amongst the Molecules of a Gas. 95 



1*1 can hardly be right when the percentage of C0 2 has been 

 increased from 32 to 38. Thus we can regard the experiments 

 on mixed gases as furnishing confirmation of the truth of the 

 law of the inverse fourth power for the attractions between 

 molecules of gas. 



The value of C which we have obtained for air and C0 2 

 (mean 17) throws an interesting light on a certain aspect of 

 the phenomenon of diffusion. Suppose a volume V A of a gas 

 A containing a molecules, and a volume Y B of another gas B 

 containing b molecules, separated by an infinitely thin partition, 

 then the potential energy of the two masses of gas is (leaving 

 out common factors) 



Aamp A -f Bbm'p B . 



When the gases are mixed together without change of 

 pressure we can see from what has gone before that the 

 potential becomes 



Y Y / V V 



^ am ?A y + y ' + B hm 'Ps y -f-V + 2C V AB ambrri /y ^_y y >- 



Subtracting this from the previous expression, we get 



Y V / V^V 

 Aam pA y +V +B hmfpB Y + Y ~~ 2C V AB amhm \'Y~+Y~) 2 



If C = 1 this is a complete square, and therefore necessarily 

 positive; therefore when C = *7 the expression is also positive, 

 and for all values of C less than 1 it must be positive ; that is, 

 the potential energy of the molecules diminishes by diffusion. 

 Hence we may regard diffusion as partly due to the tendency 

 of the molecules of the mixing gases to obey the dynamical 

 principle that a position of stable equilibrium is a position of 

 minimum potential energy. Diffusion is motion towards the 

 position of stable equilibrium for the two gases. 



However, the kinetic factor in the diffusion of gases is so 

 predominant that this aspect is not of much importance. But 

 in the case of liquids it is otherwise, and the form of our last 

 expression suggests how the tendency of some liquids to mix 

 and of others to refuse to mix may depend on the magnitude 

 of a coefficient like C. Indeed, the study of the cooling effect 

 of liquids and mixtures of liquids escaping from under pressure 

 affords a splendid field for experimental inquiry. This paper 

 will have possessed some value if it draws the attention of 

 those who have facilities for such a research, to a field whose 

 further exploration on the tracks of the pioneers must open 

 valuable ground for Physical Science. 

 Melbourne, April 1886. 



