Mr. Grant on protecting Iron from the Action of Salt Water. 129 
rosion of the zinc is very considerable: the following expe- 
riment will show to what extent: Two pieces of sheet iron 
fastened to a piece of wood, the one with nine zinc nails, the 
other with the same number of iron nails having pieces of 
zinc $ths of an inch in diameter under the head of each nail ; 
also a third piece of sheet iron fastened to the wood simply 
with iron nails: the board was then floated in the sea, and at 
the expiration of thirty days, I found that the heads of six of 
the nine zinc nails had completely disappeared, and the pieces 
of zinc corroded to such an extent that only a very small por- 
tion of zinc remained. The protected iron down to this period 
was free from oxidation, whereas the iron unprotected was 
perfectly oxidized. This experiment has been repeated several 
times with the same result, which clearly shows that although 
the zinc completely protected the iron, the zinc itself became 
corroded in exact proportion to the protection that it afforded to 
the iron. Experiment has also proved that the same evil which 
rendered Sir Humphry Davy’s system of no practical use for 
the protection of copper on ships’ bottoms from oxidation, is 
also apparent to a certain extent as regards the protected 
iron ; viz. that by rendering it slightly negative, a calcareous 
substance is found deposited on its surface; and that sea ve- 
getable matter appeared also in a short period to attach itself 
to the iron, although in a much less degree than in the expe- 
riments tried on the bottoms of boats which were subject to 
the constant friction of the water passing by them. 
In the experiments tried in still water, vegetable matter 
was found to make its appearance on the iron in six weeks 
after immersion, although a strong electrical current was 
kept up during that period. The results of the experiments, 
as far as they have proceeded, lead me, therefore, to the fol- 
lowing conclusions: in the first place, that iron and zinc in 
connexion will not protect the former from oxidation in fresh 
water; secondly, that when iron and zinc are in connexion in 
salt water, the iron will be protected, but a calcareous and 
vegetable matter is generated upon it; and, thirdly, that in 
the same proportion as the zinc protects the iron, the zinc it- 
self becomes subject to corrosion. 
I wish it, however, to be clearly understood, that although 
these experiments are not so favourable as might be wished, 
I by no means consider them so conclusive as to preclude 
the necessity of further investigation. 
I am, Gentlemen, yours, &c. 
Royal Clarence Yard, Gosport, Tuomas TassELL Grant. 
November 22, 1835. 
