138 Professor Baphael Meldola [May 16, 



the action of light on a film 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 soluble chloride, bromide, or iodide, and silver nitrate, 

 resulting in the formation of the silver haloid in a vehicle of some 

 kind, such as albumen (Nicpce de St. Victor, 1848). or collodion on 

 glass, as made practicable by Scott Archer in 1851. For twenty 

 years this collodion process was in universal use ; its history and 

 details of manipulation, its development into a dry plate process by 

 Colonel Russell in 1861, and into an emulsion process by Bolton 

 and Sajce in 1864, are facts familiar to every one. 



The photographic film of the present time is a gelatino-haloid 

 (generally 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 precipitate. [Shown.] If instead of 

 water we use a viscid medium, such as gelatine solution, the bromide 

 does not settle down, but forms an emulsion, which becomes quite 

 homogeneous on agitation. [Shown.] This operation, omitting all 

 details of ripening, washing, &c., as well known to practical photo- 

 graphers, 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 R. L. Maddox in 

 1871, previous experiments in the same direction having been made 

 by Gaudin (1853-61). Such a gelatino-bromide emulsion can be 

 spread uniformly over any substratum — glass, paper, gelatine, or 

 celluloid — and when dry, gives a highly sensitive film. 



The fundamental problem which fifty 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 the days of photography — far back in the 

 sixteenth century — Fabricius, the alchemist, noticed that native horn 

 silver became coloured when brought from the mine and exposed. 

 The fact presented itself to Robert Boyle in the seventeenth century, 

 and to Beccarius, of Turin, in the eighteenth century. The change 

 of colour undergone by the chloride was first shown to be associated 

 with chemical decomposition in 1777, by Scheele, who proved that 

 chlorine was given off when this salt darkened under water. I can 

 show you this in a form which admits cf its being seen by all. 

 [Potassium iodide and starch paper were placed in a glass cell with 

 silver chloride, and the arrangement exposed to the electric light till 

 the paper had become blue.] The gas which is given off under these 

 circumstances is either the free halogen or an oxide or acid of the 

 halogen, according to the quantity of moisture present and the 

 intensity of the light. I have found that the bromide aftects the 

 iodide and starch paper in the same way, but silver iodide does not 

 give off any gas which colours the test paper. All the silver haloids 

 become coloured on exposure to light, the change being most marked 



