OF A.RTS AND SCIENCES. 253 



a button of pure metallic silver, which we annealed and rolled out to 

 a thin ribbon. We then weighed out from two to four grammes of 

 bromide of antimony, prepared by sublimation as described above, and 

 dissolved this salt in an aqueous solution of tartaric acid, which we then 

 transferred to a litre flask and diluted to about 500 cubic centimetres. 

 "We next very accurately weighed out a quantity of silver slightly less 

 than that which calculation showed was required for complete precipi- 

 tation. This silver was dissolved in nitric acid, and the solution having 

 been evaporated to dryness over a water bath, the silver salt was 

 washed into the flask containing the bromide of antimony. As soon 

 as the supernatant liquid had cleared, the small additional amount of a 

 normal silver solution required to produce complete precipitation was 

 run in from a burette, and measured with the usual precautions. We 

 used no extraneous indicator, because it was important not to intro- 

 duce any possibly new disturbing element into the experiment, and in 

 the titration of bromine with silver the normal and familiar phe- 

 nomena, which mark the close of the process, furnish a very sharp 

 indication. The details of one of the determinations were as fol- 

 lows : — 



The weight of the bromide of antimony used amounted to 2.5032 

 grammes. To precipitate the bromine from the solution of this 

 material 2.2404 grammes of silver would be required if Sb = 122.00 

 and 2.2529 if Sb = 120.00. We weighed out, with as much accu- 

 racy as if we were adjusting a weight, the smaller of these two quan- 

 tities of metallic silver, and after dissolving the pure metal in pure 

 nitric acid, evaporating the solution to dryness and redissolving in 

 water, we added at once the whole of this silver solution to the litre 

 flask containing the solution of bromide of antimony, in the manner 

 described above. It was then found that 12 T 4 ff cubic centimetres of 

 a normal silver solution (one gramme of silver to the litre) were 

 required to complete the precipitation. It will be seen that the 

 weights of the bromide of antimony and silver used could be thus 

 determined with the most absolute precision, and we have the greatest 

 confidence in these values to the ^ of a milligramme. Moreover, it 

 will be noticed that the volumetric method is only used to estimate the 

 difference in the atomic weight which has been in question, and that 

 if the method were only accurate to the ^ of the quantity to be 

 measured it would give us the value of the atomic weight within ^ g of 

 a unit ; while if, as we had reason to believe, the process was accurate 

 within one per cent, it would fix the atomic weight within T ^ of a 

 unit. 



