258 REPORT—1840. 
angle with reference to the horizon. If the angle be formed by 
two planes of iron, the same results follow. If a cut be made 
on a plate of polished steel with a diamond, oxidation takes 
place there first. Hence, in general, a rough plate of iron or 
steel will be acted on by air and water sooner than a smooth or 
polished one; and thus we perceive, in instruments of pre- 
cision, the value of a well-polished or burnished surface. 
243. The well-known difference in rapidity of solution be- 
tween pure zinc and that containing an alloy of another metal 
in small quantity, first noticed by De la Rive, induced me to 
make a few experiments as to whether the protective power of 
zinc to iron could be exalted by alloying the former with a mi- 
nute quantity of another metal, higher or lower in the electro- 
chemical scale. The following alloys were accordingly made, 
and equal surfaces of cast iron submitted to the action of sea 
water, immersed in metallic contact with these, viz. 
50 Zn + Hg 
100 Zn + Ap 
25 Zn + Cu 
50 Zn + Cu 
100 Zn + Cu 
50 Zn + Sn 
100 Zn + Ni 
25 Zn + Fe 
100 Zn + Na 
and also with pure zinc, and alone: on examination, it was 
found that the alloy, in minute quantity of every metal which 
is electro-negative to zinc, when in contact with cast iron, in- 
creases its corrosion in sea water, including the alloy with iron 
itself. While the alloy in minute quantity of a metal electro- 
positive to zinc increases its protective power to iron, or de- 
creases the corrosion of iron in sea water when in contact 
therewith ; we shall hereafter see reason to conclude these alloys 
not to be definite combinations, at least of their entire mass, 
although fused together in atomic proportion, but mixtures of 
definite alloys with a great excess of zinc. 
244. I now proceed to notice the results contained in Tables 
IX. and X. These tables indicate the amount of corrosion of 
cast iron in sea water, when exposed in voltaic contact with 
various alloys of copper and zinc, and of copper and tin, or 
with either of those metals separately per unit of surface. The 
alloys in Table 1X. of copper and zinc, belong to the class of 
those generally called brass, those of Table X. to those usually 
denominated gun-metal. 
