424 PROCEEDINGS OF THE AMERICAN ACADEMY. 



medium for washing the silver iodide, for it was found that the precipi- 

 tate could be washed indefinitely with even one per cent nitric acid 

 without passing into the colloidal state. The washings obtained in this 

 way, when treated with an iodide, gave no trace of opalescence even 

 on long standing, showing that silver iodide is insoluble in the dilute acid. 

 Wlien silver iodide which has been washed with nitric acid only is dried 

 at a high temperature, a loss of iodine takes place, owing to action on 

 the silver iodide by the nitric acid. This was evident from the dark 

 color of the iodide. In order to avoid this difficulty, the precipitate 

 was finally transferred to the Gooch crucible with pure water, sometimes 

 after one rinsing with water. The nitric acid was so completely dis- 

 placed by this means that the precipitate did not darken even on fusion. 

 Unfortunately, even this slight washing with water in most cases caused 

 some of the precipitate to pass through the crucible in a colloidal 

 condition, consequently these last washings were collected separately. 

 In the earlier analyses the silver iodide in this colloidal solution 

 was determined by first boiling the solution with a small quantity of 

 ammonium iodide until the silver iodide was coagulated, and then 

 collecting the precipitate upon a small filter paper, together with the 

 asbestos shreds and any silver iodide which were contained in the 

 remainder of the filtrate and washings. As a rule this operation was 

 performed only after the wash waters had stood for two or three days, 

 in order that insoluble matter might have time to settle. The bottom 

 of each flask was rubbed gently with a rubber-tipped stirring rod to 

 detach adhering particles. Finally the filter paper was burned in a 

 weighed porcelain crucible at as low a temperature as possible, for if 

 a high temperature is employed, a loss of silver iodide by volatilization 

 occurs. The ash was treated with a drop of nitric acid, and, after 

 warming, a drop of ammonium iodide solution was added. The excess 

 of acid and ammonium salt was driven off and the crucible was weighed. 

 In these earlier analyses it was found necessary to detach small 

 particles of adhering silver iodide from the neck of the precipitating 

 flask by rubbing with a rubber-tipped rod. Later a better method was 

 devised for collecting this small amount of precipitate together with that 

 contained in the colloidal washings. First the flask was rinsed with a 

 small quantity of a solution of potassium cyanide, and this solution was 

 poured into the colloidal washings. Then the solution was evaporated 

 to small bulk and was electrolyzed in a weighed platinum crucible, 

 which was heated to 130° in an electric oven and was finally reweighed. 

 The film was dissolved in dilute nitric acid and the solution was precipi- 



