OF ARTS AND SCIENCES. 



209 



required to form it; and this last value was determined according to 

 two distinct methods. In the first phice, the weight of silver required 

 for a given weight of the copper bromide was calculated, and some- 

 what less than this amount was weighed out and dissolved in nitric 

 acid in the manner before described. The cupric bromide was then 

 cautiously added to the warm dilute solution, and the deficiency of 

 silver made up by very careful titration with a solution containing one 

 gram of silver to the litre.* After noting carefully this first value for 

 the amount of silver required, a slight known excess of the standard 

 solution was added, and the silver bromide was washed, collected, and 

 weighed upon a perforated crucible, as before. The excess of silver 

 in the filtrate was now carefully determined by means of a standard 

 solution of ammonic sulphocyanide, using as a standard of colorimet- 

 ric comparison solutions containing an equivalent amount of pure cop- 

 per nitrate and small known amounts of silver nitrate. This second 

 method of determining the amount of silver required to precipitate the 

 bromine is not so accurate as the first, but is of value as a check upon 

 the other. The most complete experiment tried is given in detail, as 

 an example of the method. 



Experiments 22 and 25. 



Glass-stoppered flask with CuBrg solution 

 Glass-stoppered flask alone 

 Cupric bromide solution taken 



grams. 



= 47.711 

 = 21.776 



= 25.935 



Amount of silver dissolved (cor. to vac.) 

 Amount of silver added in titration 

 First value found for 7-equired Silver 



Corrected Weights, 

 grams. 



= 0.9639 

 = 0.0001 



= 0.9640 



0.50 c.c. excess of AgNO^ solution added to fil- 

 trate. Silver present = 0.0005 



Total weight of silver used = 0.9645 



Excess of silver was precipitated by 0.07 c.c. 



NH^SCN solution. Corresponding silver = 0.0007 



Second value for required Silver = 0.9638 



* For a further description of the metliod by Prof. J. P. Cooke, see these 

 Proceedings, xvii. 18. 



VOL. XXV. (X. S. XVII.) 14 



