BAXTER AND JESSE. — ATOMIC WEIGHT OF CHROMIUM. 425 



ease with which silver dichromate decomposes. Even at the compara- 

 tively low temperature of the melting point of the dichromate, about 

 400°, oxygen is given off rapidly, while at temperatures considerably 

 below this point, 300°, and to a very slight extent at 250°, there seemed 

 to be evidence of decomposition, since salt heated to these temperatures ' 

 did not give an absolutely clear solution in dilute nitric acid. In order 

 to be on the safe side, the drying of the salt took place at 200° C. 



The heating of the dichromate was effected much as described in the 

 preceding paper in the case of silver chromate. The salt, contained in 

 a weighed platinum boat, was heated in a current of pure dry air in a 

 hard glass tube for four hours at 200° C, the air being purified and 

 dried by passing over hot copper oxide, solid potassic hydroxide, con- 

 centrated sulphuric acid containing dichromate, and resublimed phos- 

 phorus pentoxide successively. An oven composed of solid aluminum 

 blocks 4 was used, by means of which the temperature could be main- 

 tained constant within two degrees. 



After the boat had been allowed to cool in the tube, it was trans- 

 ferred to the weighing bottle by means of a "bottling apparatus," 5 

 and was reweighed. Then the dichromate was transferred to a flask and 

 was dissolved in hot 0.8 normal nitric acid, the boat and the weighing 

 bottle being carefully cleansed with nitric acid and the rinsings being 

 added to the main solution. The solution, which was always perfectly 

 clear, was quantitatively transferred to the 3-liter glass stoppered pre- 

 cipitating flask, and at a dilution of about one liter was reduced by the 

 addition of a very slight excess of sulphur dioxide. When the solu- 

 tion was cold, a slight excess of hydrobromic acid was diluted to about 

 800 c.c. and then was slowly added to the silver solution with continual 

 agitation. The flask was stoppered and vigorously shaken. After 

 twenty-four hours' standing the flask was again shaken, and then was 

 allowed to stand two days or more, until the supernatant solution was 

 clear. 



Next the silver bromide was washed at least eight times by decanta- 

 tion with pure water and collected upon a weighed Grooch crucible. 

 Then it was dried in an electric oven, first at 100° for two hours, then 

 at 175° for about eighteen hours. After cooling in a desiccator near 

 the balance for several hours, the weight of the silver bromide was 

 determined. 



The use of an asbestos mat in the Gooch crucible made it necessary 

 to collect and determine the fibres detached during the filtration. This 



* Baxter and Coffin, These proceedings, 44, 184 (1909). 

 6 Richards and Parker, These proceedings, 32, 59 (1896). 



