We have here, then, another striking instance of a great effect produced by means apparently insignificant. Who could have imagined it possible, that the spasmodic action occasioned in the limb of a frog, by the accidental contact of a pair of scissors, should have become the means of changing the whole theory of chemistry--of discovering substances, whose very existence was never suspected--of explaining the anomalous associations of mineral bodies in the veins of the earth--of protecting surfaces of metal from the corrosive action of the elements--of elucidating the theories of volcanoes and earthquakes--and, may we not add? of leading the way to a knowledge of the laws of terrestrial magnetism! Such an unexpected extension of an apparently useless fact should dispose us to entertain a kinder regard for the labours of one another, and teach us to judge with diffidence of the abstract results of science. A discovery which may appear incapable of useful application to-day, may be our glory to-morrow,--it may even change the face of empires, and wield the destiny of nations. The conic sections of Apollonius Pergæus remained useless for two thousand years: who could have supposed that, after the lapse of twenty centuries, they would have formed the basis of astronomy?--a science giving to navigation safety, guiding the pilot through unknown seas, and tracing for him in the heavens an unerring path to his native shores. Some apology may be necessary for this digression; but, I confess, the subject has always appeared to me to be capable of much interesting illustration, and I heartily concur in the opinion expressed by the accomplished author of "Lettres à Sophie"--"_L'Histoire des grands effets par les petites causes ferait un livre bien curieux._" CHAPTER VI. The History of Galvanism divided into six grand Epochs.--Davy extends the experiment of Nicholson and Carlisle.--His Pile of one metal and two fluids.--Dr. Wollaston advocates the doctrine of oxidation being the primary cause of Voltaic Phenomena.--Davy's modification of that theory.--His Bakerian Lecture of 1806.--He discovers the sources of the Acid and Alkaline matter eliminated from water by Voltaic action.--On the nature of Electrical decomposition and transfer.--On the relations between the Electrical energies of bodies, and their Chemical Affinities.--General developement of the Electro-chemical Laws.--Illustrations, Applications, and Conclusions. The History of Galvanism may be divided into six grand epochs; each being distinguished by the discovery of facts variously interesting from their novelty, and from the extent and importance of their applications.