was the complement of a slow development. The first natural philosophers, down even to Kepler and Galileo, had contented themselves with studying effects, e. g., the orbits described by the heavenly bodies, and the period of their revolutions. But, with the decay of the scholastic metaphysics, which was also physics, a new idea began to stir the minds of men—that of force. It is said to have been conceived by Nicolas of Cusa; it found tortuous expression in Descartes's Vortices; and, specialized as governing gravitation, it was perhaps first dimly seen by Gilbert little less than a century before Newton, was asserted by Kepler nine years later (1609), and in 1674 was stated by Hooke with remarkable clearness and accuracy—all before Newton had thrown out any hint of his sublime discovery. Hartley's hypothesis, on the other hand, was a chance shot, a private guess, and was no matured result of previous theorizing. It accordingly passed into the limbo to which Nature consigns her mistakes; but the gain to Psychology was, though not equally great, of fundamentally the same kind as the gain to Natural Philosophy from the establishment of the law of gravitation. The idea of force subsumed that of law, the conception of causation superseded those of sequence and conjunction; and the basis for an explanation of the phenomena of mind was for the first time sought outside the limits of these phenomena. Hartley was unsuccessful, but the mere attempt has been as a light on high to guide the uncertain steps of later inquirers, and has at last led to the physical syntheses of our own day.
Even a false, or at least a partially true, theory has the advantage of making possible a reasoned arrangement of the facts, as well as the acquisition of more. To Hartley this hypothesis of vibrations gave strength of wing to sweep the entire field of Psychology, and we accordingly find that his was the first systematic effort to explain the phenomena of mind by the law of association.
A very great advance in Psychology was made by James Mill, and it was initiated by Chemistry. During the first ten years of the nineteenth century Chemistry was revolutionized. In 1800 Nicholson and Carlisle decomposed water by means of the Voltaic pile, and enabled Davy in 1806 to make the generalizations which founded electrochemistry. The decomposition of potassa, soda, and other bodies of the same kind, soon followed. Beginning with hydrochloric acid in 1809, the discovery of the various hydracids was made. And in 1803-'04 a great synthetic addition was made to the analytic gains; Dalton's law of chemical combination was established." The influence of these brilliant discoveries upon the thought of the age was not
- Morin, in Migne's "Encyclopédie," Théologie Scholastique, s. v.
- Hallam, "Literature of Europe," iii. (edition 1872), p. 415.
- Grant, "History of Physical Astronomy," pp. 16, 17, 29.
- Bain, "Mental and Moral Science," p. 633.
- Whewell. "History of the Inductive Sciences," iii., pp. 157-159, 141, 142, 145.