68 Col. Wafcerliouso — Blectro- chemical Reversals [April, 



I have also found that reversals of the reduction products, some- 

 what 'similar to those obtained by photograjDhic methods, may be 

 obtained entirely v^ithout the agency of light by passing a current from 

 a sino-le bichromate cell through a pair of silver plates coated with 

 silver bromide and immersed in eikonogen developers prepared with or 

 without thio-carbamides. In this case the plate attached to the carbon 

 pole in the plain developer showed only a very little black deposit, 

 whilst the plate attached to the zinc pole showed a very strong dark 

 deposit all over. A pair of similar plates immersed in some of the same 

 developer to which a few drops of a solution of thio-sinamine had been 

 added showed quite different results, the plate attached to the carbon pole 

 showino* a strong black deposit, while the plate attached to the zinc 

 pole was almost clear on the face and free from deposit, showing only 

 a slio-ht tarnish, caused by sulphur. I have some plates of this kind 

 here, though the reversed effect is not quite so strong as it was on my 

 first plates. I have found it difficult to obtain such marked reversals 

 a2:ain though I quite believe they are obtainable and the best conditions 

 for securing successful results have yet to be ascertained. 



Some similar effects were produced on Eastman's bromide paper 

 and on ordinary dry plate films attached to the silver plates. 



Although results obtained with silver bromide on silver plates are 

 not quite comparable with those obtained with ordinary gelatine plates, 

 these experiments show that under favourable circumstances the action 

 of developing solutions on silver bromide is accompanied by distinct 

 electric action, and that these thio- carbamide reversals may be produced 

 by electrical methods and are attended by a reversal of current. How 

 this reversal of current is brought about is not yet quite clear, but 

 seems to be explained by some observations on metallic sulphides, by 

 W. Skey, recorded in Yol. XXIII of the Chemical News. He found 

 that sulphides which have the power of conducting can also generate 

 electricity and that silver sulphide is positive to metallic silver. In a 

 battery consisting of a sulphide and a metal in acidulated water, the 

 gas liberated is sulphuretted hydrogen, the nascent hydrogen exerting 

 a desulphurising action upon the metallic sulphide, the ultimate effect 

 of which is in some cases to completely reduce the mineral to the 

 metallic state. He shows also that these sulphides are capable of 

 performing the functions of the negative element of a galvanic couple. 



It seems probable, therefore, if electrolytic action does take place 

 in gelatine films during the process of photographic development, 

 that, according to the laws of electrolysis, with the ordinary developers 

 the exposed parts of the plate form the negative pole and attract the 

 metallic elements and hydrogen, while the bromine, or other halogen and 



