of preserving Iron from Corrosion in Sea-Water. 51 



tended to apply his interesting discoveries, is not oxidated by 

 decomposition of the sea-water, but only by the oxygen of 

 the atmosphere always taken up by the water in a certain 

 quantity, in combination with the carbonic acid gas of the air 

 contained in it in the same manner. Copper, placed by this 

 eminent philosopher in sea-water from which all the air was 

 expelled by means of an air-pump, remained perfectly bright. 



Dr John Davy principally endeavours to confirqd his opinion 

 against me by theoretical views, derived from the mutual elec- 

 trical relation of metals ; stating that his galvanometrical expe- 

 riments always shewed him tin as positive in its electrical rela- 

 tion to iron, and that, therefore, iron must be defended by tin. 



Owing to the want of a good galvanometer, I was not able, in 

 the course of my former experiments, to make these inquiries, 

 and I was now therefore anxious to determine the point to 

 which I have just alluded. 



I must confess that, at first, to my great surprise, I found the 

 fact as Dr Davy states, observing the astatic needles of the gal- 

 vanometer, by the immersion of iron and tin in sea-water, to 

 shew immediately a deviation corresponding with a positive elec- 

 trical relation of tin to iron. 



It was principally the sagacity of my friend Mr G. T. Mul- 

 der of Rotterdam, which made me notice, that, by prolonging 

 the experiment during a certain time, the needles always return 

 to zero, and afterwards shew an opposite deviation, the tin ac- 

 quiring a negative electrical relation to iron. 



The galvanometrical researches have taught us, as a constant 

 law, that where two metals are immersed together in a fluid, 

 the metal that suffers the strongest oxidation is always positive 

 in its electrical relation to the other less oxidated metal. Yet, 

 where tin and iron or steel are both placed in atmospherical air, 

 the tin is, without any doubt, positive in respect to the iron ; for 

 both metals being perfectly polished and bright, the tin is al- 

 most directly tarnished, and its surface becomes covered with a 

 thin layer of the oxide of tin, whilst the steel remains still per- 

 fectly bright. Now, it is a curious circumstance, that this elec- 

 trical relation of the two metals, acquired in the atmospherical 

 air, subsists during a certain time after their immersion in sea- 

 water, in consequence of a singular property ; and it thus ap- 

 d2 



