In this paper are collected and discussed the observations of the specific gravity of saline solutions of moderate and high dilution made during the course of the last ten years. The instrument employed was the hydrometer, of the type used by me during the cruise of the Challenger in the years 1872 to 1876. The results then obtained were such as to convince me that the instrument, in capable hands, and with strict attention to the obligatory conditions of experiment, gives more exact results than any other instrument in use for such work. In the course of the last forty years the method has been improved in the sense that more and more attention has been paid to the conditions, especially those of temperature, under which the experiment is made, because it was found that the method is so delicate that it responds to every increase of care in experimenting by greater exactness in the results which it furnishes.