﻿552 Intelligence and Miscellaneous Articles. 



silver, or by coating with a transparent resinous lac, these pictures 

 may be preserved in the light. The coating of lac probably acts by 

 enclosing the parts of the iodide of silver so firmly that they cannot 

 separate. 



These pictures, which I shall name mechanical iodide-of-silver 

 pictures, are formed upon iodide-of-silver collodion fumed with 

 iodine, which already contains iodide of silver as a precipitated fine 

 powder, appearing transparent and of a brownish-yellow colour. 

 The layer of powder, however, is still further disintegrated by light; 

 and by adequate exposure under a photographic negative, there is first 

 obtained a positive, which appears dark brown in transmitted light 

 and, by a series of beautiful shadings, ultimately changes into a nega- 

 tive. If the layer of iodized collodion be almost allowed to dry be- 

 fore immersing in the silver solution, the iodide of silver will be 

 precipitated in such fine particles that it will show these colours 

 directly without any action of light. 



It remains to be decided whether this mechanical change of the 

 haloid silver-salts effected by light plays a part in the ordinary pho- 

 tographic process, By adequate exposure a visible picture may be 

 directly obtained on the sensitive Daguerreotype plate, or on a chlo- 

 ride-, bromide-, or iodide-of-silver collodion plate which has been 

 immersed in nitrate-of-silver solution : this picture is not soluble in 

 hyposulphite of soda, and can therefore be fixed by it. The substance 

 of this picture, which is poorer in iodine than iodide of silver*, pos- 

 sesses the photographic attraction for particles of mercury or silver. 

 This chemical image is changed by excess of iodine into iodide of 

 silver soluble in hyposulphite of soda ; its colour changes, and it 

 thereby simultaneously loses its photographic property ; while the 

 above-described mechanical iodide-of-silver picture is just formed in 

 the presence of excess of iodine, is not again destroyed by it, and 

 of itself has not the photographic property. 



It thus appears to me proved that the mechanical change of the 

 haloid silver-salts in the photographically sensitive condition is indeed 

 quite parallel to the chemical change, but is extremely small, and 

 that the photographic process is inseparably bound up with chemical 

 decomposition. 



It is worthy of remark that the mechanical change of the silver 

 haloid salts (the removal of the cohesion of the molecules) is most 

 powerful when the chemical change (the separation of the atoms in 

 the molecules) is least. Possibly a disintegration by light, such as 

 has hitherto only been known in the case of realgar, takes place also in 

 other substances which are sensitive to light; apparently crystallized 

 iodide of tin undergoes this change, and probably also bichromate of 

 potash. — Berliner Chemische Berichte, No. 6, 1871. 



* This is most simply seen from the following experiment : — If a regu- 

 line surface of silver produced upon glass be superficially iodized, so that a 

 thin layer of silver remains unchanged, by exposure a picture may be pro- 

 duced upon the plate and fixed by hyposulphite of soda. In the places in 

 which light has acted, the layer of silver which gives the background of the 

 picture has disappeared, the iodide of silver which has been exposed must 

 therefore have given up iodine to silver. Moser, who first observed these 

 pictures, thought they consisted of physically modified iodide of silver. 



