HILLEBRAND.l 
FERROUS IRON. 
93 
ganic salts. The decolorization due to this cause is hence much more 
pronounced in the case of rocks high in ferrous iron than of those low 
in this constituent, because of the greater amount of manganous salt 
Fig. 13.— Cooke's apparatus for the determination of ferrous iron. 
resulting from reduction of a correspondingly larger amount of per- 
manganate. 
W hen py rite is present the bleaching is in part due to its action on 
any permanganate added in excess of the requirements of the ferrous 
