182 



Dr. C. R. A. Wright and Mr. C. Thompson. [May 3, 



impressions made with this object in view alone. But by the method 

 here adopted, the same plates which are taken for ascertaining the 

 co-ordinates of the stars, serve equally well for measuring their photo- 

 graphic magnitudes. It is perhaps unnecessary to point out that 

 practically the photometric and photographic magnitudes are, for the 

 most part, identical. The remark above will fail of application, if it 

 be possible to determine differences of right ascension and of declina- 

 tion from the traces of the stars with sufficient accuracy. 



VI. " On the Development of Voltaic Electricity by Atmo- 

 spheric Oxidation." By 0. R. Alder Wright, D.Sc., F.R.S., 

 Lecturer on Chemistry and Physics, and C. Thompson, 

 F.I.C., F.C.S., Demonstrator of Chemistry, in St. Mary's 

 Hospital Medical School. Received April 17, 1888. 



In a preliminary note on this subject (' Roy. Soc. Proc.,' vol. 42, 

 p. 212), it has been shown that when copper is immersed in an 

 aqueous solution of ammonia and opposed to an " aeration plate " of 

 some conducting material not otherwise acted upon, lying horizontally 

 on the surface of the fluid, a current flows continuously through a 

 wire, &c, made to connect the two plates, the energy manifested by 

 which is due to the absorption of atmospheric oxygen by the aeration 

 plate and the indirect combination of this with the copper forming 

 cuprous oxide which dissolves in the ammonia. Numerous analogous 

 electromotor cells are readily obtainable by suitably varying the metal 

 susceptible of oxidation and the electrolytic fluid employed, some of 

 which we have submitted to close examination ; whilst another class 

 pf voltaic cells, acting on much the same principle, we find can be 

 obtained by substituting for the oxidisable metal a platinum or other 

 incorrodible plate immersed in an oxidisable fluid, such as pyrogallol 

 dissolved in caustic soda : preferably the aeration plate is arranged 

 in one vessel on the surface of some convenient fluid (not necessarily 

 identical with the oxidisable one), and the other plate and oxidisable 

 fluid placed in another vessel, the two being connected by a siphon 

 or wet wick ; or the whole may be arranged as a gravity battery, the 

 oxidisable fluid being made the heavier one so as to preserve it 

 from direct contact with air ; or a \J -tube arrangement may be 

 employed. Thus, for example, a platinum plate immersed in an acid 

 solution of ferrous sulphate, or in sulphurous acid solution, connected 

 with a vessel containing dilute sulphuric acid, and an aeration plate 

 of spongy platinum, &c, furnishes an electromotor cell in which tbe 

 production of a current is accompanied by the virtual transference 

 of oxygen from the aeration plate to the oxidisable fluid, forming 



