Observations on Photographic Processes. 73 



Art. V. — Observations on Photographic processes ; by William 



F. Channing. 



In the photographic preparations of silver, the agency of hght 

 is confined to a change in the arrangement of the particles, or to 

 a partial decomposition. As in galvanic decomposition, it is as- 

 sisted hy coinciding chemical affinities, and these may be brought 

 to act on the salt of silver either in the preparation before expo- 

 sure to light, or afterwards to bring out the impression. In the 

 Daguerreotype, mercury seems to act mechanically in making 

 apparent the molecular change wrought in the iodide of silver by 

 the action of light. In the recent photographic processes of Tal- 

 bot, however, a chemical agent is used subsequently to complete 

 the decomposition which light had begun. 



A sensitive preparation of silver is one in which " the elements 

 are so delicately balanced as to be overturned by the slightest 

 cause."* The fulminating compounds of silver are examples of 

 such a composition and may form hereafter preparations very sen- 

 sitive to light. None of the more simple salts of silver are imme- 

 diately reduced by light, though it may afterwards be made appa- 

 rent by chemical means that they have undergone a change of 

 structure or that a subsalt has been formed. Thus chloride of 

 silver is apparently insensible to light, unless, for example, organic 

 matter containing carbon, oxygen and hydrogen be associated 

 with it. when the affinity of the carbon for the oxygen, and of 

 the hydrogen for the chlorine, immediately determines a reduc- 

 tion. Thus carbon reduces the oxide of silver in many of its 

 oxy-salts. Electro-positive bodies therefore, which are associated 

 with the salt of silver, tend directly or indirectly to decompose it 

 by removing the electro-negative principle combined with the 

 silver, while an excess of electro-negative elements, oxygen, 

 chlorine, &:c. as in the porchlorate, chlorate and hypochlorate of 

 silver, retards decomposition. Thus compounds such as phos- 

 phorus, tannic, gallic, crenic, fulminic acids, which may be called 

 deoxidizing agents, all absorbing oxygen, and also another series 

 acting mechanically, such as mercury, deserve attention, first in 

 making a photographic preparation, and second, in bringing out 



* Hunt — Griflin's Scientific Miscellany. 

 Vol. xLiii, No. 1.— April-June, 1842. 10 



