142 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



permeable septum desired. These vessels were 50 millimeters high 

 and 20 in diameter ; their walls were not much over one millimeter in 

 thickness. Their impurities were removed by boiling with nitric acid 

 and thorough washing with water. They were suspended in the solution 

 by means of a platinum wire hung upon a glass hook, which insulated the 



wire from the electric con- 

 nections. In four experi- 

 ments two concentric porous 

 cups were used, the annular 

 space between them being 

 charged with argentic oxide, 

 since Kahle found this sub- 

 stance to diminish the re- 

 ducing action in old solutions. 

 This last arrangement was 

 soon abandoned, however. 

 By means of a siphon the 

 liquid within the cup was 

 always kept at a lower level 

 than that without, so as to 

 prevent outward filtration. 



The cathodes consisted of 

 the large lipped crucibles al- 

 ready described ; the anodes 

 were bars 5x1x1 centi- 

 meters of the purest silver, 

 supported by silver wires and 

 not enclosed in filter paper ; 

 and the electrolyte usually 

 contained 10 per cent of pure 

 freshly prepared argentic 

 nitrate. 



The temperature of the 

 cell under investigation was 

 kept constant by means of 

 suitable baths. For a low temperature a mixture of pure ice and 

 a solution of aramonic nitrate answered well, and for the higher tem- 

 peratures a large beaker containing pure water was warmed from the 

 outside by enough encircling leaden steam-pipe to cause the desired 

 intensity of heat. 



^^^^ 



Figure 6. — Porous Cup Voltameter 

 (I actual size). 



A, glass hook for supporting anode. B, glass 

 ring for supporting porous cup. C, silver anode. 

 D, porous cup. E, platinum cathode. 



