In each case the volume of the precipitated milk solids is determined by the double dilution method, and the proper correction made (p. 278). The lactose remaining is determined by chemical or optical methods, but it is necessary, in all cases where invert sugar is supposed to be present, to determine the total reducing sugars in the original sample as lactose. If the quantity thus determined and the amount of sucrose found as above are sufficient to produce the rotation observed in the first polarization, it is evident that no invert sugar is present. When the polarization observed is less than is equivalent to the quantity of sugar found, invert sugar is present, which tends to diminish the rotation produced by the other sugars. In this case it is necessary to remove both the sucrose and invert sugar by a process of fermentation, which will leave the lactose unchanged. This is accomplished by conducting the fermentation in the presence of potassium fluorid, which prevents the development of the lactic ferments. For this purpose 350 grams of the evaporated milk are dissolved in water and the solution boiled to secure the normal rotation of the lactose. After cooling to 80°, the casein is thrown down by adding a solution containing about four grams of glacial phosphoric acid and keeping the temperature at 80° for about fifteen minutes. After cooling to room temperature, the volume is completed to one liter with water, well shaken and poured onto a filter. An aliquot part of the filtrate is nearly neutralized with a noted volume of potassium hydroxid. Enough water is added to make up, with the volume of potassium hydroxid used, the total space occupied by the precipitated solid, corresponding to that part of the filtrate, and if necessary, refilter. The volume occupied by the precipitated solids is easily determined by polarization and double dilution. The filtrate, obtained from the process described above, is placed in portions of 100 cubic centimeters each in 200 cubic centimeter flasks with about twenty milligrams of potassium fluorid in solution, and half a cake of compressed yeast. The yeast is broken up and evenly distributed, and the fermentation is allowed to proceed for ten days at a temperature of from 25° to 30°. At the end of this time experience has shown that all of the sucrose and invert sugar has disappeared, but the lactose remains intact. The flasks are filled to the mark with water and the lactose determined by chemical or optical methods. By comparing the data obtained from the estimation of the total reducing sugars before fermentation or inversion and the estimation of the lactose after fermentation, the quantity of invert sugar is easily calculated. The experience of this laboratory shows that invert sugar is rarely present in evaporated milks, which is an indication that the sucrose added thereto does not generally suffer hydrolysis. The mean percentage of added sucrose found in evaporated milks is about forty.