The result of this determination is that it renders possible the correction which it is necessary to make by subtracting the quantity used in the blank titration from the preceding result in order to obtain the exact strength of the uranium solution. The operation is carried on as follows: In a flat-bottomed flask of about 150 cubic centimeters capacity and marked at seventy-five cubic centimeters, by means of a pipette, are placed five cubic centimeters of the solution of sodium acetate; some hot distilled water is added until the flask is filled to the mark, and it is then placed upon a sand-bath and heated to the boiling-point. It is taken from the fire, the volume made up to seventy-five cubic centimeters with a little hot distilled water, and one or two drops of the solution of uranium are allowed to flow into the flask from a graduated burette previously filled exactly to zero. After each drop of the solution of uranium the flask is shaken and the liquid tried upon a drop of potassium ferrocyanid, as has been previously indicated. For a skilled eye, four to six drops are generally necessary to obtain the characteristic coloration; that is from two-tenths to three-tenths of a cubic centimeter. Beginners often use from five-tenths to six-tenths, and sometimes even more. The sole important point is to arrest the operation as soon as the reddish tint is surely seen, for afterwards the intensity of the coloration does not increase proportionally to the quantity of liquor employed. It is well to note that at the end of some time the coloration becomes more intense than at the moment when the solutions are mixed, so that care must be taken not to pass the saturation-point. This slowness of the reaction is the more marked as there is more sodium or ammonium acetate in the standard solutions. This is the reason that it is important to introduce always the same quantity; namely, five cubic centimeters. This is also the reason why the uranium acetate should not be employed in preparing the standard solution of uranium which ought to contain the least possible amount of acetate in order that the necessary quantity which is carried into each test should be as small as possible and remain without appreciable influence. If it were otherwise, the sensibility of the reaction would be diminished in proportion as a larger quantity of uranium solution was employed, giving rise to errors which would be as much more important as the quantities of phosphoric acid to be determined were greater. The correction for the uranium solution having been determined it is written upon the label of the bottle containing it. _Causes of Errors._—In the work which has just been described, some causes of error may occur to which the attention of analysts should be called.