14 DIFFUSION AND OSMOTIC PRESSURE 



throughout. This tendency of one liquid to diffuse into 

 another may be termed diffusion tension; it corresponds to 

 the vapor tension exhibited by an evaporating gas. Diffu- 

 sion in liquids is sometimes distinguished from that in gases 

 by the use of the term "hydro-diffusion" to denote the former. 

 They are, however, essentially the same thing. In the case 

 just cited, each liquid develops a diffusion tension independ- 

 ently of the other. Of course, above such a mixture of 

 liquids there will lie (if the chamber allow it) a stratum 

 of gas mixture in which each of the two gases has its 

 own vapor tension, just as though the other gas were not 

 present. 



III. SOLIDS 



a) Simple solids. Continuously raising the temperature 

 of a solid may result in liquefying it and then in vaporizing 

 the liquid thus formed ; or vaporization may take place 

 immediately, without the intervention of the liquid phase 

 at all. In either case the particles break away from the 

 solid mass and become more widely separated. With the 

 process of liquefaction, however, we have no concern. 



When vaporization of a solid takes place directly, it is 

 called sublimation. Gum camphor, naphthalene, and ice 

 below the temperature of melting exhibit this phenomenon. 

 If the vapor particles are prevented from escaping, an equi- 

 librium between vapor and solid is ultimately reached, at 

 which sublimation apparently ceases. At such a point sub- 

 limation is just equaled by condensation. The whole 

 process is analogous to that of evaporation from free liquid 

 surfaces. The pressure of the vapor surrounding a solid 

 mass of the same substance, when equilibrium is reached, 

 may be termed, as in liquids, vapor tension. 



b) Diffusion of two solids. If two solid masses of differ- 

 ent substances are brought together with their adjacent 

 faces in close contact, there can be demonstrated, in some 



