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ON THE ACTION OF AIR AND WATER UPON IRON. 267 
whole surface of 0°007 of an inch in depth, a result also con- 
firmed by the amount of rust contained in the vessel, and on 
the piece when reduced to peroxide. 
_ 266. The piece of soft cast. iron immersed alone was found, 
on washing off the coat of rust, to be covered with a thin coat 
_of soft, unctuous plumbago, capable also of being washed away 
_by rubbing with the finger; its surface was black, and filled 
_ with glittering minute scales of graphite, but had lost its me- 
_tallic lustre wholly. On removing the plumbago down to the 
_ solid iron, by rubbing with a piece of hard wood and washing, 
the piece was found to have lost a metallic coat over its whole 
surface of 0°01 of an inch in depth, estimated as before, and 
controlled by the amount of graphite and rust reduced to 
_ peroxide. 
_ 267. Lastly, the voltaic couple, the hard and the soft iron in 
contact, were examined; on washing off the rust, the hard 
‘specimen appeared bright and polished, and some minute file- 
marks on its surface, as sharp as when placed in the sea water. 
The surface of the soft piece of cast iron, on the contrary, was 
black, full of scales of graphite without metallic lustre, and 
eapable of being rubbed away with the finger. The two sur- 
faces, which were actually opposed to each other and in contact, 
were in both almost quite free from stain or oxidation, where 
air and water with difficulty gained access, from reasons before 
explained. On washing and cleaning perfectly the hard speci- 
men from oxide, and the soft one from oxide and plumbago, 
and weighing as before, the hard cast iron was found to have 
Tost a coat of iron over its whole exposed surface = 0:00263 of 
an inch in depth, while the piece of soft cast iron had sustained 
a loss over ifs whole exposed surface of 0°03 of an inch in 
depth, both estimated as before, by weighing and measurement, 
and the result controlled by estimation of the peroxide and 
aphite produced. 
268. Itis hence proved, that the softest dark gray cast iron is 
sufficiently electro-negative, to hard bright gray cast iron, to re- 
d the corrosion of the latter in sea water when voltaically as- 
ciated with it, to the extent of two thirds of the total amount of 
corrosion that would be experienced by the same hard gray cast 
ron, if exposed for the same time and under similar circum- 
nces alone to sea water, and that the formation of plumbago 
he softer iron or positive pole, and the collection of a coat 
ist on the surface of both irons, does not prevent, although 
may possibly in some degree interfere with, this effect. 
ence it follows, that while the voltaic relations of soft to hard 
st iron are such as will not prevent oxidation upon either, it 
