418 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Considering the size of the platinum vessels weighed, this average agree- 

 ment is all that could be expected ; hence the test of the constancy of the 

 apparatus seems to have been satisfactory. 



TABLE I. 



Test of the Constancy of the Porous Cup Voltameter. 



There is of course nothing in this table to show whether the figures 

 represent the weight of silver which ought to have been deposited by the 

 quantities of electricity employed. It may be that the error of the old 

 voltameter was merely reduced, and that a small constant error still re- 

 mained. The most probable cause of such a remaining error seemed to 

 be the possible diffusion or migration of the heavy anode-liquid through 

 the bottom of the porous cup, in spite of the fact that it was continually 

 removed by means of a pipette or siphon. In order to prevent this, the 

 bottom and a few millimeters of the sides of a porous cup were filled 

 with melted paraffin, which effectually sealed the pores. The upper part 

 of the sides only served to allow the passage of the electricity. A tenth 

 comparison showed that a current which deposited 1.83370 grams of 

 silver in this cup deposited 1.83375 grams in the ordinary porous cup 

 coulometer. This difference is no greater than a possible experimental 

 error; hence we may conclude that the effect of the diffusion is impercep- 

 tible. It will be shown later that the substance which causes the chief 



