452 



Mr. J. Brown on the Role of 



cells hitherto examined no such attraction has existed, since 

 the cation was either hydrogen or a metal with no affinity for 

 the positive plate employed. It was therefore proposed in the 

 second place to take up the question — 



In what way, if any, does a chemical attraction between the 

 positive plate and the cation influence the electromotive force of 

 the cell f 



7. To experiment on these questions it was proposed to re- 

 place the hydrogen cation by some substance having an appre- 

 ciable combining heat with both positive and negative plates. 

 Iodine and bromine in combination with chlorine suggested 

 themselves. The former was first tried, and one form of cell 

 produced would then be represented in the diagram (fig. 1), 

 where the relative attractions between the substances con- 

 cerned (as measured by their combining heats) are indicated 

 roughly by the number of the lines between each pair. The 

 numerical values of the combining heats (including heat of 

 aqueous solution) are also marked, that for copper and iodine 

 as cuprous iodide, since the cupric salt is unknown in the free 

 state. Fig. 1. 



Zinc. 



Copper. 



8. Iodine monochloride, IC1, a heavy dark liquid giving off 

 irritating fumes, was obtained from Hopkin and Williams, 

 Hatton Garden, who stated it to be fairly pure. It conducts 

 the current very well, as shown by the following experiment. 

 Two platinum wires were sealed side by side into a tube so 

 that their ends projected from its end at about half a centi- 

 metre apart. When connected in circuit with two Fuller 

 cells and a galvanometer they were immersed (with precautions 

 as to cleanliness) alternately in the chloride and in a 10-per- 

 cent, solution of sulphuric acid. The iodine chloride con- 

 ducted considerably better than the dilute acid. 



9. With zinc and platinum plates the chloride acted as an 

 electrolyte, forming a cell with an electromotive force on open 

 circuit of about 1J Daniell, and giving a current when the 

 circuit was closed. The zinc, however, became coated with 

 a solid black deposit (probably iodine) and a higher electro- 

 motive force was given by the chloride in solution. 



10. When iodine monochloride is added to water the fol- 

 lowing reaction takes place * : — 



20IC1 + 5H 2 = I 2 5 + 10HC1 IC1 + 4I 2 . 

 * Schutzenherger, Comptes Rendus, lxxxiv. p. 389. 



