Having thus disposed of the speculative part of his admirable enquiry, it will be interesting to pause in our narrative, in order to take a philosophical review of the progress of Voltaic discovery, in its relations to this particular object. It is a subject well calculated to afford a valuable lesson to the experimentalist, and at the same time to furnish illustrations, more striking even than that of the safety-lamp, of the necessity of that complicated species of machinery, without which the human mind is frequently unable to grapple with the simplicities of truth. It is true, that the fact of a galvanic effect being excited by the contact of two dissimilar metals was noticed in the earliest stages of the enquiry, but it is equally evident that the phenomena which attended it, and the laws by which it was directed, required for their discovery and elucidation the assistance of the Voltaic battery. In reference to Davy, it may be here repeated, that the power of obtaining simple results, through complicated means, was one of the most distinguishing features of his genius.
It has been stated, that the Alarm frigate, the first coppered ship in our Navy, displayed very striking evidence of the effect of Voltaic action, in the rapid corrosion of its iron.[101] As early as 1783, after copper sheathing had become general, the Government issued orders that all the bolts under the line of fluitation should, in future, be of copper; but at that period, it was not possible that any idea could be entertained as to the true nature of the operation by which the iron was thus rapidly corroded, for it was only in the year 1797 that Dr. Ash noticed, for the first time, a phenomenon which was subsequently referred to the action of a simple Voltaic circuit.
It is a very curious circumstance in the history of this subject, that, for many years after the Voltaic influence had been recognised as the agent in metallic corrosion, so far from the existence of the accompanying phenomenon of preservation being suspected, it was even supposed that the metals mutually corroded each other. At so late a date as 1813, we find Davy himself, in the letter addressed to M. Alavair, published in these memoirs,[102] dwelling upon the necessity of avoiding metallic contact, in order to prevent corrosion, without throwing out the most distant hint as to the simultaneous production of a converse effect.
The first distinct notice of a metal being preserved from oxidation by the contact of a dissimilar metal, is at once referred to a chemical law, without a reference even to its possible connection with Voltaic action; and, striking as the fact may now appear, it never attracted much attention. M. Proust observed, that although copper vessels be so imperfectly tinned, as to leave portions of the surface uncovered, still, in cooking utensils, we shall be equally protected from the poisonous effects of the former metal; because, says he, the superior readiness with which tin is oxidized and acted upon by acids, when compared with copper, will not allow this latter metal to appropriate to itself a single atom of oxygen.[103] The same chemist observes, that if lead be associated with tin, it will be incapable of furnishing to acids any saturnine impregnation, since the latter, being more oxidable than the former, will exclusively dissolve, and thus prevent the former from being attacked.
Whether the principle of Voltaic protection be applicable or not to the purpose of preserving copper sheathing, it is evident that it will suggest numerous other expedients of high importance in the arts, while it will explain phenomena previously unintelligible. By introducing a piece of zinc, or tin, into the iron boiler of the steam-engine, we may prevent the danger of explosion, which generally arises, especially where salt-water is used, as in those of steam-boats, from the wear of one part of the boiler. Another important application is in the prevention of the wear of the paddles, or wheels, which are rapidly dissolved by salt-water.
Mr. Pepys has also extended the principle for the preservation of steel instruments by guards of zinc: razors and lancets may be thus defended with perfect success. In the construction of monuments which are to transmit to posterity the record of important events, the artist will be careful in avoiding the contact of different metals: it is thus that the Etruscan inscriptions, engraved upon pure lead, are preserved to the present day; while medals of mixed metals of a much more recent date are corroded.
Numerous are the facts daily presented before us, which receive from this principle a satisfactory explanation. To the philosopher, the examination of its agencies will furnish a perpetual source of instruction and amusement; and I will here enumerate a few simple instances of its effects: in the first place, for the purpose of showing that, whenever a principle or discovery involves or unfolds a law of Nature, its applications are almost inexhaustible, and that, however abstracted it may appear, it is sooner or later employed for the common purposes of life; and in the next place, in the hope of convincing the reader, that there does not exist any source of pleasure so extensive and so permanent as that derived from the stores of philosophy. The saunterer stumbles over the stone that may cross his path, and vents only his vexation at the interruption; but to the philosopher there is not a body, animate or inanimate, with which he can come in contact, that does not yield its treasures at his approach, and contribute to extend the pleasures of his existence.
I well remember some years ago, that, in passing through Deptford, my curiosity was excited by the extraordinary brilliancy of a portion of the gilded sign of an inn in that town, while its other parts had entirely lost their metallic lustre. Having obtained a ladder, I ascended to the sign, in order, if possible, to solve the problem that had so greatly interested me: the mystery immediately vanished; for an iron nail appeared in the centre of the spot, which had protected the copper leaf for several inches around it. Any person may easily satisfy himself of the efficacy of such protection, in his rambles through the metropolis, by noticing the gilded, or rather coppered, sugar-loaves[104] so commonly suspended over the shops of grocers, when he will frequently perceive that the parts into which the iron supports have entered, unless the latter have been painted, shine preeminently brilliant. If a still more familiar example of the effect of a simple Voltaic circuit be required, it is afforded by the iron palisadoes, where the iron is constantly corroded at its point of contact with the lead by which it is cemented into the stone. These examples are not only interesting from their simplicity, but from their demonstrating the small quantity of a conducting fluid which is sufficient to transmit the electrical power, or to complete a simple circuit: a fact which, it will be remembered, the experiments of Davy had before established.[105]
As our knowledge advances, these principles will no doubt derive other illustrations, and be found capable of more extensive application; for as yet we are but in the infancy of the enquiry. I have lately been engaged in a series of experiments, the results of which, I confidently anticipate, will lead to some new facts connected with the changes produced on the negative metal of a Voltaic circuit; an account of which I hope shortly to submit to the Royal Society. I shall on this occasion merely notice one result, which appears to me to admit of an immediate application to one of the most important circumstances of life—the purity of water contained in leaden cisterns.
My attention has for several years been directed to the state of the water with which the metropolis is supplied; and upon having been lately requested to propose a remedy for preventing the action of a spring in the neighbourhood of London upon lead, which it had been found to corrode in a very rapid manner, I suggested the expediency of protecting the pipes and cisterns with surfaces of iron; but before such a plan was put in execution, I proposed to try its efficacy in the laboratory:—the first result was very startling; for, instead of preventing, as I had anticipated, I found that it greatly increased, the solution of the lead. After various experiments, I arrived at the conclusion, that lead, when rendered negative by iron, and placed in contact with weak saline solutions,—such, for instance, as common spring water,—was dissolved; in consequence of the decomposition of the salts and the transference of their elements according to the general law, the acid passing to the iron, and the alkali to the lead; and so powerfully is this latter body acted upon by an alkali, that, if a slip of it be immersed in a solution of potash or soda, its crystalline texture is so rapidly developed, that its surface exhibits an appearance similar to that presented by tin-plate, and which is designated by the term moirée.