature during fusion, and in such cases the melting- 

 point is taken as the average of the temperatures at which fusion and 

 solidification begin. 



The determination of the melting-point is carried out by introducing 

 a column about J inch long of the finely powdered substance into a 

 capillary glass tube sealed at one end, and attaching this to the bulb 

 of a thermometer. The latter is then dipped into a liquid of high 

 boiling-point and the temperature is slowly raised. The instant the 

 substance melts, the temperature is noted, which is the melting-point, 

 often abbreviated m.-p. 



Latent heat of fusion. This is the number of heat units (calories) 

 required to make 1 gramme of a substance fuse. Different substances 

 have different latent heats of fusion, but water (ice) has the greatest. 

 To melt 1 gramme of ice at C. to water at C. requires 80 calories, 

 or as. much heat becomes latent as will raise 1 gramme of water from 

 C. to 80 C. 



Change of volume by fusion. When any solid fuses, a change in 

 volume always occurs. Some substances expand during fusion, for 

 example, bismuth, wax ; others contract, for example, brass, cast-iron, 

 ice. When a solid floats in its own liquid, this is evidence that it con- 

 tracts on fusion, because the density of the solid is less than that of its 

 liquid. When a solid sinks in its own liquid, there is expansion on 

 fusion. 



Evaporation and boiling-. Many liquids, even some solids, 

 evaporate or assume the gaseous state at nearly all temperatures. 

 Water and ice, mercury, camphor, and many other substances vapor- 

 ize at temperatures which are far below their regular boiling-points. 

 This fact is to be explained by the assumption that during the rapid 

 vibratory motion of the particles of these masses, some particles are 

 driven from the surface beyond the sphere to which the surrounding 

 molecules exert an attraction, and thus intermingle with the mole- 

 cules of the surrounding air. 



