In order to determine the quantity of ferric oxid which it contains, fifty cubic centimeters are evaporated to dryness, ignited, and weighed. A second operation like the above is carried on by adding fifty cubic centimeters of the standard solution of phosphoric acid, and the strength of the solution thus obtained is marked upon the flask. If the operation have been properly carried on, three or four duplicates will give exactly the same figures. If there are sensible differences, the whole operation should be done over from the first. =98. Titration of the Solution of Uranium.=—In a 150 cubic centimeter flask marked at seventy-five cubic centimeters, are poured ten cubic centimeters of the standard solution of phosphoric acid measured with an exact pipette; five cubic centimeters of the acid sodium acetate are added, and distilled water enough to make about thirty cubic centimeters, and the whole carried to the boiling-point. The titration is then carried on by allowing the solution of uranium to fall into the flask from a graduated burette, thoroughly shaking after each addition of the uranium, and trying a drop of the liquor with an equal quantity of a ten per cent solution of potassium ferrocyanid upon a greased white plate. Since the quantity of the uranium solution present will be very nearly ten cubic centimeters at first, nine cubic centimeters can be run in without testing. Afterwards, the operation is continued by adding two or three drops at a time until the test upon the white plate with the potassium ferrocyanid shows the end of the reaction. When there is observed in the final test a slight change of tint, the flask is filled up to the mark with boiling distilled water and the process tried anew. If in the first part of the operation the point of saturation have not been passed, it is still usually necessary to add a drop or two of the uranium solution in order to produce the characteristic reddish coloration, and this increase is rendered necessary by the increase in the volume of the liquid. Proceeding in this manner two or three times allows the attainment of extreme precision, inasmuch as the analyst knows just when to look for the point of saturation. _Correction._—The result of the preceding operation is not absolutely exact. It is evident indeed that in addition to the quantity of uranium necessary for the exact precipitation of the phosphoric acid, it has been necessary to add an excess sufficient to produce the reaction upon the potassium ferrocyanid. This excess is rendered constant by the precaution of operating always upon the same volume; namely, seventy-five cubic centimeters. It can be determined then once for all by making a blank determination under the same conditions but without using the phosphoric acid.