420 PROCEEDINGS OP THE AMERICAN ACADEMY. 



zinc nitrite. The NO3' ion must have been decomposed into NO.,' and 

 oxygen. This same reaction takes place when silver serves as an anode 

 in its nitrate solution, although to a much smaller extent. 



In spite of the irregularities just described, the deposition on the 

 kathode proceeded in a perfectly regular manner, and the figures show 

 that as much silver was deposited in one cell as in the other. 



Still another means of testing the porous cup voltameter was found in 

 its comparison with a device which eliminates the porous cup wholly, 

 but which nevertheless keeps the anode solution quite away from the 

 cathode. This device consists in placing the anode at the bottom of a 

 tall beaker filled with a concentrated solution of argentic nitrate (200 

 grams of the salt in a litre of solution), and arranging the kathode in 

 the upper part of the vessel.* The anode solution becomes heavier 

 and remains around the anode, while the kathode solution becomes 

 lighter and rises to the surface. In order to prevent this dilution 

 around the kathode from diminishing too much the concentration of the 

 contiguous liquid, it is well to sink the kathode at least two centimeters 

 below the surface. A circular disk of platinum wire gauze,t six centi- 

 meters in diameter, was used as the kathode, since many holes in gauze 

 permitted the ready escape upward of the impoverished electrolyte. 

 The gauze was bent around a stout circular platinum wire, and the disk 

 was stiffened by four radial wires, and was hung rigidly from the centre. 

 The vertical distance between this kathode and the anode was about 

 seven centimeters. The anode consisted of a plate of pure silver, and its 

 platinum connecting wire was protected from the solution by an enclos- 

 ing glass tube. 



The chief trouble encountered in manipulating the voltameter thus 

 constructed is the danger of losing fine crystals of silver from the flexi- 

 ble gauze. In the two experiments described below every precaution 

 was taken to avoid this source of error, and it is believed that no appre- 

 ciable weight was lost. Another disadvantage of the gauze is the fact 

 that metals deposited upon it are very apt to include minute quantities of 

 electrolyte because of the interstices arising from its woven structure. 

 Even silver deposited in a crucible contains some included mother 

 liquor, and that deposited on the gauze contains much more. In the 

 two experiments given below, the first deposit on the gauze lost 0.42 

 milligram on gentle ignition in a large porcelain crucible, and the 



* Merrill, Pliys. Rev., X, 169 (1000). 



t Paweck, Zeitsch. fiir Berg. u. Hiittenwesen, 46, 570 (1898) ; Winkler, Ber. d. 

 d. ch. Ges., 32, 2192 (1899). 



