12 DIFFUSION AND OSMOTIC PRESSURE 



will be a uniform mixture of the two gases. If this pro- 

 cess of diffusion is obstructed by a wall placed between 

 them, the pressures of both gases will of course be exhibited 

 independently upon the opposite sides of this wall. 



II. LIQUIDS 



a) Simple liquids. When a liquid is heated, the kinetic 

 energy of its particles is increased, until at length the cohe- 

 sive force which held them together is overcome ; then they 

 fly off from the main mass and tend ever to increase their 

 distance apart. This is the process of vaporization by heat. 

 As long as the temperature remains high enough, such mat- 

 ter will remain in the gaseous state. Also many substances 

 which are usually liquids can be vaporized at ordinary tem- 

 peratures. Water, alcohol, and ether are examples of this. 

 This process, however, is a slow one. It is explained 

 theoretically in this way: Although the majority of the 

 liquid particles cannot break away from the main mass at 

 ordinary temperatures, yet some of them, which reach the 

 surface with greater kinetic energy than the others, do suc- 

 ceed in breaking through the firmer surface layer (see p. 7), 

 and so escape as gas particles. If the chamber above the liquid 

 be a closed one, so that the evaporated liquid cannot escape, 

 evaporation soon apparently ceases. If some of the liquid 

 particles come against the surface layer with sufficient force to 

 pass through it, it is reasonable to suppose that, after escap- 

 ing into the chamber above, some of them may again pass 

 through this film in the opposite direction, and so re-enter 

 the liquid. Here they come under the influence of the force 

 of cohesion, which holds the liquid particles together, and, 

 since they are unable to break forth at once, they remain in 

 the liquid state. The number which thus re-enter the liquid 

 will gradually increase as the pressure of the vapor (i. e., 

 the number of vapor particles, for the temperature is sup- 



