90 Sir H. Davy’s further Researches 
tin. The oxides of tin and copper which form upon the head 
of the nail and in the space round it, defend the metal from 
the action of sea water; and being negative with respect to it, 
a stronger corroding effect is produced in its immediate vici- 
nity, so that the copper is often worn into deep and irregular 
cavities in these parts. 
When copper is unequally worn, likewise in harbours or 
seas where the water is loaded with mud or mechanical depo- 
sits, this.mud or these deposits rest in the rough parts or de- 
pressions in the copper, and in the parts where the different 
sheets join, and afford a soil or bed in which sea weeds can fix 
their roots, and to which zoophytes and shell fish can adhere. 
As far as my experiments have gone, small quantities of 
other metals, such as iron, tin, zinc, or arsenic, in alloy in 
copper, have appeared to promote the formation of an insolu- 
ble compound on the surface; and consequently there is much 
reason to believe must be favourable to the adhesion of weeds 
and insects. 
I have referred in my last paper to the circumstance of the 
carbonate of lime and magnesia forming upon sheets of cop- 
per, protected by a quantity of iron above 1-120th part, when 
these sheets were in harbour and at rest. 
The various experiments that I have caused to be made at 
Portsmouth show all the circumstances of this kind of action ; 
and I have likewise elucidated them by experiments made on 
a smaller scale, and in limited quantities of water. It appears 
from these experiments that sheets of copper, at rest in sea 
water, always increase in weight from the deposition of the 
alkaline and earthy substances when defended by a quantity 
of cast iron under 1-150th of their surface; and if in a limited 
or confined quantity of water, when the proportion of the de- 
fending metal is under 1-4000th. With quantities below 
these respectively proportional for the sea, and limited quan- 
tities of water, the copper corrodes; at first it slightly in- 
creases in weight, and then slowly loses weight. Thus a 
sheet of copper 4 feet long, 14 inches wide, and weighing 
9lbs. 60z., protected by 1-100th of its surface of cast iron, 
gained in ten weeks and five days 12-drachms, and was coated 
over with carbonate of lime and magnesia: a sheet of copper 
of the same size protected by 1-150, gained only 1 drachm 
in the same time, and a part of it was green from the adhering 
salts of copper; whilst an unprotected sheet of the same class, 
both as to size and weight, and exposed for the same time, 
and as nearly as possible under the same circumstances, had 
lost 14 drachms: but experiments of this kind, though they 
* See Phil. Mag. vol. Ixy. p. 205. 
agree 
