THE PURITY OF THE MATERIALS. 1 5 



The factor -=J = 0.995 approximately when a zinc amalgam containing 



0.9 per cent of zinc is diluted with nine times its weight of mercury ; the 

 same dilution of 3 per cent cadmium amalgam will introduce a factor of 

 0.987. Neglect of this consideration makes the calculated values for it too 

 high in all cases, and the error may be a serious one. 



THE PURITY OF THE MATERIALS. 



All the materials used in potential work were purified with great care. 

 " Chemically pure " zinc sulphate was dissolved in twice-distilled water to 

 form a fairly strong solution. This was allowed to stand in Jena glass for 

 a month over electrolytic zinc made from another sample of the same salt. 

 Fresh portions of zinc were added every week, and the flasks were frequently 

 shaken to bring all parts of the solution into contact with the metal. The 

 last portion of zinc added was made from a salt prepared for atomic-weight 

 work in this laboratory. 30 The product was filtered on pure filter papers and 

 recrystallized three times in platinum. The feathery crystals obtained were 

 each time separated from the mother liquor with great completeness by a 

 small centrifugal 37 similar to those used in urine analysis. The efficiency 

 of this device was so great that a cubic centimeter of liquid could easily be 

 separated from three times that bulk of pressed crystals. 



The final product was dried on a watch glass and used in the concentra- 

 tion cells without further treatment. 



A generous supply of very pure cadmium sulphate prepared for atomic- 

 weight work used was kindly placed at our disposal by Professor Baxter 

 and Mr. Hines. 38 The sample in question had been twice precipitated as 

 sulphide, dissolved in nitric acid, evaporated with excess of sulphuric acid, 

 three times recrystallized, and dried at ioo. It was used for the concen- 

 tration cells without further purification. 



The double precipitation as sulphide must have excluded those metals, 

 zinc especially, which could become sources of error in the later part of the 

 work. 



Crude mercury was shaken with sulphuric acid to remove the major part 

 of its metallic impurities. Then it was vigorously shaken for a long time 

 with a solution of mercurous nitrate and nitric acid prepared from a sample 



"Richards & Rogers, Proc. Am. Acad., 31, 158 (1895). This treatment is important 

 in order to separate any metal of less solution tension, which would later replace zinc 

 in the amalgam and lower its electromotive force. 



37 Journ. Am. Chem. Soc, 27, 109 (1905). 



38 Ibid., 222 (1905). 



