The author, after explaining the methods hitherto proposed for purifying the phosphoric acid of bone earth, and pointing out their inconveniences, mentioned that the chief difficulty was the separation of the magnesia always present in bone earth. The lime, it has been for some time known, may be entirely removed by means of sulphuric acid. In repeating the process of Liebig, which did not succeed in his hands, and which requires the use of alcohol, he found that, after the separation of the lime, there is obtained, by evaporation and heating, a clear and colourless glass, containing all the magnesia: that this glass dissolved completely by boiling in water; but that the solution, if again evaporated and heated to 600° for a quarter of an hour in a platinum capsule, became turbid, and deposited the whole magnesia as an insoluble salt. When water was added, so as to dissolve the phosphoric acid, and the insoluble salt separated by the filter, the filtered liquid was found absolutely free from magnesia, and was a solution of pure phosphoric acid.