
ON THE ACTION OF AIR AND WATER UPON IRON. 13 
323. In Table XI. is given the results of corrosion of wrought iron in vol- - 
taic contact with the alloys of copper and zine, and in Table XII. those with 
copper and tin. The alloys are the same as those whose reactions are given in 
Tables 1X. and X. of second Report, with which the present tables coordinate. 
Corrosion of wrought iron is accelerated by the presence of either brass 
or gun-metal ; most so by the latter. With equal surfaces and conditions, 
copper produces greater corrosion than any of its alloys with zinc. Most of 
those which constitute the metals used in commerce, however, do not greatly 
accelerate the corrosion of wrought iron. 
_ All alloys of copper and tin do accelerate it considerably, and even more 
than copper itself, while tin produces a still greater effect than copper; thus 
in most respects wrought iron is acted upon by air and sea water in presence 
of these alloys in a similar way to cast iron. 
These tables now give numerical measures of the amount of loss of metal 
that will occur in practice in the given conditions. The results obtained long 
since by Sir H. Davy, as to the small amount of positive metal requisite to 
protect copper sheathing, indicate that within very wide limits in the relative 
proportions of the iron, to either the brass or the gun-metal, these results will 
be very nearly exact. In the present case the surface of wrought iron was 
always = 3:07 square inches, and of the brass or gun-metal = 1:99 square 
inch, and the experiments were made in vessels containing a proportionally 
large volume of sea water, and frequently renewed. 
_ $24. In Table XIII. the average loss by corrosion is given of all the varie- 
_ ties of cast iron at the second period of exposure of 732 days, and of the 
wrought iron and steel exposed for the same period, but for the first time, 
_ and also of both cast and wrought iron exposed to the weather; and in ge- 
_neral the average results of the whole investigation, reduced into form for 
_ practical use, will be found in this and the two following tables. Table XIV. 
gives at one comparative view the results of all the classes of experiment. 
$25. In Table XV., which coordinates with Table VIII. of second Report, 
_ the average results for wrought iron and steel are extended to a period of a 
century for clear sea and fresh water, and for exposure to weather, &c. The 
numbers here give absolute measures of the loss of metal taking place in the 
several conditions, and from the extended base of induction from which they 
have been obtained, may, I think, be relied on in practice. 
_ 326. In each of the classes of experiment will be found included some 
made for the same lengthened period of 732 days, or about two years, on 
iron zinked in the ordinary way or “ galvanized,” and on iron coated with the 
zine paint before spoken of (2nd Rep. 195). These results quite confirm the 
Statements made in my second Report respecting these modes of protection ; 
_ in every case zinking is but a partial preservative to iron in any sort of water. 
Referring to Table XIII. it will be seen, that in clear sea water the corro- 
sion of such zinked iron is rather more than one-half of unprotected iron in 
like conditions, while in foul water, whether fresh or salt, it is fully as great. 
Tn fact, in foul water the zinc becomes wholly converted into a black, brittle, 
crystalline crust, which is found to be sulphuret of zinc united to sulphuret of 
iron, in fact to be an artificial blende, having the composition (Zn + Fe) + S. 
poo. Zinked iron exposed merely to the weather, however, seems to be 
more permanently protected ; and it does appear that a coat of zinc, although 
thin, if its integrity be not injured mechanically, will protect iron from rust 
when exposed to the ordinary atmospheric influences. 
_ 328. Zine paint appears to be, as predicted, an extremely durable covering, 
‘More so than any one tried, except the asphaltic varnishes or coal-tar laid on 
hot ; it is desirable its use should become better known, and be extended to all 
