The P.D. between the interior of a cell of Valonia macrophysa and concentrated or diluted sea water applied externally is given by the empirical equation:

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where P.D.s.w. is the value observed when the external solution is sea water. The sign is that of the interior of the cell. In the chain:

sea water, concn.1 | Valonia | sea water, concn.2

therefore, the more concentrated solution is positive in the external circuit to the more dilute solution.

This holds for the concentration range 1.5 sea water to 0.2 sea water, when the dilute solutions are made isotonic with sea water by the addition of a suitable non-electrolyte (such as glycerol). Prolonged exposure to these solutions, or brief exposure to very concentrated or very dilute sea water, or to hypotonic solutions, produces in the cell secondary changes which are made evident by the reversal of the sign of the P.D. These changes are to a certain extent reversible, but if allowed to proceed too far they become irreversible, involving permanent injury to the cell.

The concentration effect with sea water is shown to be practically the concentration effect for NaCl, the part played by the other salts in sea water being relatively small.

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