200 M. C. Lea—Influence of Color upon Reduction by Light. 
Art. XXIII —On the Influence of Color upon Reduction by 
Light; by M. Carey Lea, Philadelphia. 
In all metallic salts which suffer reduction by exposure to 
light, the facility of reduction varies with the acid to whic 
the base is united. Thus in the case of iron, ferric oxalate and 
citrate are much more easily reduced than ferric sulphate. 
Further, the reducibility of any salt is generally much in- 
fluenced by substances placed in contact with it. Thus silver 
chloride exposed to light alone, changes somewhat slowly to a 
violet color; in contact with silver nitrate it changes more rap- 
idly and assumes a deeper color; and if, in addition, we place 
certain sorts of organic matter in contact with it, the change 1s 
still more rapid and an intense black color may result. Pure 
silver iodide, as I have shown elsewhere, undergoes no chemical 
decomposition when exposed, completely isolated, to the sun, 
but in contact with silver nitrate, and also with many other sub- 
stances, it is extremely impressible. The sensitiveness to light 
of silver bromide also is greatly modified by substances placed 
in contact with it. 
It seemed therefore a matter of interest to determine whether 
any general law existed that when a metallic compound capa- 
ble of reduction by light was placed in contact with a body 
capable of bein oxidized (or of uniting with Cl, Br or I, as the 
ease may be), the capacity of reduction of the metallic com- 
and by any particular portion of the spectrum would be in- 
uenced by the color of the body placed in contact with it. If 
