1881.] Spectrum, on the Haloid Salts of Silver, SfC 181 



Whether the two molecules Ag 2 BrI and Ag 2 IBr have the same value 

 is a moot point, but the evidence tends to show that such is the case. 

 If the equivalents of bromide and iodide were equal, that is, if the 

 bromide and iodide of silver were equally distributed, supposing both 

 the above actions took place, the locality of the spectrum in which 

 the iodide and bromide are equally sensitive should show an almost 

 entire destruction of a developable image, and also of a printed 

 image. 



This locality is doubtless about G, and when we come to analyse the 

 curve in fig. 48 we see that there is very small effect about G, whilst 

 there is an increased effect between G and F. Now, to test the matter 

 further, paper prepared with washed silver bromide was exposed 

 to light till it darkened thoroughly, and such paper was treated 

 with a very dilute solution of iodine, and then exposed in the 

 spectrum, with the result given in fig. £4, in which it will be seen that 

 the new molecule is more sensitive to the green between G and F than 

 above G ; in fact, we have very little action comparatively at G and 

 above it. In this case we have then a paper prepared in which there 

 is an absolute imitation of the action that takes place in the mixed 

 iodide and bromide. It cannot be said that by this treatment we have 

 Ag 2 I 2 + Ag 2 Br 3 , since the molecule formed by light is Ag 2 Br, and the 

 addition of the iodine is simply to form Ag 2 BrI, which is very different 

 from a simple mixture. This experiment then 'seems to show that 

 this new molecule is more sensitive to the blue-green than it is to the 

 violet. The point then comes as to how, when the original paper is 

 exposed to the spectrum, we have not only a fall of sensitiveness at 

 G and beyond it, but also a greater sensitiveness in the green. Now, 

 silver iodide, as has already been shown, is not in the least sensitive 

 to beyond a very small region below G ; therefore, in the green the 

 only component of the mixture of bromide and iodide that can be 

 acted upon is the bromide. As we see when bromide is acted upon 

 one atom of bromine is liberated from the molecule ; thus, 



Ag 2 Br 2 =:Ag 2 Br + Br. 



The liberated atom of bromine immediately attacks the molecule of 

 iodide in its immediate neighbourhood and forms a new bromo-iodide 

 molecule liberating iodine. Thus 



Br + Ag 2 I 2 = Ag 2 IBr + 1, 



and the iodine either escapes or else forms the molecule Ag 2 BiT ; thus 



Ag 2 Br + I=Ag 2 BrI. 



Here then it is probable that we have a new saturated molecule 

 formed by the action of light, which on formation is susceptible of 

 being acted on by light in its turn. Whether iodine or bromine is 



