DIFFUSION AND DIFFUSION TENSION 11 



is obvious from the theoretical consideration already pre- 

 sented that this must be true. In this case the kinetic 

 energy of the particles is not altered, but their number 

 has been increased, hence the increase in pressure. Also, 

 for a given concentration and temperature, all gases exhibit 

 the same pressure. This is called the principle of Avo- 

 gadro. It is usually stated in a somewhat different way, 

 namely: Equal volumes of gases, at equal temperature 

 and pressure, contain the same number of particles. This 

 principle holds rigorously true only for gases whose con- 

 centration is rather low. As a gas approaches the liquid 

 state, the principle of Avogadro, and also those of Boyle 

 and Gray-Lussac, have to be modified. They apply only to 

 a theoretically perfect gas. 



b) Mixed gases. In a mixture of several gases each 

 gas practically exerts its own pressure independently of the 

 others. Thus the total pressure of a mixture of gases in a 

 chamber is the sum of the pressures which would be exhib- 

 ited were the gases separated and each put into a chamber 

 of the same size as the first one, the temperature of course 

 remaining constant. The pressures which would be thus 

 shown are called partial pressures, and the above fact may 

 be stated more directly, by use of this term, as follows: The 

 total pressure of a gas mixture is practically the sum of the 

 partial pressures of its component gases. As a gas nears 

 the liquid state, this principle also breaks down in part, it too 

 applying rigorously only to perfect gases. 



Also, if two gases be brought together so as to form two 

 horizontal strata in a chamber, diffusion of each gas will 

 take place just as completely as if the other gas were not 

 present. Particles of the lower gas will pass up from the 

 lower stratum until that gas is equally distributed through- 

 out the chamber. Downward diffusion of the upper gas will 

 occur simultaneously, and the result of the two processes 



