1826.] Preservationof Metals hy Electro-chemical Means. 249 



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 vicinity, 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 deposits, this 

 mud or these deposits rest in the rough parts or depressions 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 insoluble compound on 

 the surface ; and consequently there is much reason to beheve 

 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 copper, 

 protected by a quantity of iron above ^i^ parts, when these 

 sheets were in harbour and at rest. 



The various experiments that 1 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 alka- 

 hne and earthy substances, when defended by a quantity of cast 

 iron under -^ of their surface, and if in a limited or confined 

 quantity of water, when the proportion of the defending metal is 

 under t-qVo- With quantities below these respectively propor- 

 tional for the sea, and limited quantities of water, the copper 

 corrodes ; at first it slightly increases in weight, and then slowly 

 loses weight. Thus a sheet of copper, four feet long, 14 inches 

 wide, and weighing 9 lb. 6 oz. protected by -j- l_ 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 -^^, gained only one 

 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 

 agree when carried on under precisely similar circumstances, 

 must of necessity be very irregular in their results, when made 

 in different seas and situations, being influenced by the degree 

 of saltness, and the nature of the impregnations of the water, the 

 strength of tide and of the waves, the temperature, Sec. 



