194 Dr. C. R. A. Wright on the Determination of 



such as the reduction of ferric to ferrous salts, &c. One of 

 the most sensitive was found to be that used by Osann (Pogg. 

 Ann, xcv. pp. 311 and 315), viz. washing the electrode 

 (in recently boiled just cool distilled water), and then im- 

 mersing it in a clear brown-yellow solution of mixed ferric 

 chloride and potassium ferricyanide. In afew minutes, according 

 to the quantity of hydrogen occluded, a distinct blue preci- 

 pitate forms on the surface of the platinum, due either to the 

 conversion of ferri- into ferrocyanide, or to the reduction of 

 ferric to ferrous chloride, or both together. In this way the 

 presence of hydrogen can be distinguished in a platinum 

 electrode (ignited in oxygen previously to use in the volta- 

 meter), even when no visible evolution of hydrogen gas 

 from its surface has occurred, the E.M.F. of the battery used 

 being too small to admit of a steady current passing at a rate 

 more rapid than that representing the rate at which removal 

 of the aura of condensed gas by the effects of diffusion (§ 70) 

 takes place — for instance, when the battery-E.M.F. is not 

 greater than 1*4 to 1*5 volt (Part iv. § 84 et seq.). In 

 applying this test, however, too much reliance must not be 

 placed on the formation of a blue precipitate after the lapse of 

 a long time ; for organic matters and reducing gases and 

 vapours from the air are apt to be absorbed by the ferric 

 ferrocyanide liquor, causing reduction, especially under the in- 

 fluence of light. Moreover even platinum recently ignited 

 and cooled in oxygen, when placed in the solution, becomes 

 covered with a thin blue film after a considerable time (some 

 hours or clays), suggesting the possible reduction of ferri- 

 cyanide to ferrocyanide with simultaneous production of 

 platinocyanide. Gold acts in this way much more rapidly 

 than platinum. In applying this test in the above-cited ob- 

 servations, check experiments were always made with a second 

 piece of platinum foil of about the same size recently ignited : 

 the blue deposit on the foil used as electrode was then found 

 to be notable or considerable when none at all was visible on 

 the check piece. 



76. From the general theory of electrolysis and the ex- 

 periments above described, and the much more numerous 

 analogous observations made but not described in detail for 

 the sake of saving space, it results that the condition of the 

 aura round each electrode of a voltameter that has been used 

 for decomposing water at a given period since rupturing the 

 original current is influenced by many circumstances. The 

 aura round the — electrode, for example, has a particular 

 mean density at the moment of breaking circuit, whilst that 

 round the + electrode has some other mean density, the 



