THE PHOTOGRAPHIC IMAGE. 381 



Of all the substances kuown to chemistry at the present time, the 

 salts of silver are by far the most imj)ortant in photography on account 

 of the extraordinary* degree of sensitiveness to which they can be 

 raised. The i)liotographic image with which it is my privilege to deal 

 on this occasion is that invisible impression jjroduced by the action of 

 light on a tihn of a silver haloid. Many methods of producing such 

 films have been in practical use since the foundation of the art in 1839. 

 All these depend on the double decomposition between a soluable chlo- 

 ride, bromide, or iodide, and silver nitrate, resulting in the formation 

 of the silver haloid in a vehicle of some kind, such as albumen (Niepce 

 de St. Victor, 1848), or collodion on glass, as made practicable by Scott 

 Archer in 1851. For 20 years this collodion process was in univ^ersal 

 use; Its history and details of manipulation, its development into a 

 dry plate process by Colonel Russell in 1801, and into an emulsion 

 process by Bolton and Sayce in 1864, are facts familiar to every one. 



The photographic film of the present time is a gehitinohaloid (gen- 

 erally bromide) emulsion. If a solution of silver nitrate is added to a 

 solution of potassium bromide and the mixture well shaken, the silver 

 bromide coagulates and rapidly subsides to the bottom of the liquid as 

 a dense curdy i)recipitate. [Shown.] If instead of water we use a 

 viscid medium, such as gelatine solution, the bromide does not settle 

 down, but foruis an emulsion, which becomes quite homogeneous on 

 agitation. [Shown.] This operation, omitting all details of ripening, 

 washing, etc., as well known to practical photographers, is the basis of 

 all the recent photographic methods of obtaining negatives in the 

 camera. The use of this invaluable vehicle, gelatine, was practically 

 introduced by K. L. Maddox in 1871, previous experiments in the same 

 direction having been made by Gaudiu (1853-Gl). Such a gelatiuo- 

 bromide emulsion can be S{)read uniformly over any sub-stratum— glass, 

 pai)er, gelatine, or celluloid — and when dry gives a highly sensitive 

 film. 



The fundamental problem which 50 years' experience with silver haloid 

 films has left in the. hands of chemists is that of the nature of the chemical 

 change which occurs when a ray of light falls on such a silver salt. Long 

 before tiiedaysof photogra[)]iy, far back iu the sixteenth century, Fabri- 

 cius, the alchemist, noticed that native horn silver became colored when 

 brought from the niinc and ex|>()S('d. The fact presented itself to Ttobert 

 Boyle in the seventeenth century, and to Beccarius, of Turin, in the 

 eighteenth century. The change of color undergone by the chloride 

 was first shown to be associated with chemical dec()m[)ositiou in 1777 

 by Scheele, who proved that chlorine was given off when this salt dark- 

 ened under water. I can show you this in a form which admits of its 

 being seen by all. [Potassium iodide and starch paper were placed in 

 a glass cell with silver chloride, aiul the arrangement expose<l to the 

 electric light till the paper had become blue.] The gas which is given 



