188 . HISTORY OF PHOTOGRAPHY. 



diately acquires an intense color, which shoiikl then be stopped 

 directly by plunging it into water sJ/f/htli/ acidulated with muriatic 

 acid, or the blackening will extend all over the paper. It may be 

 fixed by being soaked in water, and then dipped into a solution of 

 bj'posulphite of soda and again soaked in water as in the other 

 processes. 



Mr. Bingham has the following remarks on this process, and he 

 gives a modified form, into which a new photographic element is 

 introduced : 



We find it is better to add to the protosulpbate of iron a little acetic or 

 sulphuric acid ; tins will be found to prevent the darkening of the lights of the 

 picture to a great extent, and it will be found better not to prepare the paper 

 long before it is required for use, this being one reason why the picture often 

 becomes dusky on application of the protosulpbate. 



Reasoning upon the principle that the action of light is to reduce the salts of 

 silver in the paper to the metallic state, and that any substance which would re- 

 duce silver would also (juicken the action of light, we were led to the following 

 experiment : The protochloride of tin possesses the i)roperty of reducing the 

 salts both of silver and gold. A paper was prepared with the bromide of silver, 

 and previously to exposing it to light it was washed over with a very weak solu- 

 tion of the chloride of tin. The action of light iipon the paper was exceedingly 

 energetic; it was almost instantaneously blackened, and a copy of a print was 

 obtained in a few seconds. 



The use of fluorides has been recently introduced as a novelty by 

 some French })h()t()graphers, but reference to the author's Researches 

 on Light, published in 1844, Avill distir.ctly show that I w^as the first 

 to employ these salts as photographic agents. 



BROMIDE or SILVER AND ]MERCITRIAL VAPOR. 



In my first publication on this subject, in (iriffin's Scientific Mis- 

 cellany, I introduced the following process, which, although it has 

 never yet been properly worked out, involves many points of interest : 

 Some extremely curious results led me to examine the effect of the 

 mercurial vapor on the pure precipitated iodides and bromides. I 

 was long perplexed Avith exceedingly anomalous results, but, being sat- 

 isfied from particular experiments that these researches promised to 

 lead to the discovery of a sensitive pi'eparation, I persevered. 



To prepare this sensitive paper we proceed as follows: Select the 

 most perfect sheets of Avell-glazed satin post, quite free from specks 

 of any kind. Placing the sheet carefully on some hard body, wash it 

 over on one side by means of a very soft camel's-hair pencil with a 

 solution of 60 grains of the bromide, of potassium in 2 fluid ounces of 

 distilled water and then dry it quickly by the fire. Being dry, it is 

 again to be washed over with the same solution and dried as before. 

 Now, a solution of nitrate of silver, 1'20 grains to the fluid ounce of 

 distilled water, is to be applied over the same surface and the paper 



