OF ARTS AND SCIENCES. 29 



pared. It consists in determining in one series of analyses the bromine 

 of the compound as bromide of silver by the iisual gravimetric method 

 with the precautions already described (page 19) ; and in another series 

 of experiments, or in the same analyses, when practicable, determining 

 the amount of silver required to precipitate the bromine. We thus 

 obtain not only the relation of the atomic weight sought both to that 

 of bromine and to that of silver ; but also the relation between the 

 atomic weight of bromine and that of silver ; and since all experi- 

 menters agree on this last ratio to within one ten-thousandth of its value 

 it is evident that the comparison of the two series of results gives a 

 sharp control of the accuracy of the work. 



Professor Cooke assigned to me the atomic weight of cadmium as 

 my portion of the work he had planned on the revision of the atomic 

 weights, and this investigation was made with his aid and under his 

 immediate direction. Bromide of cadmium fulfils all the conditions 

 which the new method requires ; and, since the accepted value of the 

 atomic weight of cadmium is a whole number, it seemed probable that 

 a revision of this value by a more exact process would bring additional 

 evidence in support of the hypothesis of Prout. 



Having found that bromide of cadmium could not readily be puri- 

 fied by repeated crystallizations on account of its very great solubility 

 in water, we sought to obtain a pure compound by preparing pure car- 

 bonate of cadmium on the one hand and pure hydrobromic acid on the 

 other. 



To prepare pure carbonate of cadmium the commercial metal was 

 first dissolved in pure hydrochloric acid. From this solution, still 

 strongly acid, sulphide of cadmium was precipitated by sulphide of 

 hydrogen, and the precipitate thoroughly washed with hot distilled 

 water. The sulphide having been redissolved in hydrochloric acid, 

 and the resulting sulphide of hydrogen expelled by boiling, the cad- 

 mium was next precipitated as carbonate by carbonate of ammonia, 

 and the precipitate digested with a large excess of this reagent. The 

 white carbonate thus obtained was thoroughly washed and redissolved 

 in hydrochloric acid ; and the same series of precipitations repeated. 

 Lastly, in order to remove any possible trace of adhering chloride, the 

 carbonate of cadmium which had thus been twice precipitated by car- 

 bonate of ammonia, and twice digested with a large excess of this re- 

 agent, was dissolved in pure hydrobromic acid, and a third time precipi- 

 tated and digested with pure carbenate of ammonia. 



The hydrobromic acid used in this investigation was made by the 

 process described by Dr. Edward R. Squibb, of Brooklyn, in the 



