COPPER AND IRON IN SALT WATER. 193 
never had been the slightest adhesion of either weed or shell-fish to her copper, but that a 
few small barnacles had once appeared on the loose oxide of iron in the neighborhood of the 
protectors, which, however, were immediately and easily washed off. 
‘¢ The Canebrea Castle, a large vessel of upwards of 650 tons, was furnished with four 
protectors, two on the stern and two on the bow, equal together to about 7)z of the surface 
of the copper. She had been protected more than twelve months, and had made a voyage 
to Caleutta and back. She came into the river perfectly bright, and when examined in the 
dry dock was found entirely free from any adhesion, ard offered a beautiful and almost pol- 
ished surface, and there seemed to be no greater wear of copper than could be accounted for 
from mechanical causes.” 
It follows from the preceding facts that, whether in sea water or the salt 
water of the laboratory, the copper sheets which are at rest in salt water in- 
crease in weight by becoming covered with earthy or alkaline deposits when 
they are protected by a proportion of iron below +35; and if this proportion be 
contained between ;1, and 7,455, the surface seems to be preserved without 
receiving either deposits or zodphytes or shells. 
It appears that Davy devoted himself to determining the limits on the sur- 
face within which the protection takes place, but not at all in the thickness. 
He neither takes into account the layer of oxychloride of zine nor of copper, 
oxychloride mixed with particles of one of these two metals, which, by op- 
posing the reaction of salt water, stops the protection. The whole question 
lies there, and Davy studied only the theory of contact. ; 
Although several of the preceding experiments have furnished favorable 
results, nevertheless the process of protection was not adopted; the reason 
assigned for it was the negative condition of cepper, which favored the deposit 
of marine bodies to such an extent as to diminish the rate of sailing. 
We shall see presently that the deposit of marine bodies was not to be 
attributed to this cause, since the major part of the protection had disappeared. 
It has been remarked, however, that in order to preserve the copper the pro- 
tecting metal must be oxidized. He had such a strong belief in this theory 
that he asserted that a piece of zinc of the size of a pea, or the point of an iron 
nail, was suflicient to protect copper plates of 256 to 320 square centimetres of 
surface, immersed in sea water; this preservation could last only for a short 
period, as the piece of zinc, or the point of a small iron nail, was rapidly de- 
stroyed. There is, withal, in this nothing to inform us whether, in the means 
of preservation employed at sea, a thought was given to the disadvantages 
resulting from the destruction of the oxidable metal; it is not, therefore, aston- 
ishing that the copper-sheathing became foul, to use the seaman’s phrase, and 
was covered with organic bodies. In addition to the above, it is the same with 
regard to the production of electricity for preserving copper or iron at sea as 
with regard to the production of heat; in the latter case it is necessary to keep 
up the supply of the combustible material, and in the former to provide for the 
replacement of the oxidable metal according as it is destroyed; this is an indis- 
pensable precaution to secure its preservation. 
The causes of change in metals are numerous. We will cite particularly the 
heterogeneousness of parts : the difference in the mode of aggregation of particles, 
the presence of any bodies whatever on the surface of metals, of grains of sand 
for example, or spots of rust, strokes of the hammer falling here or there, pres- 
sure, a simple fold or corrugation, &c., are so many causes which give rise to 
voltaic couples on the surface, and which destroy a protection otherwise suitably 
selected. The friction of water must still be added, as M. Ed. Becquerel has 
shown in the interesting experiments which he repeated at Toulon, while aiding 
me in my investigations. 
From this it may be seen why it is that metals like iron, that are forged, 
wrought, and hammered, present so many causes of change, which are removed 
by means of protectors, arranged according to electro-chemical principles. This 
explains, what experiment proves, why it is that the electrical condition of the 
protected metal does not always follow a regular law. 
18 s 
