1824.] R e pty to an erroneous Assertion 143 



a preservative at all ? If he did know all this, he knew much 

 more than one of the ablest chemists of the day; for Dr. Watson, 

 in the seventh edition of his Essays, published in 1800, insists 

 very strongly on the danger of tinned copper vessels, in case of 

 abrasion of the tin; and so apprehensive was he of the consequences 

 of the minutest portion of copper being uncovered, that he says 

 that " a new copper vessel, or a copper vessel newly tinned, is 

 more dangerous than after it has been used; because its pores, 

 which the eye cannot distinguish, get filled up with the sub- 

 stances which are boiled in it, and all the sharp edges of the 

 prominent parts become blunted, and are thereby rendered less 

 liable to be abraded." Dr. Watson, therefore, was so far from 

 being aware of the principle on which Sir Humphry Daw's 

 invention is founded, that he obviously was not even aware of 

 the fact alluded to, and the Birmingham patentee was probably 

 not much better informed than the Bishop of Llandaff. At the 

 date of Mr. Wyatt's patent, and for many years after, all the 

 world was ignorant of the principle of action of the defending 

 metal ; nor was it developed till the instrument of Volta, in the 

 hands of Davy, furnished the clue ; and its present important 

 application is, in fact, an extension of the same train of reason- 

 ing that led to his preceding discoveries in electro-chemical 

 science. 



It is in the principle, therefore, I repeat, that, the merit and ori- 

 ginality of Sir H.Davy's method is founded, and the importance of 

 the principle is confirmed by a circumstance which would have 

 rendered a mere mechanical covering, like Mr. Wyatt's, useless 

 and abortive. The defended copper is more liable to become 

 foul from the adhesion of barnacles, weeds, &c. than the unde- 

 fended. Had Mr. Wyatt's tinned sheeting been adopted, it 

 would have been subject to the same pest, nor is it probable that 

 in the then state of chemical science, he could have suggested 

 a remedy for the evil. With the light thrown on the subject by 

 Davy, the antidote is obvious. Barnacles, &c. do not adhere to 

 the undefended copper, because the oxide on its surface poisons 

 them, but the clean metallic surface of the defended copper does 

 them no harm. All that is necessary, therefore, is to weaken 

 the defensive action, by diminishing the extent of defending 

 surface, to such a point as to allow a slight oxidation of the 

 copper, sufficient to repel the animalcule, but not sufficient to 

 occasion a serious waste of the metal. J. G. C. 



