ON THE PHOTOGRAPHIC IMAGE. 115 



of silver forms the grey deposit, but that the addition of a little acid pro- 

 duces the white and brilliant form of the metal. 



If now we would take a result opposite to this from the experience of the 

 photographist, we may select an ordinary collodion plate prepared by the 

 usual negative process, and we shall find that protacetate of iron developes 

 the image of a black colour. Now Rose, in the remarkable experiments on 

 the production of argentous compounds with the higher oxides of iron, &c, 

 to which we have called attention, shows that whereas the argentic salts con- 

 taining strong mineral acids are precipitated as grey metal by ferrous salts 

 containing similar acids, the deposit formed by uniting the ferrous oxide and 

 the argentic oxide, or the compounds of these with organic weak acids, con- 

 tain the suboxide of silver and are black. 



When to this is added the circumstance that the white and grey photo- 

 graphic images are with facility amalgamated with mercury, but that the 

 coloured and black images are not, it may be treated as a matter of high 

 probability that the black and coloured images are formed by compounds of 

 the suboxide of silver. 



A directive energy is exercised upon the nature of the deposit by the 

 various kinds of organic matter employed in the development. These all 

 seem to restrict the limits of variation to the dark bluish-black (given by 

 citric acid when present), on the one hand, and various reds and browns upon 

 the other; while, again, the presence of the albuminous and other substances, 

 so often before referred to, is, as was above remarked, a sure means of 

 forming these darker and coloured images. Indeed, albumen will determine 

 such images notwithstanding that even free nitric acid be present with it. 

 If it be a suboxide that causes the dark precipitate, that suboxide must go 

 down in combination, and so resist the action of the fixing solvents. 



But, 3. The character of the light has also a remarkable influence in 

 inducing a grey or a dark character on the developed image. 



If the picture has been produced by an intense light, as by a lens of 

 large aperture, or as in the case of an exterior as contrasted with an interior 

 view of a building, or as on a dull, misty day in contrast with a bright and 

 sunny one, it will be found that, cceteris paribus, the tendency of the weaker 

 action of the light is to allow the reduction of the silver in the metallic form. 

 On the other hand, the more intense light has given to the molecules of the 

 sensitive film a controlling energy which they exercise on the deposit, and 

 which appears analogous to that of the light in the direct process, in its 

 modifying the reduction and giving it the form of a production of an argen- 

 tous compound ; as though the iodic compound became in a certain sense 

 phosphorescent to the chemical rays of the light, and operated on the mixed 

 silver-salt and reducing agent as "they float over it in the manner that the 

 direct light might be supposed to do. 



Of course, the materials must be nicely balanced, as regards their tenden- 

 cies to produce the black or the grey images, for the peculiar action of an 

 intense or a weak light to be made fully evident. Albumen or powerful 

 organic agents will usually destroy this balance. 



One fact remains to be observed. Whatever may have been the character 

 of the first particles deposited on the plate, that character will be maintained 

 thenceforward, and fresh deposits may be, so to say, piled upon the first by the 

 singular agglutinative tendency of crystalline deposits, so long as the neces- 

 sary conditions of fresh silver solution and of fresh stores of the reducing 

 agent be supplied to keep up the action. 



Our task has been, by an investigation of the chemistry of the image in 

 its different varieties, to afford some data, at least, by which the further step 



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