452 Thermal Properties of Carbonic Acid and Nitrogen. 



experimentally, that as the admixture becomes greater in 

 proportion, the slope of isothermals between the gas- and 

 liquid-lines against the line of constant pressure will become 

 greater. 



Graphically speaking, those intermediate portions of iso- 

 thermals in case of an absolutely pure gas shift parallel to 

 themselves as the temperature becomes higher, and, preserving 

 their straight form till the critical point is neared, at last 

 vanish there, to make up the continuity of curvature of those 

 portions which lie on both sides of them. For a mixture, the 

 slope of those intermediate portions seems to become greater 

 as the temperature becomes higher, and thus that continuity 

 to be smoothly made up. In such a discussion one will get 

 much help from experiments already referred to as made by 

 Cailletet and, if any, others. 



The curious phenomenon that a gaseous mixture presents 

 an optically homogeneous appearance once for a while within 

 the region bounded by the gas- and liquid-lines, attracted 

 much attention from physicists. Jamin ascribes it to the mutual 

 approach to equality of densities of the mixed gases due to 

 high pressures ; while Duhem * tries to deduce the existence 

 of, and to explain graphically, that homogeneous appearance 

 from his mathematical equations of thermodynamic potentials. 

 Jamin's explanation is physical to a certain extent, yet seems 

 to be somewhat deficient, while Duhem's may not be deficient, 

 yet seems to be not quite physical. At the same time, 

 Andrews was undoubtedly the first to observe the phenomenon, 

 and his descriptions and experiments seem to contain the right 

 means towards understanding the true physical processes by 

 which the phenomenon under consideration is occasioned. 

 Dewar's experiments on the critical points of mixed vapours 

 seem to direct us for an explanation towards the same point 

 as Andrews's ; so also do the experiments of Cailletet. 



The course of isothermals of a gaseous mixture much below 

 the critical point may probably not be much different from 

 the usual course. Andrews observed near the point of com- 

 plete condensation how a little mass of nitrogen diffuses into 

 the condensed liquid. A little increase of volume due to that 

 diffusion must be of the same nature, as was pointed out by 

 Angstrom f. In the meantime, evaporation, a special type o f 

 diffusion, of the condensed liquid into the little atmosphere of 

 nitrogen must have been going on under the special conditions 

 of temperature and pressure. 



These interdiffusions of one part into another of the hetero- 



* Journ. de Phys. vii. 1888. 

 t Wied. Ann. Bd. xvii. 1882. 



