FACTORS INFLUENCING DIFFUSION OF GASES 89 



Principle of Independent Diffusion. — In the first experiment described 

 in the preceding section it was shown that while one gas (hydrogen) was 

 diffusing in an inward direction through the pores of a clay cylinder, other 

 gases (oxygen and nitrogen) were simultaneously diffusing in an outward 

 direction through the same pores. This exemplifies one of the most important 

 principles governing diffusion phenomena, namely that the direction in which 

 any substance will diffuse is controlled entirely by its own differences in dif- 

 fusion pressure, and is not influenced by either the direction or rate of diffusion 

 of other substances in the same system. Hence, in any given system, as for 

 example, two adjacent plant cells, a number of substances may be diffusing 

 in one direction across the intervening membranes, while simultaneously other 

 compounds may be diffusing in the opposite direction across the same mem- 

 branes. Each one of these individual substances will diffuse in the direction 

 determined by its own differences in diffusion pressure, and at a speed which is 

 determined by the factors which are influencing the diffusion of that par- 

 ticular substance. 



Factors Influencing the Rate of Diffusion of Gases. — i. Density of the 

 Gas. — Different gases diffuse at different rates even when influenced by the 

 same set of environmental factors. Hydrogen, for example, diffuses more 

 rapidly than any other gas. The same Thomas Graham who conducted some 

 of the earliest studies on the properties of colloidal systems also was one of the 

 first investigators to study quantitatively the phenomenon of gaseous diffusion. 

 He discovered the principle, often called "Graham's Law of Diffusion," that 

 the relative speeds of diffusion of different gases are inversely proportional 

 to the square roots of their relative densities. By relative density is meant 

 the \veight of a given volume of gas as compared with the weight of the same 

 volume of hydrogen.^ The relative densit>' of oxygen is 16. Hence the rate 



of diffusion of hydrogen is proportional to-T^^, while that of oxvgen is pro- 



VI 



portional to . Hvdrogen gas will therefore diffuse four times as rapidly 



V 16 

 as oxygen gas under the same conditions of temperature and pressure. The 



relative density of carbon dioxide is 22; hence by similar reasoning it is ap- 

 parent that hydrogen gas will diffuse nearly five times as rapidly as carbon 

 dioxide gas. 



^ Since a molar weight of any gas occupies a volume of 22.4 liters at standard 

 conditions, molar weights of gases are in themselves a measure of the relative 

 density of gases. Since the molar weight of hydrogen (Ho) is 2.016, the relative 

 density of any gas on the basis H = I, is equal to its molar weight divided by 

 2.016 (usually rounded off to 2). 



