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] [104] There is an excellent example at this time in the London Road leading to the Elephant-and-Castle. [105] Page 248 of this Volume. 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.