Electromotive Force of different Forms of the Clark Cell. 127 



exactly as therein described. When cool, the supernatant liquid 

 may be decanted off to be used for the preparation of crystals. A 

 supply of suitable crystals is easily obtained at any time by taking 

 this solution, or any other solution of zinc sulphate which has been 

 neutralised and treated with mercurous sulphate at 30° C, and cool- 

 ing it down to 0° C. The liquid is then decanted off, and the crystals 

 drained on a piece of filter-paper. This method leaves them suffi- 

 ciently moist for the purpose. The crystals are filled into the cell 

 through a glass tube or funnel to a depth of about 2 cm., and a hollow 

 is made in the surface with a glass rod to facilitate the introduction 

 of the zinc. When the cell has settled it should be free from air- 

 bubbles, and should show the merest film of liquid above the surface of 

 the crystals. Inasmuch as zinc sulphate does not tend to form any 

 hydrate higher than the hepta-hydrate between the limits 0° and 

 30° C, the crystals will remain equally moist, or very nearly so, 

 between these limits. 



After soldering the platinum wire on to the zinc rod, we prefer to 

 seal the end of the zinc rod with marine glue into a glass tube 

 which nearly fits it. The object of this is to make the best possible 

 seal to protect the solder joint, which may otherwise be injured by 

 the creeping of the solution. The upper part of the glass tube is 

 then sealed on to the platinum wire to form a mercury cup. The 

 glass tube also forms a convenient handle to use when inserting the 

 cork and the zinc rod into the cell. We prefer to amalgamate the 

 zinc rod, as this proceeding appears to protect the zinc from local 

 action, and to give more uniform results. We may here remark that 

 in the B.O.T. cells containing solution the zinc rod, even if amalga- 

 mated, rapidly becomes corroded near the top by local action. In 

 the cells filled with crystals, on the other hand, the zinc remains 

 perfectly bright and clean. 



§ 11. Decomposition of Mercurous Sulphate. 



After neutralising the zinc sulphate solution with zinc oxide, we 

 prefer to filter in a jacketed funnel at 40° C, for the sake of obtain- 

 ing a stronger solution. Mercurous sulphate is then added to remove 

 any traces of zinc oxide or other impurities which have any action 

 upon it. We have observed that if, after the addition of the mer- 

 curous sulphate, the solution be heated to between 35° and 40° C, a 

 slight change may be noticed in the appearance of the mercurous 

 sulphate. The filtrate, when cooled to 15° C, may remain clear, but, 

 if further cooled to 0° C, a yellow turbidity makes its appearance, 

 showing that the mercurous sulphate has, in all probability, been 

 partly decomposed by exposure to the higher temperature. If, on 

 the other hand, the zinc solution has not been heated above 30° C. 



