1881.] Spectrum on the Haloid Salts of Silver, tyc. 171 



sium or other soluble iodide, the silver compound is at once decom- 

 posed, and silver iodide formed in its place. If, then, a film of 

 iodide of silver in collodion (whether prepared from an emulsion or 

 by the bath process) be washed from silver nitrate, and be then im- 

 mersed in a weak solution of potassium iodide (it must not be strong 

 or it will dissolve out the silver iodide from the film) or other soluble 

 iodide, it may be seen that there will be nothing but silver iodide in 

 the film, all impurities being decomposed. If the film be washed well 

 with distilled water, and again immersed in the bath, or flowed over 

 with some sensitiser, such as potassium nitrite, sodium sulphite, beer, 

 pyrogallic acid, &c, it may be exposed with the certainty that only 

 pure silver iodide is under examination. It was necessary to make 

 these remarks, since the whole of the utility of the research depends 

 on the use of the pure substance, the collodion being absolutely 

 inert as regards the silver salt. The silver iodide emulsion made from 

 the purified potassium iodide proved to contain nothing but the pure 

 iodide, but that prepared with the cadmium and other iodides, as will 

 be seen, proved untrustworthy as to purity. It was owing to this 

 that I was led into a mistake in a paper which appeared in the " Pro- 

 ceedings of the Royal Society," wherein I stated that owing to the 

 oxygen-absorbing properties of potassium nitrite, I was able to 

 obtain an image lower than ordinary. It seems now that this may 

 have been due to a contamination of bromide or chloride, or to the 

 formation of silver nitrate, any of which would have given me the 

 same results. 



One word also as to the neutral or alkaline developer employed. 

 It has been customary to state that silver iodide is unamenable to 

 alkaline development. This is, however, not the case. The ferrous 

 oxalate and the ferrous citro-oxalate bring out a distinct image, as 

 does pyrogallic acid and ammonia, when no restraining iodide is 

 employed. In all dry plates prepared with the iodide and other silver 

 haloids, the iodide is developable (though it gives a weakly image 

 compared with that due to other salts) by the alkaline or organic 

 iron developer. 



A plate was coated with cadmium iodised collodion, and placed in 

 the bath for a couple of minutes, and exposed to the spectrum. The 

 top half of the slit was uncovered for one second, the bottom half for 

 ten seconds; the results are seen in figs. 14 and 15. The develop- 

 ment took place by the acid developer. Plates similarly prepared 

 and washed, and then, similarly exposed, also gave as results figs. 

 14 and 15. When using ferrous oxalate, the cadmium emulsion also 

 gave the same result. Plates coated with a film of the same collodion, 

 washed, and then immersed in a weak solution of potassium iodide 

 or cadmium iodide, again washed clean with distilled water, and 

 finally treated with silver nitrate, beer, pyrogallic acid, potassium 



