300 



Mr. J. Brown. 



[Nov. 18, 



of the hydrogen sulphide was found still present in it, and the metals 

 were much corroded ; the copper brown with lighter patches ; the 

 iron greyish with patches also, but having a thinner coating of sul- 

 phide than the copper. On allowing the metals to remain exposed to 

 the air, the iron side became again gradually more positive; the 

 difference of potential rising during 26 hours to 0"26D, after which 

 it fell in about a week to 0*22D. 



13. As the electrochemical relation of silver and iron in water is 

 also reversed when solution of potassium sulphide is added, I had 

 little doubt that an addition of hydrogen sulphide to the air about 

 these metals when in contact would reverse the potentials near them. 



To test this the upper surface of the copper quadrant used in the 

 last experiment was covered with a piece of thin silver sheet cemented 

 on, thus forming a silver-iron pair with the iron quadrant. The 

 difference of potential just after cleaning the metals with emery- and 

 glass-paper was 023D, iron side positive. In three minutes after 

 admission of the gas the next reading showed the silver side positive; 

 difference about 0*4D. The following readings were then taken, 

 t as before the time after admission of the gas. 



t. 12 m. 16m. lh.Om. 23 h. 40m. 33 L. 30 m. 58 h. 40m. 

 P/D. 042 044 043 038 035 035 



84h. 20m. 94h. 1 

 0365 0-38 



The sulphide formed on the inside of the metal case of the instru- 

 ment by the action of the* gas, now began to peel off in scales and 

 jammed the vanes of the weight. After cleaning these out fresh gas 

 was sent in, and P/D became 0\36D ; silver side still positive. 

 Measurements were then made at intervals, the difference of potential 

 falling gradually to 0*23D, on the fourteenth day after admission of 

 the gas at the outset. 



By that time, however, the gas was found to have been all or 

 nearly all absorbed by combination with the metals and the sides of 

 the instrument. The conclusion of the experiment is therefore in- 

 complete, but it indicates that the difference of potential decreases 

 with these metals also, although more slowly than with copper and 

 iron . 



14. It was suggested by Mr. Cross, of Barrow-in-Furness, that the 

 difference of potential near a copper-iron pair in air and in ammonia 

 gas should be compared. The electromotive force of a cell formed by 

 these metals immersed in distilled water is about 0'35D, copper the 

 positive pole, but if ammonia solution be added, this electromotive 

 force reverses and becomes - 27D. 



Three experiments were made with copper-iron quadrants in this 

 gas, which was sent into the instrument by boiling strong liquor; 



