486 IMr. R, Sabine on Electricit)/ disengaged 



the mercury was caused to rise to the top of the funnel, when 

 an excursion to the loft was again observed*. 



A series of observations were then made, taking the readings 

 of the steady deflections when the mercury was at the top and 

 at the bottom of the funnel alternately, the difference between 

 two consecutive readings being proportional to the increase or 

 decrease of electromotive force between the surfaces. 



Change of Deflection after the mercury surface, /, had 

 been 



Increased. 



Decreased 





172 divisions. 



— 227 divisions 



178 „ 



-198 



V 



136 „ 



-162 



)) 



137 „ 



-123 



j; 



140 „ 



-115 



)7 



116 „ 



-130 



j? 



143 „ 



-121 



j? 



178 „ 



-132 



j? 



163 „ 



-171 



?J 



The mean of the two series represented a difference of elec- 

 tromotive force equivalent to 0*115 volt. When a small 

 quantity of sulphuric acid was added to the water, similar re- 

 sults were obtained. When the acid water was strong, the 

 effect was much sooner over. This was probably due to the 

 combination of the acid with the mercurous oxide as the latter 

 was formed. 



It was observed that the change in the electromotive force 

 took place more rapidly while the mercury surface was ex- 

 panding than while it was contracting. While expanding, 

 the light-spot invariably made an excursion far beyond the 

 point to w T hich it finally settled, whereas while the mercury 

 was being contracted the light moved steadily up to its posi- 

 tion of rest and swung only very few degrees beyond it, — indi- 

 cating that the film of suboxide, while contracting, underwent 

 a tolerably regular thickening ; but when expanding, the film, 

 not being elastic, split and suddenly exposed a surface of me- 

 tallic mercury. The expanded or disturbed surface was always 

 positive to the still surface ; that is, it took the place of the 

 zinc in a voltaic couple f . 



* If instead of letting the mercury rise from the bottom its surface was 

 scraped, a deflection to the left was observed. 



t This effect is the reverse of that said to be observed when the mercury 

 surfaces are polarized with hydrogen. Mr. "Varley says, " During this 

 change of surface-dimensions [of mercury previously hydrogenized] cur- 

 rents of electricity will be found to pass from one to the other, the dimi- 

 nishing surface acting as the zinc plate, and the increasing surface as the 

 copper plate of a voltaic couple." 



