192 HEAT. 



pressure ceased to be one-valued and became dependent on the amount 

 of water present. Up to the critical point the pressure of vapour in con- 

 tact with the water was definite at a definite temperature, but as soon 

 as that point was passed the contents of the vessel were homogeneous, 

 and the pressure depended on the amount of water dealt with. They 

 found a critical temperature of 365 C., with a critical pressure of 200-5 

 atmospheres. Traube and Teichner (Ann. der Physik, xiii., 1904, p. 620), 

 found the critical temperature to be 374" 0. They used a quartz tube 

 filled to J with distilled air-free water and heated by mercury boiling 

 under pressure. The meniscus disappeared at 374 and reappeared 

 when the temperature was again brought down to that point. 



The refractive index has also been used to find the critical point. 

 In the researches of Prinz Galitzine and Wiliss (Congres International 

 de Physique, 1900, i. p. 668), the refractive index of a substance near 

 the critical point was measured either by using the substance as a 

 cylindrical lens and observing the refraction, or by inserting a small 

 angled prism in a tube containing the substance and finding the refrac- 

 tion due to the prism. The refractive indices of the liquid and vapour 

 approach each other as the critical point is approached, and at that point 

 are equal. For ether the meeting point of the refractive indices was 

 found to be pl-12 at 1937 C. 



In these experiments as in others it was found that near the critical 

 point very great variations in density occurred in the successive layers, 

 variations only eliminated by stirring. 



The isothermal is very nearly horizontal at the critical point and 

 very minute changes of pressure may bring about large volume changes, 

 and with the volume changes temperature changes may ensue, so that 

 it might be expected that there would be difficulty in obtaining homo- 

 geneity of the contents of the vessel even just above the critical point. 

 But the variations are so large that many observers doubt whether the 

 liquid and the vapour do actually meet in all their physical qualities at 

 the critical point. They regard the critical point as that at which the 

 densities coincide. They suppose that slightly above the point the 

 liquid is still liquid, and may have a different density from the gas and 

 a different molecular grouping ; but the evidence is hardly conclusive. 



Ramsay and Young, and later Young (Phil. Mag., xxxiii., 1892, 

 p. 153, and in other papers) have measured a number of critical con- 

 stants. The pressures and temperatures were obtained by the direct 

 method of raising the temperature and observing the vapour pressure 

 at the critical point, when the liquid meniscus disappeared. The 

 critical volume was found by heating the substance above the critical 

 temperature, and then cooling it by sudden expansion till there was 

 a temporary separation between liquid and vapour. The volume 

 at which this just took place on slight expansion was taken as the 

 critical volume. MM. Cailletet and Mathias (C.R., cii. p. 1202 ; 

 civ. p. 1563) have found that in many cases the critical volume can 

 be calculated on the assumption that the mean of the liquid density 

 and saturated vapour density changes uniformly with the temperature, 

 so that on a density temperature curve it would be a straight line. 

 If these densities for a lower range of temperature and the critical tem- 

 perature are known, the critical density is easily determined. 



