80 Intelligence and Miscellaneous Articles. 



sn 2 — , and thereby A, B, C, D, and the arbitrary point P of the 



rectangle fall upon points a, b, c, d, p, the electric potential of the 

 rectangle in the point P is 



V=SE t log(pa l4j p& t .pc t ._K) (y) 



Quincke * bases his experiments, on the potential in very large 

 square plates when the points of inflow are in the diagonal, upon 

 a formula of approximation which in our notation would be 



Y =2 E t log (PA t . PB t . PC t . PD t ). 

 It is now apparent, if this be compared with the exact formula (y), 

 that it results from the latter, rf snz may be supposed propor- 

 tional to z, therefore with very large rectangular plates — or, better, 

 under the supposition that P and the A's lie near an angular 

 point of the rectangle. The approximation-formula therefore holds 

 also when the rectangle is not a square and when the inflow-points 

 do not He on the diagonal. 



The derivation of these expressions I intend to communicate, in 

 a connected form, to Borchardt's Journal fur Maihetnatik. For 

 this reason I omit here the exhibition in a purely analytical form, 

 without the aid of geometry, of the relations expressed by (/3) and 

 (y). — Monatsbericht der Tconiglich preussischen AJcademie der Wis- 

 sensch. zu Berlin, March 5, 1874. 



ON THE NATURE OF THE ACTION OF LIGHT UPON SILVER BRO- 

 MIDE. BY M. CAREY LEA, PHILADELPHIA. 



When silver bromide is exposed for a moment to light, it under- 

 goes no visible change, but has acquired the property of passing to 

 an intense black when treated with pyrogallic acid and an alkali. 



As to the nature of this black substance, there has existed con- 

 siderable diversity of opinion. In a paper published on the subject 

 about a year since by Captain Abney, P.C.S., he expressed the 

 opinion that it was an oxide of silver. 



Some years since, while investigating the action of light upon 

 silver iodide, I succeeded in proving that the black substance 

 which is produced when silver iodide is exposed to light in presence 

 of silver nitrate contains iodine, and is therefore either a sub- 

 iodide or an oxy-iodide. The quantity obtained was too small to 

 enable me to ascertain which. Yfhen this black substance was 

 treated with nitric acid, normal yellow silver iodide was left behind, 

 and silver was found on solution. 



I have recently applied the same treatment to the bromine com- 

 pound with similar results. I find that when silver bromide is 

 treated with pyrogallic acid and alkali after exposure to light, the 

 black substance which remains contains bromine, and is resolved by 

 nitric acid into normal silver bromide (left behind as a pale yellow 

 film) and silver, which passes into solution. It is therefore either 

 a subbromide or an oxy-bromide, not an oxide, probably the former. 



The existence of these compounds is evidently an argument for 

 doubling the atomic weight of silver, as has recently been proposed 

 on other grounds. — Silliman's American Journal, May 1874. 

 * Pogg. Ann. voL xcvii. p. 382. 



