= Carbon-dioxide viewing-cell phase-transition experiment
The institutional account <CO2 viewing cell> describes an optical cell at the University of Stuttgart: a fixed-<volume (thermodynamics)> <carbon dioxide> sample cools from above its <liquid-gas critical point> into <phase coexistence>. The apparatus supplies a view through the cell and displayed <temperature> and <pressure> readings. Near roughly $31\,{}^\circ\mathrm C$, the account reports <critical opalescence>: enhanced <scattering of light> makes reflected light appear whitish and transmitted light dark. This readout reveals growing density <thermal fluctuations>; it is not a direct count of molecules or a measurement of an infinite <correlation length>. <CO2 viewing cell>{ref}
Fixed <volume (thermodynamics)> constrains the experimental path; the <pressure> varies and is a readout, not an independently held control. The account explicitly identifies the clip as part of a longer cooldown and says its endpoints are not steady states. To test an equilibrium interpretation more closely, one would hold <temperature> until the readout settles, compare cooling and warming, and check optical background away from the <liquid-gas critical point>; these are proposed controls, not measurements reported on that page. The account does not provide cell dimensions, filling <number density>, calibration uncertainties or such control runs. The sample is finite: its striking appearance motivates the <thermodynamic limit> as a model of macroscopic sharpness, rather than demonstrating a literal equilibrium singularity.
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