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This material is echoed from http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA2/MAIN/BENZENE/CD2R1.HTM :

The Critical Point of Benzene

A sealed tube containing benzene liquid and vapor is heated. As the critical temperature is approached, the meniscus flattens. At the critical temperature the meniscus disappears, and separate liquid and vapor phases can no longer be detected. When the apparatus is cooled below the critical temperature of benzene, the meniscus reappears.

As the temperature rises, the liquid meniscus flattens. This flattening is due to the increased energy of the benzene molecules. The forces due to increased molecular movement are beginning to overcome the attractive molecular forces at the surface. At the edges of the vial, near the liquid surface, the benzene can be seen to be boiling. When the critical point is reached, the meniscus disappears and only diffraction irregularities are present due to temperature gradients. Here the forces due to molecular movement overcome the surface tension, and the separation between liquid and vapor disappears.

Above the critical temperature, only the gaseous phase exists. A gas above the critical temperature cannot be condensed into a liquid, since the molecular attractive forces are dominated by the rapid molecular motion.

As the temperature decreases, the meniscus reappears at the critical temperature. The molecules in the gas move more slowly, allowing the attractive forces between the molecules to become dominant. At this point separate liquid and vapor phases form.

See the slideshow

The cooling-down part of the process can be seen as a QuickTime movie (download)
(mirrored from jchemed.chem.wisc.edu/jcesoft/cca/cca0/MOVIES/BENZENE3.MOV)

Critical opalescence can also be seen in freon
(mirrored from U. Minnesota)