The Bakerian Lecture, read before the Royal Society on the 20th of November 1806, not only set this question for ever at rest, but unfolded the mysteries of general Voltaic action; and, as far as theory goes, may almost be said to have perfected our knowledge of the chemical agencies of the pile. This grand display of scientific light burst upon Europe like a splendid meteor, throwing its radiance into the deepest recesses, and opening to the view of the philosopher new and unexpected regions. I shall endeavour to offer as popular a review of this celebrated memoir, as the abstruse and complicated nature of its subjects will allow; and I shall be careful in pointing out the successive stages of the enquiry; for we are all too much in the habit of exclusively looking after results; whereas an examination of the steps by which they were attained is far more important, not only to the fame of the discoverer, but to ourselves, as the means of instruction. The subjects investigated in this memoir are arranged under the following divisions. 1. "On the changes produced in Water by Electricity. 2. "On the agencies of Electricity in the decomposition of various compound Bodies. 3. "On the transfer of certain constituent Parts of Bodies by the action of Electricity. 4. "On the passage of Acids, Alkalies, and other Substances, through various attracting chemical menstrua, by means of Electricity. 5. "Some general Observations on these Phenomena, and on the mode of Decomposition and Transition. 6. "On the General Principles of the chemical changes produced by Electricity. 7. "On the Relations between the Electrical Energies of bodies and their Chemical Affinities. 8. "On the mode of action of the Pile of Volta, with Experimental Elucidations. 9. "On some General Illustrations and Applications of the foregoing facts and principles." With respect to the first of these divisions, comprehending a history of the changes produced in water by electricity, it is worthy of particular notice, that as early as the year 1800, while residing at Bristol, Davy had discovered that when separate portions of distilled water, filling two glass tubes connected by moist bladders, or any moist animal or vegetable substance, were submitted to the electrical action of the Voltaic pile, by means of gold wires, a _nitro-muriatic_ solution of gold appeared in the tube containing the positive wire, and a solution of soda in the opposite tube; but he soon ascertained that the muriatic acid owed its appearance to the animal or vegetable matters employed; for when the same fibres of cotton were used in successive experiments, and washed after every process in a weak solution of nitric acid, the water in the apparatus containing them, though acted upon for a great length of time with a very strong power, produced no effect upon a solution of nitrate of silver.