A large quantity of hydrobromic acid was prepared according to the method described by Dr. Edward R. Squibb (Trans. of Med. Soc. of the State of N. Y.). One part of water was added to seven parts of strong sulphuric acid (Sp. Gr. = 1.83) and the mixture cooled. Then six parts of potassium bromide were dissolved in six parts of hot water and the diluted sulphuric acid added to this hot solution. It was set aside until cold in order to allow the sulphate of potassium to crystallize out. The crystals were drained on a filter-plate and quickly washed with two parts of water. The mother-liquor and washing were then distilled until no more acid was obtained on further heating. The acid thus obtained was distilled three times from potassium bromide, twice from cadmium bromide formed by adding a piece of pure cadmium to it, and twice without the addition of anything. It was tested and found to be free from sulphuric acid. Cadmium bromide was prepared from it, in exactly the same way that the cadmium chloride used in the chloride method was prepared from pure metal and hydrochloric acid. While the crystalline mass of cadmium bromide was still moist, it was transferred to a combination tube and dried at 300°C for several hours in a current of nitrogen. It was then sublimed in a vacuum as the chloride had been. This specimen served for the first three determinations. About nine grammes of it was placed in a platinum boat in a combustion tube, and part of it distilled in a current of nitrogen. The distillate, a portion of which had been tested with tropaeolin and found neutral, was used for determination I. The residue in the boat was used for determination II. Another portion of the main sample was resublimed in a vacuum and used in determination no. III. Cadmium bromide is not hygroscopic or at least only slightly, therefore the sublimed cadmium bromide can be transferred to a weighing-glass without taking up water. This cannot be done in the case of the chloride. It is probable that the hydrobromic acid as above prepared was perfectly free from hydrochloric acid. Chlorine in cadmium bromide would cause the atomic weight to be found lower than it really is. It was thought desirable, however, to prepare an acid which would certainly be free from chlorine. The method described by Stas (Aronstein’s German translation, p. 154.) was employed with the additional precaution that the above purified acid was used to start with and all reagents employed had been especially prepared so as to be free from chlorine. Pure silver was prepared according to Stas’ description (see Aronstein’s translation, page 34, also page 104) by the action of ammonium sulphite on an ammoniacal solution of silver nitrate and copper sulphate. The silver was dissolved in nitric acid free from chlorine, and then slowly added to a dilute solution of the above-described hydrobromic acid, and the precipitated silver bromide thoroughly washed. It was then digested for a long while in a strong