98 PROCEEDINGS OF THE AMERICAN ACADEMY. 



his measurements are so inaccurate that no conclusions can be drawn 

 from them. The results of Paschen* with dropping mercury elec- 

 trodes show signs of the same regularities which are found in Table I. ; 

 but the variations in the observations are so great that the matter 

 cannot be said to be definitely settled. It seemed to me necessary to 

 make measurements in order to clear up the following points. Does 

 the effect of the negative ion disappear in certain cases? Is this a 

 characteristic of the halogens only ? What is the effect of the concen- 

 tration ? May one look upon these cells as the limiting cases of the 

 constant, reversible cells of the Daniell type? 



Most of the experimental work recorded in this paper was done in 

 the chemical laboratory at Amsterdam, and I am much indebted to 

 Prof, van 't Hoff for assistance in carrying out this research. 



The metals used came from the collection of the laboratory. I did 

 not purify them further owing to lack of time. For this reason the 

 absolute value of some of the figures may not be correct ; but that has 

 no effect on the general laws, which alone interested me. I purified 

 the mercury myself ; but I did not succeed in keeping the sur- 

 face bright very long. The salt solutions were made by weighing 

 out carefully the amounts of salt necessary for the concentrated solu- 

 tions, and the others were made from these by diluting to the required 

 volumes. The measurements were made with the small Lippmanu 

 capillary electrometer. As normal element I used a Latimer Clark 

 cell; its value was found by Ilerr Barendrecht to be 1.426 volts. I 

 compared it from time to time with a Gouy cell, and found no change 

 which amounted to more than a millivolt. As working cell I had 

 also a Leclanche with about the same electromotive force as the 

 Clark cell. Its variations were quite considerable, about 3%. The 

 measurements against mercury were made in a small test tube which 

 had a platinum wire melted into it; the others were made in a U tube 

 or in an ordinary beaker. The metals, with the exception of platinum, 

 were used in the form of rods or heavy wire. At first I polished them 

 with sand-paper ; but this proved disadvantageous, as the surface is 

 brought into a state of stress by this which is incompatible with accu- 

 rate measurements. I have found it profitable to cut away the surface 

 with a sharp knife, and then to wipe the metal with a cloth. There is 

 the danger of a piece of steel getting on the electrode; but in most 

 cases this will not cling firmly, and will be removed by wiping. It is 

 true that the surface tension of a metal is often changed temporarily 



* Wied. Ann., XLIII. 5G8, 1801. 



