300 THE POPULAR SCIENCE MONTHLY, 



was tlie complement of a slow development. The first natural philos- 

 ophers, 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 scholas- 

 tic 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 Ke23ler 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 Psy- 

 chology 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 con- 

 ception of causation superseded those of sequence and conjunction ; 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 ac- 

 cordingly 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 ena- 

 bled Davy in 1806 to make the generalizations which founded electro- 

 chemistry. 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 " Encyclopedic," Theologie Scholastlque, s. v. 



2 Hallam, " Literature of Europe," iii. (edition 1872), p. 415. 



3 Grant, "History of Physical Astronomy," pp. 16, 17, 29. 



4 Bain, " Mental and Moral Science," p. 633. 



5 Whewell. '* History of the Inductive Sciences," iii., pp. 157-159, 141, 142, 145. 



