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We measured high-speed sound propagation in a near-critical fluid using a ultra-sensitive interferometer to investigate adiabatic changes of fluids on acoustic timescales. A sound emitted by very weak continuous heating caused a stepwise adiabatic change at its front with a density change of order 10 −7 g/cm 3 and a temperature change of order 10 −5 deg. Very small heat inputs at a heater produced short acoustic pulses with width of order 10µsec, which were broadened as they moved through the cell and encountered with the boundaries. The pulse broadening became enhanced near the critical point. We also examined theoretically how sounds are emitted from a heater and how applied heat is transformed into mechanical work. Our predictions well agree with our data. Thermal equilibration in one-component fluids takes place increasingly faster near the gas-liquid critical point at fixed volume [1,2,3,4,5,6,7,8,9,10], despite the fact that the thermal diffusion constant D tends to zero at the criticality. This is because the thermal diffusion layer at the boundary expands and sounds emitted cause adiabatic compression and heating in the whole cell after many traversals in the container. This heating mechanism is much intensified near the critical point due to the critical enhancement of thermal expansion of the layer. If the boundary temperature T w is fixed, the interior temperature approaches T w on the timescale of the piston time [2],where L is the cell length and γ = C p /C V is the specificheat ratio growing near the critical point. This time is much shorter than the isobaric equilibration time L 2 /4D by the very small factor (γ − 1) −2 [11]. The previous experiments have detected only slow temperature and density changes in the interior region on timescales of order 1 sec. The aim of this letter is to report ultra-sensitive, high-speed observation of sound propagation through a cell filled with CO 2 on the critical isochore close to the critical point T c = 304.12K. We can detect density changes of order 10

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The mechanism of phosphate coprecipitation with calcite is proposed on the basis of homogeneous distribution of phosphate in the crystal at 20°C and pH 7.9-9.5. At a constant pH, phosphate content of calcite is proportional to total phosphate concentration in aqueous solution. PO' ions are uptaken into the calcite lattice, where 3 CO3 ions are replaced by 2 P04 ions in aqueous solution, as described by the ion exchange equilibrium:where S and L represent calcite and aqueous solution, respectively.