256 REPORT— 1840. 
decomposition of the water itself; hence results local action 
on the zinked surface, between the portions of it in the metallic 
state and in the state of suboxide or protoxide; and hence its 
removal in patches, by which the iron is soon laid bare in 
spots, on which, when once peroxide of iron has formed toa 
certain extent, the protective power of the remaining zinc is at 
an end; for as has been shown*, the original difference in 
electric condition between clean iron and clean zine is so small, 
that the former ceases to be negative with reference to the 
latter as soon as it has been rendered more positive by the pre- 
sence of its own peroxide. 
235. We have seen that the conditions the most favourable 
possible for rapid oxidation of iron consist in its exposure to 
‘* wet and dry,” or to air covered with an indefinitely thin 
film of water constantly renewed ; thus circumstanced, zinc has 
no protective power over iron in fresh water ; and on the whole 
it may be affirmed, that under all circumstances zine has not yet 
been so applied to iron to rank as an electro-chemical protector 
towards it in the strict sense; hitherto ithasnot becomea prevent- 
ive, but merely a more or less effective palliative to destruction +. 
236. There are some contingent circumstances in the re- 
actions of zine and iron, in presence of air and water, which 
require a brief notice. All the surfaces of a parallelopiped of 
iron, in contact with zinc, do not lose alike by oxidation; in 
these circumstances, that surface which is nearest to the source 
of absorbed atmospheric air, especially if it be parallel to the 
plane of the surface of the fluid, loses the most by oxidation in 
a given time. 
237. All other circumstances being equal, the upper surface 
of a parallelopiped of iron loses more in a given time than 
either of the others. The reason of this is, that the bubbles of — 
hydrogen escape freely from the upper surface as soon as 
formed, and leave it constantly exposed to the action of the air 
and water; but they cling to the lateral and under surfaces, 
and so defend them more or less from the reaction. The same 
result is frequently observable in a piece of iron exposed to a 
moist atmosphere, but from a different reason ; here, zn general, 
dew deposits first and most copiously on the upper surface, and 
* § 207. 
+ It is scarcely therefore necessary to notice, in way of contravention, a 
paper in Poggendorff’s dAnnalen for last year, vol. xlvii. p. 213, giving an 
account of the complete preservation of certain salt-pans by bands of zine, 
which are said not to have been in contact with the saline solution. The 
paper in question is a curious instance of the “ sophisma non causz pro 
causa.” A recently-boiled saturated solution of common salt has no action on 
iron, whether zinc be present or absent. 
