14 LECTURES TO SCIENCE TEACHERS. 



in its place we applied some iodine absorbent (after washing] 

 which is less crystallizable, such as tannin (as we did in our 

 last experiment), salicin, tea, coffee, &c., it will again be sensi- 

 tive even when dry. The application of such bodies is the 

 foundation of any dry plate process in which the iodide plays 

 a part. At the same time these absorbents are given to the 

 iodide, there are usually added compounds which soak into 

 the film and keep the pores of the collodion open. After 

 washing the exposed film these of course dissolve away, and 

 allow free access to the developer. The same procedure holds 

 good when both bromides and iodides are present in the sen- 

 sitive film, and we may develop with iron or pyrogallic acid 

 and silver, as in the wet collodion process, building up the 

 image on the irritated haloids. 



When silver bromide is present alone, or in conjunction 

 with the iodide, however, another method of development 

 may be resorted to, viz., one that is primarily independent of 

 the deposition of metallic silver. 



Pyrogallic acid has an affinity for bromine as well as for 

 oxygen, and the affinity for both is multiplied many fold 

 by the addition of an alkali to it. Pyrogallic acid and an 

 alkali such as ammonia (say) form the pyrogallate of the 

 alkali. When such a solution is poured over the exposed 

 bromide film it meets with molecules of the sub-bromide 

 (Ag 2 Br) which appear to be in an unstable state, and ready 

 to part with any of their atoms. The pyrogallate speedily 

 separates the remaining atoms of bromine from them, and 

 leaves the metallic silver behind. When ammonia is 

 the alkali employed with the pyrogallic acid, we have a 

 further action. Over this glass plate, on which is a layer 

 of silver bromide in collodion, I allow a few drops of 

 strong ammonia to trickle. Notice the transparent track they 

 leave. From this we learn that the bromide is soluble in 

 ammonia. Hence, whilst the pyrogallate is still acting in 

 the manner indicated, if there be sufficient ammonia present 

 it dissolves a portion of the silver bromide ; this the pyro- 

 gallate decomposes, and causes the metal to deposit as in 

 the ordinary development. By this means a greater density 

 is given to the image than would otherwise exist. There 

 is a further density given by a somewhat curious action 

 (apparently catalectic) the cause of which I have not time to 

 explain. 



