with the material of the latent Photographic Image. 485 
It.is a matter of interest that sodium hypophosphite which pro- 
duces the above described effects, has no developing power 
whatever. 
i: ue 
The two impressions, that formed by light and that by hypo- 
phosphite, are similarly affected by reagents 
As an example of this identity of effect produced on the two 
impressions, I first take the action of nitric acid. 
Chloride, bromide and iodide papers were exposed to moderate 
diffuse light under a screen with openings, for a proper time to 
form a latent image, the chloride and bromide for four or five 
seconds, the iodide for twen ty or twenty-five. They were then 
cut into halves and one half of each was soaked in strong nitric 
acid for five minutes. These halves were then washed for some 
hours and were developed along with the halves not so treated. 
Result was, latent image on silver chloride almost if not quite 
uninjured; on silver bromide, somewhat affected buz still 
strong ; on silver iodide entirely destroyed. 
Similar portions of the same papers were then marked with 
hypophosphite and were cut into halves and one-half was sub- 
jected to the action of nitric acid in exactly the same way as 
the previous. The result was exactly as before. The hypo- 
phosphite marks on the half of the chloride paper that had been 
treated with acid came out in development as vigorously as on 
the half that had not been treated. ‘The bromide paper showed 
the marks weakened by the acid but still strong; on the iodide 
not a trace appeared. In all respects the result was the saine, 
what the one resisted, the other resisted, what destroyed the 
one destroyed the other. 
Another confirmation is presented by the action of the alka- 
line baloids upon the latent image. 
The latent images produced by light on chloride, bromide 
and iodide of silver were all treated with cold and moderately 
strong solutions of potassic chloride, bromide and iodide for 
half an hour, and were then subjected to development. It was 
found that all three silver salts bore the action of potassic chlo- 
ride and bromide fairly well, the images were somewhat weak- 
ened but strong developments were obtained without difficulty. 
But when potassic iodide was applied the latent image was 
totally destroyed. 
The yisible effect of the alkaline haloids on the photosalts 
exactly corresponds with their action on the latent image. 
Potassic chloride and bromide, applied in moderately strong 
solution and cold have little effect, but potassic iodide quickly 
destroys them. 
Therefore the action as well of nitricacid as of KCl, KBr and 
