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Berkeley and Hartley measured the osmotic pressures of concentrated solutions with an apparatus
 | Fig: Apparatus of Berkeley and Hartley (diagrammatic) |
in which the porous tube C carrying the semipermeable membrane is filled with water and surrounded by the solution to which a pressure P is applied to balance the osmotic pressure. The two pressures are equal when no water passes through the membrane, the meniscus M remaining stationary. The membrane is subjected to equal pressures on both sides and hence is less likely to fail under the high pressures measured.
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