180 



Capt. Abney. On the Effect of the [Dec. 15, 



Paper was next washed, and portions were treated with a solution 

 of potassium bromide, and again washed. Strips of these two speci- 

 mens were dried and exposed to the spectrum, and in both cases the 

 printing action is seen in fig. 48. Similar papers, in a moist state, 

 were also exposed without any deviation of the result. Again, paper 

 which had been prepared as above was allowed to dry with the excess 

 of silver nitrate on it, and exposed, and fig. 48 again approximately 

 resulted ; as also it did when the washed paper treated with potassium 

 nitrite was dried. 



The difference between curves 47 and 48 is very remarkable, and at 

 first sight might not seem to admit of explanation. A study of the 

 experiments described, however, affords a clue to the apparent incon- 

 gruity of the results. According to text-books on chemistry, bromine 

 will displace iodine in combination, whilst iodide displaces bromide. * 

 Later researches seem to modify the first statement to a certain extent. 

 Bromine will only displace a definite proportion of iodine when it is 

 in excess ; but for our purpose we may take the text-book statement 

 as practically correct. When the paper was exposed wet with either 

 silver nitrate or potassium nitrite (I may remark that other halogen 

 absorbents gave the same result) the iodine and bromine liberated by 

 the action of light would be at once absorbed by them ; in the one' 

 case silver iodide (or bromide) and silver iodate (or bromate) being 

 formed, and in the other potassium iodide (or bromide) ; so that each 

 of the two kinds of sensitive salt would have its full action. When 

 the paper was washed and exposed in a dry state the result would be 

 different, and the question would arise, what would become of the 

 iodine and bromine liberated by light ? 



If silver iodide be exposed to light and treated with a trace of 

 bromine, the sub-iodide combines with the bromine, and all trace of 

 the action of light is destroyed. Thus when the mixture of iodide and 

 bromide is exposed to light, both iodine and bromine being liberated, 

 the bromine will at once combine with the sub-iodide and destroy it. 

 Thus, 



Ag 2 I + Br=Ag 2 IBr, 

 the only factor remaining being the sub-bromide, which is develop- 

 able. Now it maybe said that the iodine liberated should also destroy 

 the sub-salts ; but it is a matter of fact that, in the presence of light, 

 it has no power of destroying the sub-iodide, since it is immediately 

 again shaken off from the molecule.* Iodine can destroy the sub- 

 bromide molecule, and form a new saturated molecule ; thus, 



Ag 2 Br+I=Ag 2 BrI. 



* [At the same time it must be noted that the iodide is much less sensitive of 

 light when no absorbent of iodine is present. This is fully accounted for by the 

 immediate recombination of, at all events, a portion of the iodine liberated with the 

 sub-iodide molecule. — Dec. 19.] 



