PHOTOGRAPHY. 9 



whole of the details are brought out, and now we will place 

 it in a dish of water for a while. 



This operation of causing the invisible to become visible, 

 how is it effected] We must set ourselves to solve the 

 problem by referring to a kindred action. If a rod of zinc 

 be placed in a strong solution of acetate of lead, by degrees 

 this latter becomes decomposed, and crystals of lead deposit on 

 the rod and completely cover it; but the action does not 

 cease when the covering is effected : the lead solution still 

 keeps depositing the metal, and a beautiful network of leaves 

 formed by metallic crystals is built up. The first particles 

 of lead deposited on the zinc attract other particles from the 

 solution till we have what is known as a lead-tree. 



In the development of the image, as our last operation is 

 called, we have simply an example of the laws of crystal- 

 lisation, like crystals tending to adhere to like, and to be 

 attracted by them. Now in our exposed paper we had some 

 excessively minute portions of silver iodide (Ag 2 I 2 ) reduced 

 to the state of sub-iodide (Ag 2 I). Only one silver atom of 

 these more elementary molecules is saturated as it were, and 

 the other is free, and it is this free atom that is capable of 

 attracting metallic silver from a solution of silver nitrate, 

 when this latter is in an unstable state. In the present 

 instance the instability is caused by the gallic acid, for this 

 body tends to absorb oxygen, and as it absorbs oxygen it 

 liberates from the silver nitrate the metallic silver, and as 

 quickly as the separation is effected the free silver atoms 

 attract it. We thus get an image built up on the sub- 

 iodide ; for after one small particle of silver has been attracted, 

 it, in its turn, attracts others, as in the case of the lead 

 in the lead-tree. This then is the secret of development ; it 

 is the attraction exercised by the sub-iodide for freshly- 

 separated silver. 



The developed image is therefore a metallic image ; but in 

 order to render it permanent, or perhaps I ought to say, more 

 clear, it was necessary to get rid of all the silver iodide. 

 To Sir J. Herschel belongs the discovery (in 1819) of the 

 solvent property of sodium hyposulphite on the silver chloride, 

 and it was by the application of this salt to the iodide that 

 the desired fixing of the image was effected. It is a matter 

 of surprise that this solvent was not employed at an earlier 

 date. 



