as Photographic Agents. 263 



changed; darks faded and reddened. Green. Lights un- 

 changed; shadows less faded, not so red. Yellow. Lights un- 

 changed; dark parts a blue-black. Red. Lights unchanged; 

 dark parts become very black. 



52. From a careful perusal of these results it will appear 

 that this curious darkening of the finished picture is most evi- 

 dent under the influence of red light, but that this property 

 extends up to the green rays, beyond which a different power 

 is exercised ; the deoxidizing influence appearing to be great- 

 est in the blue rays, whilst the yellow iodide of silver suffers 

 decomposition in the most refrangible rays. 



53. The fading of Hydriodic Photographs. — I have before 

 noticed (30.) the want of absolute permanence in these pictures. 

 The study of the modus operandi of solar light in its action on 

 them opens some very remarkable facts in relation to the 

 iodide of silver, which when first observed, led me to believe 

 the existence of two distinct salts, whereas I now entertain a 

 different opinion. The drawing fades first in the dark parts, 

 and as they are perceived to lose their definedness, the lights 

 are seen to darken, until at last the contrast between light and 

 shadow is very weak. 



54. If a dark paper is washed with an hydriodate and ex- 

 posed to sunshine, it is first bleached, becoming yellow ; then 

 the light again darkens it ; if, when quite dry, it is put away 

 in the dark, it will be found in a few days to be again restored 

 to its original yellow, which may be again darkened, but not 

 so easily as at first, and the yellow colour is again restored in 

 the dark. The sensitiveness to the influence of light dimi- 

 nishes after each exposure, but I have not been enabled to ar- 

 rive at the point at which this entirely ceases. 



55. If a dark paper, bleached by an hydriodate and light, 

 be again darkened, and then placed in a bottle of water, the 

 yellow is much more quickly restored, and bubbles of gas will 

 escape freely, which examination will show to be oxygen. 



56. By inclosing pieces of hydriodated paper in a tube to 

 darken, we discover, as might have been expected, some hy- 

 drogen is given off. If the paper is then well dried and care- 

 fully shut up in a warm dry tube, it remains dark; moisten 

 the tube or the paper, and the yellowness is speedily re- 

 stored. 



57. Take a photograph thus formed and place it in a vessel 

 of water, in a few days it will fade out, and bubbles of oxygen 

 will accumulate around the side. If the water is examined, 

 there will be found no trace of either silver or iodine; thus it 

 is evident the action has been confined to the paper. 



58. We see that the iodide of silver has the'power of sepa- 



