86 DIFFUSION 



the concentration of the diffusing substance is not uniform throughout the 

 system, and the process can continue only as long as dift'erences of concentra- 

 tion are maintained. 



Diffusion dependent upon the kinetic energy of molecular motion and 

 thus resulting solely from differences in concentration is often called "simple 

 diffusion" in order to distinguish it from more complex types of diffusion 

 phenomena in which other forces come into play. Examples of such more 

 complex phenomena will be encountered in some of the following chapters. 



The phenomenon of diffusion is exhibited by the molecules of liquids, 

 solutes, and even solids, as well as by those of gases. Diffusion of ions also 

 occurs, and even colloidal particles diffuse, but only at very slow rates 

 (Chap. V). 



Diffusion phenomena should be clearly distinguished from mass movements, 

 in which the moving units are not single molecules, but more or less extensive 

 assemblages of molecules. Winds and air currents generally are examples of 

 mass movements of gas molecules. The draft of warm air ascending a 

 chimney is another. All of the phenomena just listed, and many others, are 

 due primarily to differences in the density of the gases in various parts of a 

 system. Heavy gases are more strongly attracted by gravitational forces than 

 light gases. Hence, as in a chimney, cold (relatively heavy) air, displaces 

 hot (relatively light) air, forcing the latter to rise. Similar phenomena on 

 a grand scale are the principal cause of winds and air currents. They are 

 representative of the physical process called convection. Such phenomena also 

 occur in liquids. Mass movements of gases and liquids can also be caused in 

 many other waj's. 



Diffusion Pressure. — That diffusing gases sometimes result in the de- 

 velopment of measurable pressures can be readily shown by certain simple 

 experiments. In Fig. 17 is depicted an apparatus which can be used to 

 illustrate a number of aspects of the phenomenon of diffusion of gases. This 

 apparatus consists essentially of a vertically arranged glass tube, to the upper 

 end of which is attached, by means of rubber stopper, a cylindrically shaped 

 porous clay cup. This cylinder is hollow and its thick walls are pierced by 

 numerous minute capillaries. These pores are fine enough to prevent mass 

 movement of gases at any appreciable rate, but the porous clay is not a mem- 

 brane in the usual sense of the word. The lower end of the vertical glass 

 tube dips in some colored water. If such a porous clay cylinder, containing 

 air, be enclosed within a bottle containing hydrogen gas a rapid bubbling of 

 gas will occur from the lower end of the tube through the dye solution into 

 which it dips. When the bottle is removed a rapid and sudden rise of liquid 

 up the glass tube will ensue. This is followed by a slow subsidence in the 



