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 
metallic 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 d 
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 the 
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 ues as- 
certained by trial, that so great was the power of these oxidable met- 
als over copper, that a piece of tin soldered oe sheet of was cae 
would protect a surface one-hundred and fifty times as large <teg ite 
perfectly, and would partially protect a surface one thousand times as 
large as its own. After various trials, cast iron was found to-be = 
all accounts, the most eligible substance for protecting copper from 
* Phil. Trans. 1824, p. 153. 
