1891.] with thio-carh amides. 69 



acid radicals, with the hydroxyl go to the unexposed parts forming* the 

 positive pole. 



On the other hand, with the alkaline thio- carbamide developers, at 

 the same time that silver is reduced on the exposed parts, silver 

 sulphide is formed on the unexposed parts which then become the ne- 

 gative pole and attract the sulphur, the hydrogen and some of the 

 silver from the exposed parts ; while the halogen and hydroxyl pass to 

 the positive pole and transform part of the remaining silver into silver 

 haloid which is dissolved in the fixing bath. 



Although this theory, as stated in a rough way, seems to agree 

 fairly well with the facts, and from the experiments I have made seems 

 probably the correct explanation of the reversals, I do not feel myself 

 yet able to put it forward authoritatively, and a great deal of further 

 investigation is required. The subject is a very difficult one beset 

 with uncertainties, and requires more time and close attention than I 

 have been able to give for working it out fully. 



The theory that photographic action is accompanied by electrical 

 phenomena is no new one. Becquerel found that if silver plates coated 

 with silver bromide or other haloid silver salts were electrically con- 

 nected and immersed in dilute acid, and light was allowed to fall upon 

 one plate while the other was screened, the effect of the light was 

 marked by distinct electrical action capable of deflecting the needle of a 

 galvanometer. Prof. Minchin has recently found that the same effect 

 is produced on silver plates coated with silver haloids in emulsions 

 of gelatine or collodion, and immersed in very dilute solutions of alka- 

 line bromides, iodides or chlorides. He has also found that if silver 

 plates coated with a silver bromide emulsion in gelatine are attached to 

 the poles of a battery, and half immersed in a weak solution of potas- 

 sium bromide, the film attached to the carbon pole was visibly black- 

 ened on its immersed part, while no visible effect was produced on 

 the other, but on developing this plate with pyrogallic acid and am- 

 monia, its immersed part also became dark, exactly as if it had been 

 exposed to light for a few seconds. 



There is a good deal of other more or less direct evidence that an 

 electrolytic action takes place during development, although so far as 

 I can ascertain no observations of the currents with a galvanometer 

 are recorded. Comparatively little attention has, however, been given 

 to the question, and electrical or electrochemical action has never been 

 looked upon as a principal factor in the production of the photographic 

 image. From these new results it would appear that, at any rate as 

 regards the silver haloids, the formation and development of the pho- 

 tographic image is to a very great extent influenced by electrical action^ 



