HENRY'S LAW 29 



But from this a conclusion may be drawn if the nitrogen 

 present in B consist entirely of molecules N 2 , i. e. is of the 

 same constitution as in the gas. For if the quantity per 

 unit volume in A is doubled, that in B must be so too, for, 

 with sufficient dilution, this change would double both the 

 number of gaseous molecules passing from A into B and 

 the number of dissolved molecules passing from B into A. 

 So that in this case the concentration C in the solution is 

 proportional to the pressure P of the gas 



a law that, under the name of Henry's law, is known to 

 be applicable to the majority of gaseous solutions. 



Let us now suppose a different case : first, that nitrogen 

 is present in water as N, not as N 2 ; then the law of absorp- 

 tion becomes different also. In order to deduce this modified 

 law, let us assume that in the gas too a certain quantity, 

 even though vanishingly small, of nitrogen occurs as N ; 

 then between that and the nitrogen in the water there 

 must be an equilibrium as in the preceding case, and pro- 

 portionality as before 



0=*!^*, 



where P N is the very small partial pressure of the gaseous 

 nitrogen in the form N. This partial pressure is not, 

 however, proportional to the total pressure P, since the 

 equilibrium in the gas between the two forms of nitrogen, 

 according to the equation 



is subject, according to the laws of chemical equilibrium 

 (cf. Part I, p. 109), to the relation 



where ./V a and PN are the partial pressures of the nitrogen 

 in the forms N 2 and N respectively, and consequently P y 



to nitrogen, but not to water vapour; although if the vapour were 

 present the reasoning would still hold. 



im 



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