S/J^ HUMPHRE Y DA FY. 65 



to enter. The safety lamp has been the means of saving 

 many lives, and has enabled extensive mines, or portions of 

 mines, to be wrought, which but for its assistance must have 

 remained unproductive. The coal-owners of the northern dis- 

 tricts invited Sir Humphrey, in 1817, to a public dinner, and 

 presented him with a service of plate of the value of ;^2ooo, in 

 testimony of what they felt to be the merit of this invention. 



We will mention only another of this eminent individual's 

 ingenious practical applications of those scientific truths with 

 which he enriched the philosophy of his age. About the year 

 1823, the attention of the Commissioners of the Navy was so 

 strongly excited to the fact of the rapid decay of the copper 

 sheathings of ships when exposed to the action of the salt-v/ater, 

 that they applied to the Royal Society to take the subject into 

 consideration, and endeavour to devise a remedy for the evil. 

 On this occasion, Davy again had recourse to those principles 

 of electro-chemistry, of which he had himself been the dis- 

 coverer, and by the application of which he had already 

 obtained so many brilliant results. One of the laws of elec- 

 trical agency which he considered himself to have ascertained, 

 was that two substances can only combine by what is called 

 chemical afifinity or attraction when they are in opposite elec- 

 trical states, — that is to say, when the one is positively, and the 

 other negatively, electrified. The copper and the water, there- 

 fore, he concluded, were naturally in these circumstances ; and 

 all that would be required, consequently, to prevent the action 

 of the one upon the other, would be to change the electrical 

 condition of that one of them — namely, the copper — which 

 it was possible to submit to the necessary treatment He 

 thought of various ways of effecting this object; but, at last, 

 he determined to try the effect of merely placing a quantity ot 



