340 Mr. M. Carey Lea's Notes 



into a 1-per-cent. solution of potassium ferridcyanide, when 

 the part exposed colours much sooner and more strongly than 

 that which was covered. The effect is shown still better by 

 placing the film in a frame, covering part with paper rendered 

 absolutely opaque by coating it with thick tin-foil, part with 

 translucent paper (thick white writing-paper or very thin 

 brown paper), and leaving part wholly exposed. After four 

 or five hours' action of strong summer sunshine, the film is to 

 be treated with weak ferridcyanide. The part wholly exposed 

 having passed into the gold- coloured crystalline condition (if 

 the exposure has been sufficient) is wholly unacted upon ; the 

 part covered by the translucent paper is rapidlv attacked, that 

 wholly protected is attacked slowly. So that the portion 

 moderately acted on by light has very markedly increased in 

 sensitiveness thereby. 



It follows that upon this form of silver light has a reversing 

 action, first exalting its sensitiveness, then completely de- 

 stroying it. 



It is impossible to overlook the analogy which exists between 

 this action of light and that which light exerts on silver 

 bromide. The latter substance, though incomparably more 

 sensitive to light, is subject to the same reversing action ; 

 first gaining in sensitiveness to reducing agents, and then, by 

 continued exposure, becoming less sensitive than originally, a 

 change commonly known as solarization. 



Causes determining whetlier, in the reduction of Silver, the 

 Allotropic or the Normal form shall be produced. — I have 

 examined the phenomena connected with the reduction of 

 silver under a great variety of conditions. These for the 

 most part do not deserve particular mention, but seem to lead 

 up to this generalization : that the reduction of silver may be 

 direct or indirect ; direct when it passes from the condition 

 of the normal salt or oxide to that of the metal, indirect when 

 the change is first to suboxide or to a corresponding subsalt. 

 So far as my observation has gone, when the reduction is 

 direct the reduced silver always appears in its ordinary form ; 

 but when the reduction is indirect the silver presents itself in 

 one of its allotropic states. 



The following reactions support this view. 



Three of the principal modes of formation of allotropic 

 silver are: — (1) Reduction of silver citrate or tartrate by 

 ferrous citrate or tartrate ; (2) acting on silver nitrate or 

 oxide by dextrine and fixed alkaline hydroxide ; (3) acting on 

 silver nitrate or carbonate by tannin and fixed alkaline car- 

 bonate. Now if, in either of these three cases, we interrupt 

 the action before it is complete by adding an excess of dilute 



