382 KEYES 



ART. J 



vanished, and this assumption is sometimes made when, as is 

 often the case, there is a practically complete lack of heat 

 capacity data. The leading term is of course ZvEi/Rt and is very 

 large in the usual case of gas reactions. 



The equation (114) [309] contains the generalization set 

 forth in equations [311] to [318]. It includes also the case 

 referred to in the sentence following [318]; "graded" dissocia- 

 tion illustrated by the reaction HI ^ H2 + I2 -^ 2H + 21. 

 It is clear also that the presence of a neutral gas in the reaction 

 mixture is without influence on the value of the equilibrium 

 constant (114) [309] provided p is understood to be the total 

 pressure diminished by the pressure the neutral gas would 

 exert if it alone occupied the volume of the mixture. The 

 influence of a gravitational field of the magnitude available on 

 the earth is exceedingly small and equation [234], Gibbs, I, 146 

 provides the basis for investigating such effects. 



V. Gas Mixtures with Convertible Components 



{Gibbs, I, 172-184) 



The equation (114) [309] of the previous section includes 

 the case of interest here developed. The term convertible com- 

 ponents refers to the formation of multiple molecules such as 

 (N02)2; a case which would also be included under the term 

 reversible polymerization or association. The painstaking 

 justification of the application of the principles established for 

 the treatment of mixtures of chemically related components 

 to the present case may seem unnecessary. On the other 

 hand it should be recalled that one of the former axioms of 

 chemistry was that substances of the same qualitative and 

 quantitative composition must possess the same physical 

 properties. Reference may be made to Liebig's discovery of 

 the identity of composition of silver fulminate and silver 

 cyanate as the first definite fact invalidating the axiom. Had 

 NO2 been colorless the explanation of the considerable change in 

 density of the gas with pressure would probably not have been 

 ascribed to association and dissociation for a long time. As a 

 matter of fact it was the change in color on change of pressure 



