1887.] Voltaic Electricity by Atmospheric Oxidation. 



213 



the cupric oxide thus formed as the first product becoming subse- 

 quently reduced (in the absence of air) to cuprous o dde, thus — 



CuO + Cu = Cu 2 0. 



When a sheet of copper is kept out of direct contact with air by 

 heing immersed in ammonia solution, oxidation of the metal is 

 gradually effected by virtue of the dissolving of oxygen from the air 

 at the surface of the fluid, and diffusion of the oxygen solution to the 

 vicinity of the copper. This action is an extremely slow one if the 

 copper be covered by some depth of fluid, and if the setting up of con- 

 vection currents through heating or evaporation be prevented by 

 keeping the vessel perfectly at rest and at an equable temperature, and 

 well closed to prevent escape of ammonia ; but if these precautions be 

 neglected it goes on much more rapidly, and the liquid compara- 

 tively soon becomes blue ; it can, however, be also materially acce- 

 lerated by arranging horizontally on the surface of the fluid a plate of 

 platinum or other electrically conducting material not chemically acted 

 upon by the fluid, and connecting this by means of a wire, &c, with the 

 copper plate. The upper conductor, or aeration 'plate as it may be con- 

 veniently termed, being simultaneously in contact with the atmosphere 

 and fluid, attracts to its surface a film or aura of condensed gases, the 

 oxygen of which becomes gradually transferred to the copper, a voltaic 

 current circulating through the fluid and connecting wire. Cuprous, 

 and not cupric, oxide thus results, dissolved in the ammonia 

 solution in contact with the copper plate, the mechanism of the 

 reaction being conveniently represented by the scheme — 



f Copper plate Cu 3 | OH 2 | OH 2 | Aeration plate, 



L Copper plate | Cu 2 | H 2 | H 2 | Aeration plate, 



water being represented as the electrolyte for simplicity's sake. The 

 air film on the aeration plate being constantly renewed by absorption 

 from the atmosphere, the process goes on continuously as long as the 

 two plates are connected together by the wire. This wire may be 

 lengthened at will so as to make the current which passes through it 

 whilst the action goes on relatively stronger or weaker according to 

 the amount of resistance introduced into the circuit ; and by includ- 

 ing a galvanometer or silver voltameter in the circuit the ordinary 

 phenomena due to the passage of currents are readily recognisable. 



A voltaic cell thus produced " runs down " very rapidly when the 

 resistance in circuit is diminished, more or less recovering when the 

 resistance is again increased ; with a large resistance (e.g., sufficient to 

 reduce the current density to a micro-ampere or less per square 

 centimetre of aeration plate surface), a very notable E.M.F. is main- 



