156 RESEARCHES UPON ATOMIC WEIGHTS. 



since the water content of our material was carefully investigated, the presence 

 of hydrochromate in traces could do no harm; for the latter substance upon 

 sufficient heating would yield dichromate and water according to the following 

 equation: 



2AgHCr04 = Ag2Cr207 + H20. 



Although the presence of polychromates other than the dichromate seemed 

 improbable, their absence from our material was shown by crystallizing silver 

 dichromate from nitric acid of different concentrations. Since this variation 

 was without effect, it may be reasonably supposed that more highly acid salts 

 than the dichromates were neither precipitated as solid phases nor occluded. 



PURIFICATION OF MATERIALS. 



Only slight changes were made in the methods of purifying the materials used 

 in the various preparations of silver dichromate and in the analyses from 

 those described in the preceding paper. 



Nitric acid was freed from chlorine by several distillations through a platinum 

 condenser. 



Hydrochloric acid also, after dilution, was purified by distillation with a 

 quartz condenser. 



Hydrobromic acid was prepared from bromine which had been twice distilled 

 from solution in potassium bromide, the bromide in the second distillation being 

 essentially free from chlorine. The hydrobromic acid was synthesized by passing 

 carefully cleansed hydrogen (made from the lead-sodium alloy "hy drone" and 

 water) through the bromine at about 40° and then over hot platinized asbestos, 

 the acid being collected in pure water. Iodine was eliminated from the acid by 

 boiling with free bromine several times. Finally the acid was redistilled 

 through a quartz condenser three times with rejection of the extreme frac- 

 tions. The acid, diluted to normal concentration, was kept in a well protected 

 glass bottle. 



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 crystallization, 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.^ Heating was carried out over electric stoves in order to avoid con- 

 tamination by the combustion products of illuminating gas, both in this and 

 in all other preparations in this research. 



The best commercial potassium dichromate was crystallized four times, 

 once from aqueous solution in Jena glass, and three times in platiniun vessels. 



Chromic acid was three times recrystallized in platinum vessels as described 

 in the preceding paper. 



* Baxter: /cwr. Amer. Chem. Soc, 30, 286 (1908). 



