Sir Humphry Davy. 245 



least dependent on the same cause ; that, consequently, substances 

 will combine only when they are in different electrical states ; and 

 that, by bringing a body naturally positive into a negative state, its 

 usual powers of combination are altogether destroyed.* He informs 

 us, that he was conducted by these principles, to the discovery of the 

 metalUc bases of the alkalies. For, if potash, for example, were a 

 compound, and its constituents were held together in consequence of 

 their being in opposite electrical states, then, according to known 

 laws of electrical attraction and repulsion, it was only to apply to them 

 electrical powers of greater intensity, and they would be separated 

 from each other, — the negative constituent would leave the positive 

 for a body positive in a higher degree, and the positive constituent 

 would leave the negative for a body negative in a higher degree. 

 Now the voltaic apparatus, (which admits of indefinite extension,) 

 affords the means of producing the opposite electricities, at the two 

 poles, of a degree of intensity exceeding that which exists between 

 the constituents of any given compound, and thus all the combina- 

 tions of matter are brought under its dominion. Although this hy- 

 pothesis was adopted by Berzelius, certainly one of the most compe- 

 tent judges in the world, yet it had not been generally received by 

 chemists. Its author, however, had some reason to feel attached to 

 an hypothesis, which had conducted him to so successful a result in 

 his galvanic researches, and he resolved once more to follow its sug- 

 gestions. He reasoned thus: — the cause of the corrosion of tlie 

 copper sheathing of ships is a chemical action of sea water upon 

 copper, and this is owing to their being in opposite electrical states, 

 the copper positive and the water negative. How shall this action 

 be destroyed ? By rendering the copper negative ; for then the metal 

 and the water being in the same electrical state, no affinity can ex- 

 ist between them. Now tin, and zinc, and iron, being respectively 

 positive in a much higher -degree than copper, have each the power, 

 when joined to copper, of rendering it negative. Indeed it was as- 

 certained by trial, that so great was the power of these oxidable met- 

 als over copper, that a piece of tin soldered to a sheet of copper, 

 would protect a surface one hundred and fifty times as large as itself 

 perfectly, and would ^ar?;ia% protect a surface one thousand times as 

 large as its ov^^^. After various trials, cast iron was found to be on 

 all accounts, the most eligible substance for protecting copper from 



* Phil. Trans. 1S21, p. 153. 



