ON THE ACTION OF AIR AND WATER UPON IRON. 235 
experiments show that the voltaic action produced at the sur- 
face of chilled cast iron, by its want of homogeneity, increases 
the corrosion of the metal by menstrua, to a greater extent than 
its great density and hardness, and small amount of uncombined 
carbon, are capable of retarding its corrosion, in comparison 
with other sorts of cast iron. In confirmation of this it will be 
observed, that in almost every case the condition of the corroded 
surface of the “ chilled’ specimens has been “ tubercular,’”’—a 
form of rust which, whenever it is found on iron, is an unfailing 
index of a want of uniformity of substance. This want of ho- 
mogeneity, however, is less in every chilled casting as we recede 
from its surface towards the interior; and, accordingly, I ex- 
pect that the result of the next two years’ corrosion of those 
specimens, the whole of which are now immersed, and proposed 
to remain so for that period, will show rather a diminished excess 
in the index of corrosion of ‘ chilled’’ over the other sorts of 
cast iron; the difference also diminishes with increase in bulk 
of a chilled casting, which must be more uniform the larger 
it is. 
179. This leads to the second fact brought to light by these 
results, viz. that the size (and perhaps the form) of castings in 
iron influences their rate of corrosion. With the view to deter- 
mine whether any difference in this respect might exist, it will 
be seen that in the series «, immersed in clear sea water, nearly 
every sort of cast iron has duplicate specimens experimented 
on; viz. of one inch in thickness by five inches square, and of 
one quarter of an inch in thickness by five inches square, but 
differing in no other respect whatever save in this one dimension 
only. Yet it will be observed, that throughout the amount of 
corrosion of the quarter of an inch, or thinner pieces, is greatly 
more than that of the thicker, or one-inch pieces of each sort 
of iron. 
The difference is greatest in the softest and most carbonace- 
ous, or rather “graphitic”? cast irons, and least in the hard, 
dense, silvery cast irons. Thusin the Vartey Hill (No. 2) hot 
blast iron, the index of corrosion of the quarter-inch casting to 
that of the one-inch, is about 5°5 : 1; in the Cinderford (No. 1) 
cold blasts as 10°35; in the Muirkirk (No. 2) cold blast as 
11°5 : 3, while in the Calder (No. 4) hot blast it is only as 
6°76 : 6°20, or nearly in ratio of equality. 
This very striking circumstance would scarcely have been 
predicted before the present results forced it on our notice; 
yet its rationale is easy upon principles just applied to the case 
of “chilled’’ castings. These thinner castings have cooled 
much faster, and more irregularly than the thicker, or one-inch 
