340 



Chemical Corrosion and Voltaic Current. [Feb. 21, 



decreased the rate of total corrosion of the positive plate, and the 

 amount of the latter was five times that of the former ; and in seven 

 instances it increased and in eleven decreased the loss of weight of 

 the comparison plate, and the amount of the latter was fourteen times 

 that of the former. 



The rate of total corrosion of the positive plate appeared to be 

 related to the degree of electromotive force. By comparing together 

 Columns 4 and 6 of the table it was found that the instances of least 

 electromotive force and lowest rate of corrosion coincided in the case 

 of magnesium, zinc, tin, aluminium, iron, nickel, and copper; and of 

 greatest coincided in the case of lead and copper. With magnesium 

 the order of electromotive force was almost the same as that of total 

 corrosion, and with copper it was partly so. No. 24 in the table 

 further illustrates the influence of decreased electromotive force. 



The simple and chief explanation of the great variation in the 

 proportion of corrosion by " local action " to that attended by external 

 electric current, appears to be difference of electric-conduction-resist- 

 ance. Where there is no external circuit and the external resistance 

 is infinite, as in cases of deposit of metal or hydrogen by simple 

 immersion, the whole of the electricity flows through the local 

 circuits and deposits its complete equivalent of metal, hydrogen, or 

 both, on the negative parts of the positive plate ; but when the 

 external resistance is least, and the resistance to circulation of local 

 currents is greatest, nearly the whole of the electricity passes through 

 the external circuit, and nearly the whole of the deposition of 

 hydrogen or metal occurs on the negative plate. In either case the 

 full equivalent of electricity circulates. With zinc and platinum in 

 dilute sulphuric acid, the "local action" was diminished by closing 

 the circuit. By diminishing also the external resistance in relation to 

 that in the battery by the employment of two voltaic cells in series, 

 instead of one (see No. 24), the proportion of silver deposited was 

 increased from 90'02 to 93'83 per cent, of the theoretical quantity. 



Rate of corrosion depends chiefly upon the particular combination 

 of metal and liquid, and is limited by the substance of weakest mole- 

 cular action. Thus the rates of corrodibility of magnesium in solution 

 of potassic cyanide (or in dilute hydrofluoric acid) and in dilute 

 sulphuric acid, are just the opposite of those of aluminium in the same 

 liquids ; while magnesium dissolves very slowly in the former liquid 

 and very rapidly in the latter, aluminium does quite the reverse (see 

 " Relations of Heat to Voltaic and Thermo-Blectric Action," " Proc. 

 Roy. Soc," vol. 36, p. 50) ; therefore, either a difference of metal or 

 of liquid in this case reverses the effect. Two of the most extreme 

 cases of difference of rate of corrosion observed at 60° F. were 

 aluminium in 3^ per cent, pure hydrofluoric acid, and in very dilute 

 sulphuric acid. 



