Shortly after a new light was struck by Harriott and Descartes, with their contemporaries, or immediate predecessors, and the restoration of ancient geometry, aided by the modern invention of algebra, placed the science of mechanism on the philosophic throne. How widely this domination spread, and how long it continued, if, indeed, even now it can be said to have abdicated its pretensions, the reader need not be reminded. The sublime discoveries of Newton, and, together with these, his not less fruitful than wonderful application, of the higher mathesis to the movements of the celestial bodies, and to the laws of light, gave almost a religious sanction to the corpuscular system and mechanical theory. It became synonymous with philosophy itself. It was the sole portal at which truth was permitted to enter. The human body was treated of as an hydraulic machine, the operations of medicine were solved, and alas! even directed by reference partly to gravitation and the laws of motion, and partly by chemistry, which itself, however, as far as its theory was concerned, was but a branch of mechanics working exclusively by imaginary wedges, angles, and spheres. Should the reader chance to put his hand on the "Principles of Philosophy," by La Forge, an immediate disciple of Descartes, he may see the phenomena of sleep solved in a copper-plate engraving, with all the figures into which the globules of the blood shaped themselves, and the results demonstrated by mathematical calculations. In short, from the time of Kepler(3) to that of Newton, and from Newton to Hartley, not only all things in external nature, but the subtlest mysteries of life and organization, and even of the intellect and moral being, were conjured within the magic circle of mathematical formulæ. And now a new light was struck by the discovery of electricity, and, in every sense of the word, both playful and serious, both for good and for evil, it may be affirmed to have electrified the whole frame of natural philosophy. Close on its heels followed the momentous discovery of the principal gases by Scheele and Priestly, the composition of water by Cavendish, and the doctrine of latent heat by Black. The scientific world was prepared for a new dynasty; accordingly, as soon as Lavoisier had reduced the infinite variety of chemical phenomena to the actions, reactions, and interchanges of a few elementary substances, or at least excited the expectation that this would speedily be effected, the hope shot up, almost instantly, into full faith, that it had been effected. Henceforward the new path, thus brilliantly opened, became the common road to all departments of knowledge: and, to this moment, it has been pursued with an eagerness and almost epidemic enthusiasm which, scarcely less than its political revolutions, characterise the spirit of the age. Many and inauspicious have been the invasions and inroads of this new conqueror into the rightful territories of other sciences; and strange alterations have been made in less harmless points than those of terminology, in homage to an art unsettled, in the very ferment of imperfect discoveries, and either without a theory, or with a theory maintained only by composition and compromise. Yet this very circumstance has favoured its encroachments, by the gratifications which its novelty affords to our curiosity, and by the keener interest and higher excitement which an unsettled and revolutionary state is sure to inspire. He who supposes that science possesses an immunity from such influences knows little of human nature. How, otherwise, could men of strong minds and sound judgments have attempted to penetrate by the clue of chemical experiment the secret recesses, the sacred adyta of organic life, without being aware that chemistry must needs be at its extreme limits, when it has approached the threshold of a higher power? Its own transgressions, however, and the failure of its enterprises will become the means of defining its absolute boundary, and we shall have to guard against the opposite error of rejecting its aid altogether as analogy, because we have repelled its ambitious claims to an identity with the vital powers.