BAXTER AND JESSE. — ATOMIC WEIGHT OF CHROMIUM. 423 



tilled through a quartz condenser three times with rejection of the 

 extreme fractions. The acid, diluted to normal concentration, was kept 

 in a well protected glass bottle. 



Silver Nitrate. — Silver nitrate was prepared from silver which had 

 been precipitated once as chloride, and then reduced with invert sugar. 

 The nitric acid solution of the fused product was evaporated to crystal- 

 lization, and the salt was then three times more crystallized from nitric 

 acid solutions, the crystals being drained centrifugally in a centrifugal 

 machine employing platinum Gooch crucibles as baskets. 3 Heating 

 was carried out over electric stoves in order to avoid contamination by 

 the combustion products of illuminating gas, both in this and in all 

 other preparations in this research. 



Potassium Dichromate. — The best commercial material was crystal- 

 lized four times, once from aqueous solution in Jena glass, and three 

 times in platinum vessels. 



Chromic Acid. — This substance was three times recrystallized in 

 platinum vessels as described in the preceding paper. 



Silver Dichromate. — Silver dichromate was prepared by combining 

 either potassium dichromate or chromic acid with silver nitrate in 

 nitric acid solution in platinum vessels. Precipitation was carried out 

 in fairly concentrated solution, since in the subsequent crystallization 

 of the silver salt from nitric acid solution any included substance was 

 sure to be eliminated. Although the inclusion of nitric acid during 

 the crystallization was to be feared, and was actually found to have 

 taken place, a method was devised for the determination of this nitric 

 acid, together with the moisture retained by the solid. 



Sample I. Silver nitrate and potassium dichromate were dissolved 

 in equivalent proportions in 3 normal nitric acid, the concentration of 

 each salt being about 0.7 normal. The cold silver nitrate solution was 

 added very slowly, with constant vigorous stirring, to the dichromate 

 solution. After the precipitate had been allowed to settle, the mother 

 liquor was decanted, and the precipitate was centrifugally drained, and 

 rinsed in the centrifugal machine with 3 normal nitric acid. 



The salt was then five times recrystallized from solution in 3 normal 

 nitric acid with centrifugal drainage after each crystallization. Owing 

 to the small solubility of silver dichromate in nitric acid solutions the 

 following scheme of crystallization was adopted. The dichromate was 

 heated with the nitric acid solution upon the electric stove until the 

 acid was saturated with silver dichromate. Then the hot solution was 

 decanted into a dish through a platinum Gooch crucible without a mat 



8 Jour. Amer. Chem. Soc, 30, 286 (1908). 



