4 REPORT—1843. 
that this almost uniformly occurs in those specimens which had the stnallest 
amount of corrosion at their first immersion. Thus there is a tendency to a 
greater equality in the index of corrosion in all the varieties of iron evidenced 
by the second than by the first immersion. 
289. In the first period of immersion the amount of corrosion of all the 
thin cast specimens, those of 0°25 inch in thickness, was much greater than 
that of the thick or 1-inch specimens of the same iron in the same conditions, 
as remarked (2nd Rep. 178, 179), where this was shown to arise from a less 
homogeneity of surface in the thin than in the thicker castings. 
‘The difference in their respective rates of corrosion is however much less 
on the second period of immersion, which arises from the fact that the re- 
moval of metal by the corrosion of the previous immersion had bestowed a 
much more uniform or homogeneous surface upon all the specimens. 
290. The conclusions previously given therefore (2nd Rep. 175—187), as 
to the connexion between the size, form, method of casting, with consequent 
surface and amount of corrosion, are not only borne out, but we shall see 
reason to conclude that homogeneity of surface and texture, or the contrary, 
are by far the most important circumstances which vary the amount of cor- 
rosion in cast iron by air and water; that the rapidity of this is not so much 
dependent upon the chemical constitution of the metal as it occurs in com- 
merce, as it is upon its state of molecular arrangement and the condition of its 
constituent carbon. 
291. Upon collating the tables of the first and second immersions, it will 
be found that the specimens of cast iron, whose analyses are subjoined, are 
those presenting the maxima and minima corrosion. 
It was to be presumed, that if the extremes of corrosion were connected 
with the constitution of the metal, a careful analysis would elicit that upon 
which the best qualities depended. 
292. TasLrt A.—ANALYSES OF CAST IRONS. 
Of Maximum and Minimum Corrosion. 









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Hot. Hot. Cold. Cold. Cold. Hot. Hot. Hot. Cold. Cold. 
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graphite 
cap } 213] 054] 0:38) 080] O11 | O34 | O21 | 096] 027] 035| 4¢€ 
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Sulphur ...... aoe +». | traces. ane tes traces. | traces. | traces. Ses soe Pe 
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ETON cscsvaeti 95:06 | 93°95 | 95:25) 95-00 94-61 93°77 93°22 93°48 92-83 | 94:29] 92: 
100-00 |100-00 |100-00 | 100-00 | 100-00 | 100-00 | 100-00 | 100-00 | 100-00 {100-00 |100: 
On inspecting these results, however, in connexion with the tables, it will 
be evident that corrodibility does not depend upon the proportion of consti- 
tuent carbon, and still less upon that of the other foreign matters usually 
found in cast iron, but upon the state in which the carbon exists in the com- 
pound, upon the state of aggregation of the whole mass, and upon the voltaic 
uniformity or otherwise of the surfaces exposed to corrosion. This is practi- 
— 

