﻿Characteristic Curve of a Photographic Emulsion. 353 



further, and showed it possible to observe the silver reduced 

 by the developer only around certain centres in the grain, 

 and a recent paper of Svedberg's (Phot. Jour. April 1922) 

 leaves little room for doubt that the possibility of a grain 

 being made developable depends on the existence in it of 

 some kind of reduction centre. 



Opinion as to the nature of these centres seems at present 

 to be divided. There are those who assert that they are 

 formed by the light-action, and that they do not exist before 

 exposure is made. Such, for example, is the case if the 

 centre is really a molecule of silver halide which has lost an 

 electron, as is believed by H. S. Allen (Phot. Journ. 1914, 

 liv. p. 175). On the other hand, there are those who believe 

 that the centres are actual particles other than silver halide 

 formed in the grains during precipitation and subsequent 

 ripening, and that these only become susceptible to the action 

 of developer after exposure to light. 



There certainly is considerable evidence to show that 

 silver halide is not the only substance in the grains. Luppo- 

 Oramer (Kolloidchemie una 1 Photographie) was led, as a result 

 of his work, to the conclusion that, at any rate in the most 

 sensitive emulsions, nuclei are present which probably consist 

 of a colloidal solution of silver in the halide. Renwick 

 (J. S. C. I. 1920, xxxix. No. 12, 156 T.) extends this idea, 

 and says : " In our most highly sensitive photographic plates 

 we are dealing with crystalline silver bromide in which, 

 besides gelatin, some highly unstable form of colloidal silver 

 exists in solid solution, and it is this dissolved silver which 

 first undergoes change on exposure to light." These silver 

 particles are negatively charged, and Renwick believes that 

 the action of light is to discharge, and hence to coagulate 

 into larger groups, those particles of colloidal silver which 

 existed in the grain before exposure ; it is these groups of 

 coagulated electrically neutral particles which are the re- 

 duction centres. This view is supported by the ultra-micro- 

 scopic observations of Gralecki {Koll. Zeit. 1912, x. pp. 149- 

 150), who showed that X-rays have a coagulating effect on 

 the particles in gold sols ; by Svedberg {Koll. Zeit. 1909, 

 iv. p. 238), who has similarly shown that ultra-violet light 

 agglomerates ultra-microns to larger aggregates; by Spear, 

 Jones, Neave, and Shlager (J. Ainer. Chem. Soc. 1921, xliii. 

 p. 1385), who have observed the same kind of effect with 

 colloidal platinum; and by recent experiments of Weiger 

 and Scholler {Sitz. Preuss. Akad. Wiss. Berlin, 1921, 

 pp. 641-650). 



PA/7. Maq. S. 6. Vol. 44. No. 260. Aug. 1922. 2 A 



