130 Pro/. Kuenen and Mr. Robson on Mixtures with 



For ~t_ a j ^i we g n j -209 : the volume of one gramme 



of the gas calculated by the gas laws is 91 c. c. : subtracting 

 from this the approximate volume of the liquid, and allowing 

 for a small deviation from the gas law we find for H per 

 gramme of the mixture 89 calories ; the latent heat of 

 carbon dioxide and of ethane at the same temperature being 

 95 and 114 respectively. 



If we subtract from the latent heat the external work, we 

 obtain the internal latent heat which should give us a more 

 accurate measure of the relative values of the internal 

 energies. The value of this quantity for the mixture is 79 

 calories, and for the components 86 and 100. Moreover, we 

 ought to compare not a gramme of the mixture with a 

 gramme of the components, but with the sum of the quantities 

 actually contained in the mixture ; in this way we find 91 

 calories compared with 79 for the mixture. 



In whichever way we make the comparison, the latent heat 

 of the mixture appears to be small, as might have been 

 expected : for the maximum mixture is characterized by a 

 comparatively small mutual attraction between the component 

 substances, on which attraction the latent heat doubtlessly 

 to a large extent depends. On the theory of van der Waals 



the internal latent heat is equal to a( J where a is the 



attraction-constant and vi and v v are the volumes of liquid 

 and vapour*. As Vi, the volume to which the vapour con- 

 tracts on condensation, depends on the volume-constant b in 

 the equation of condition, we see that in general the latent 

 heat depends on both constants, but we know from previous 

 results f that h for mixtures of carbon dioxide and ethane has 

 the normal value, and that the maximum vapour-pressure is 

 due to a small a 12 the mutual attraction constant. 



Without actually calculating the value of the heat of 

 transformation at other points on the three-phase curve, we 

 can see how the value of H/V gradually diminishes at tem- 

 peratures beyond M, becomes zero at where dp/dt = 0, and 

 is negative between and T where d2?/dt<0. 



The meaning of these changes is the following : — If we 

 take V positive, i. e. if we increase the volume in which the 

 mixture in its three phases is contained, the transformation 

 (fig. 5) consists in an evaporation of the liquid mixture, L. 

 This mixture contains more carbon dioxide than the vapour 



* Bakker, Dissertation, Schiedam 1888. 

 t Kuenen. Phil. Mag. [5] xliv. p. 195. 



