in Mercury by Deoxidation. 489 



the superficial suboxide by the current, producing a cleaner 

 metallic surface, decreases the adhesion of the water, and this 

 causes the drop to contract ; and in the other direction, that 

 the current oxidizing the surface increases the adhesion of the 

 water, which spreads out in consequence. The second explana- 

 tion is that suggested by Dr. J. W. Draper, in 1845, in a 

 paper * entitled " Is Capillary Attraction an Electrical Pheno- 

 menon ? " According to his view, the capillary constant at 

 the contact-surface of mercury and dilute sulphuric acid is 

 supposed to be altered by the current; and hence the contraction 

 or expansion, the surface of mercury behaving like an elastic 

 membrane. It appears to me that both adhesion and capilla- 

 rity play very insignificant parts in the phenomena in question. 

 The explanation which I oifer is that these contractions and 

 expansions are due simply to the different atomic volumes of 

 mercury and its suboxide. 



If small particles of dry solid matter be placed upon the 

 bright surface of the mercury just outside the water-drop, 

 before connecting the battery, their distances from the peri- 

 phery of the drop remain practically unchanged both when the 

 drop contracts and when it expands. From this it is obvious 

 that the expansion of the water-drop is not due to its over- 

 running fresh mercury surface, which would correspond with 

 an increased adhesion, but simply to the expansion of the mer- 

 curial area originally covered by the water — and that the con- 

 traction is not a retiring from the mercury surface, such as 

 would result from diminished adhesion, but a contraction of 

 the covered surface. In other words, the water is the index 

 of the contraction or expansion of a certain area of the mer- 

 cury which is undergoing some chemical change. When the 

 water-drop is placed upon the mercury surface, that surface is 

 already (or immediately afterwards becomes) converted into 

 suboxide. When the drop is connected with the battery so 

 tha£ the suboxide is reduced, the base of the drop becomes 

 metallic and necessarily contracts to an area corresponding 

 with the reduced atomic volume of the molecules which form 

 the surface. On the other hand, when the battery-current is 

 broken, the metallic mercury surface, sensitive to oxidation, is 

 ready to return at once to its state of suboxide and resumes its 

 original dimensions. And, finally, when the battery-current 

 is reversed so that the mercurial surface underneath the drop 

 is still further oxidized, the drop will go on expanding in con- 

 sequence of the formation of new oxide underneath that first 

 formed, the oxide being porous and allowing the water to per- 

 colate through it. 



* Draper, Phil. Mag. May 1845, p. 185. 



