Dr Fj'fe on Daguerreotype. 207 



tested for copper, gave indications of its presence. It is evi- 

 dent, then, that the acid acts merely by washing off impuri- 

 ties, and thus allowing a purer surface to be afterwards ex- 

 posed to the iodine. 



Different opinions have been entertained with regard to the 

 action of the iodine. Some suppose that the golden-coloured 

 coating is merely a thin film of iodine, adhering to the sur- 

 face of the silver ; Avhile others maintain that it is an iodid of 

 silver. Were it the former, we should expect it to be easily 

 rubbed off, and more particularly that it would be dissipated 

 by heat ; neither of which occurs. When heated — excluded 

 from light — the plate still retains the golden-yellow colour. 

 Potassa has no action on it, Avhich it should have, were it a 

 mere film of iodine. From these and other experiments of a 

 similar nature, it is evident that the iodine does not merely 

 adhere to the silver, but that it combines with it and forms 

 an iodid ; the colour of which depends on the time the plate 

 has been exposed to its action.* 



When the iodinized plate is kept in the camera /o/- the pro- 

 j)er time, it does not appear to be affected in the slightest 

 degree, and it is not till after it is exposed to the mercurial va- 

 pour that the image appears. It is evident, however, that 

 the light must exert some peculiar action on the iodid. 

 Either the afl&nity between the iodine and the silver must be 

 overcome, and the iodine be set free from the silver, or the 

 constitution of the iodid must be altered. When the plate is 

 kept long in the camera, it may be so affected as to bring out 

 an indistinct image, a circumstance which I have occasionally 

 observed ; but this result is more easily obtained by exposing 

 the plate to the direct rays of the sun, or even to a dull light, 

 with an object — say an engraving — placed on it, as in the com- 

 mon photographic process, with paper prepared wdth the chlo- 

 rid or phosphate of silver, in which case a distinct representa- 

 tion of the object on the engraving is pi-oduced — ^those parts 

 exposed to the light becoming dark, while those excluded from 



* When the plate is long exposed to the vapour of iodine, it presents a 

 rich display of prismatic colours, sometimes ia concoutric rings, sometimes 

 passing from side to side. 



