66 HEROES OF INVENTION AND DISCO VEK \ \ 



zinc or iron in contact with the copper; the former metals 

 being more positive than the letter, and therefore fitted by 

 induction to repel a portion of its electricity, and so to render 

 it negative like the water. The result surpassed his expecta- 

 tions. So powerfully did the one metal act in reversing the 

 electrical state of the other, that a bit of zinc or iron, no larger 

 than a pea, was found sufficient to protect from corrosion forty 

 or fifty square inches of copper. Nothing, therefore, could be 

 more perfect than the success of this contrivance for the partic- 

 ular purpose it was intended to serve. But, unfortunately, it 

 has been found by experience, that, although Davy's method 

 completely answers for preventing the wasting of the copper, 

 the sea-weeds and marine insects accumulate in such quantities 

 upon the bottoms of ships so protected, that they become, after 

 a short time, scarcely navigable. At the time, therefore, the use 

 of the zinc and iron was of necessity abandoned. It is by no 

 means improbable, however, that some expedient may be con- 

 trived for counteracting this consequence of the application of 

 Davy's invention — in which case it will be entitled to rank as 

 one of the most valuable discoveries ever made. 



We have thus, guided chiefly by the Memoir of which men- 

 tion has been made above, pursued the principal triumphs of 

 Sir Humphrey's splendid career, and described what he 

 achieved, although cursorily and briefly, in such a manner, we 

 trust, as to put even the unscientific reader in possession of a 

 tolerably just view of the great discoveries on which his fame 

 rests. In 1827, as we have already mentioned, his health had 

 become so bad, that he found it necessary to resign the presi- 

 dency of the Royal Society. Immediately after this he pro- 

 ceeded to the Continent. During his absence from England, 

 he still continued to prosecute his chemical researches, the 



