26 
of sulphuretted hydrogen water to precipitate the mercury. After 
being well shaken, filtration not being necessary at this point, 
excess of carbonate of bismuth is added to precipitate the 
residual sulphuretted hydrogen, well shaken, filtered and washed, 
and the filtrate titrated with silver nitrate after the addition of 
a little potassium iodide to sharpen the end reaction which is 
indicated by a faint permanent opalescence. One molecule of 
silver nitrate used indicates two molecules of cyanogen present. 
