1881.] Spectrum on the Haloid Salts of Silver, fyc. 179 



be definite proportions between each, supposing that each salt was 

 entirely decomposed by the silver nitrate. Unfortunately this is never 

 absolutely the case, and hence the results obtained with the paper 

 must be received with some caution. Chemists know that silver 

 bromide or silver chloride cannot exist in the presence of a soluble 

 iodide, nor can silver chloride in presence of a soluble bromide. 

 Hence when we have an iodide and bromide impregnating paper, the 

 silver iodide will first be formed, and then the bromide ; or, again, with 

 iodide and chloride, the silver iodide will first be formed and then the 

 chloride ; and, finally, with bromide and chloride, the bromide will 

 first be formed and then the chloride. 



It was necessary to make these remarks, as a right conception of the 

 results might not be taken on casually looking at them. 



The same remarks apply with equal force when a sensitive film of 

 the double salts is prepared by the ordinary silver bath when very 

 short immersion is given to the plate. The only true way of obtaining 

 definite results seems to be by means of separate emulsions, in which 

 a definite amount of soluble chloride, bromide, or iodide is fully con- 

 verted into silver chloride, bromide, or iodide, and then to mix these 

 emulsions, after proper washing, in the required proportions. It was 

 in this manner that the emulsions which will be discussed presently 

 were prepared. 



I would here call attention to a somewhat remarkable behaviour of 

 silver iodide. It is well known that if silver iodide be prepared with 

 ■an excess of soluble iodide, it is totally insensitive to light. Thus 

 if we prepare (say) an emulsion in collodion with an excess of 

 iodide, and wash it thoroughly in the usual manner, and after redis- 

 solving the pellicle resulting from the washing operations, expose it in 

 the camera, no amount of development will bring out an image. If, 

 however, to such an emulsion but a drop of a bromide or chloride 

 emulsion be added sensitiveness will appear. This seems to be due 

 to the last trace of soluble iodide being converted into silver iodide. 



Mixtures of Silver Iodide and Bromide. 



Equal Equivalent Proportions of Iodide and Bromide. — Paper was 

 soaked in a solution of equivalent proportions of soluble iodides and 

 bromides, and, after drying, was sensitised on a 10 per cent, solution 

 of silver nitrate for such a time that the back of the paper became 

 thoroughly damp. The silver nitrate solution was acidified in order 

 to prevent the formation of any sub -salts. 



A strip of such paper was exposed to the spectrum whilst moist, 

 and the printing actio a noted. The result is given in fig. 47. Similar 

 paper was washed and treated with potassium nitrite, and exposed 

 whilst moist ; the effect of the action of the spectrum is seen in the 

 same figure. 



