sW 



vn in 



the 



accompa- 



^S- 



of the Photographic Image, 4 7 



usually been assumed that their sole function is to reduce to the 

 metallic state the particles of silver bromide which have been 

 acted upon by light. 



The alkaline developer consists of pyrogallic acid or other 

 oxygen-absorber, an alkali such as ammonium hydrate, and a 

 restrainer such as potassium bromide. These are generally 

 mixed together and applied to the film on which has been im- 

 pressed an invisible image in the camera. Those parts acted 

 upon by light darken under the influence of the solution; whilst, 

 if the surface be in a proper condition and the proportion of 

 the restrainer to the alkali be well balanced, the portions un- 

 acted upon by light remain unchanged. The image thus 

 formed is soluble in nitric acid ; and further tests show it to be 

 metallic silver. In order to discover the part which the alka- 

 line developer played to cause this reduction, a large series of 

 experiments have been conducted in the laboratory of which I 

 have charge ; and the results appear sufficiently interesting to 

 be published. 



Bulbs were made of the shape 

 nying figure. In A was placed, 

 whilst carefully excluded from all 

 light, thoroughly washed silver 

 bromide and pyrogallic acid. In 

 B was placed a solution of potas- 

 sium hydroxide or other alkali ex- 

 cept ammonium hydrate. If ammo- 

 nium hydrate were required, the bulb 

 B was made double ; in the first was 

 placed ammonium nitrate and in the 

 other potassium hydroxide, calcu- 

 lated so that the ammonium salt 

 should always be in excess when 



the potassium was brought into contact with it. The tube C was 

 attached to a Sprengel pump ; and when the apparatus was 

 exhausted, C was sealed, and the whole of the alkali caused 

 to enter A. After from 2 to 60 hours the bulbs were broken, 

 and the solids and liquid kept for analysis. As far as can be 

 judged, the silver bromide was instantaneously attacked by 

 the alkaline pyrogallate. (There is no action due to pyro- 

 gallic acid on the silver bromide without the presence of an 

 alkali, as was proved by keeping them in contact for weeks and 

 noting their appearance.) After a large number of experi- 

 ments, it was found that the amount of silver bromide capable 

 of reduction was primarily dependent on the amount of pyro- 

 gallic acid present, and in a secondary degree on the amount 

 of alkali present. Thus 1 equivalent of pyrogallic acid can 



f 



