Density Discontinuities at the Critical Temperature

A technique for measuring densities in various parts of a one- or two-component system, raised above its critical temperature, is described. Considerable improvements over a method for this purpose recently described by one of the writers consist in greater flexibility of manipulation and in making possible a variation in the volume of space confining the medium during the experiment. Propylene and methyl-ether were the two media examined. From the results the following generalizations regarding aberrations from the continuity of state were found to hold for both.When either liquid was heated above the temperature at which the visible meniscus disappeared, the density below this point of disappearance was found to be greater than that above. The density was uniform throughout each portion, undergoing a relatively sharp change in the small region where the meniscus was last seen. After one hour of temperature equilibrium, the difference in density between top and bottom became constant and remained unaltered for six hours. Constant stirring or temperature fluctuations of the order of 0.02 °C. do not alter this density difference. A decrease in the volume available for the medium increases the density difference between the top and bottom, a continuous relation existing between available space and density difference. With decrease in available space, the densities of both upper and lower portions of the medium increase, the density of the lower more rapidly than that of the upper. These results were reproducible quantitatively in the experiments carried out to date. The density difference for a fixed available space decreases with rise in temperature and is measurable up to at least 10 °C. above the critical temperature. A number of miscellaneous experiments are described which form the basis of work now being carried out. The theoretical significance of aberrations from the continuity of state is discussed to the extent warranted by the present stage of the experimental investigations, and tentative conclusions are drawn.