There has been considerable discussion as to whether any physical constant of sea-water can be used to give an accurate measure of its total salinity, and from theoretical reasoning it would appear obvious that, in so complex a mixture, no physical constant can give an accurate measure of either the total amount of dissolved salts or of the chlorine present. In a paper communicated to this Society, Mr J. J. Manley showed that the optical method for distinguishing between various samples was as delicate as the relative density method, but the refractometer used was expensive and not fitted for use on board ship.