320 HISTORY OF SCIENCE. 



The last paper which Gray communicated to the Royal Society was 

 dictated by him on his death-bed to Dr. Cromwell Mortimer. He 

 died in 1736; and had he lived to revise his experiments, he would 

 doubtless have come to different conclusions. He, like Guericke, at- 

 tempted to assimilate the motions of the planets to the movements of 

 electrified bodies. He was persuaded that an electrified body could 

 cause another to revolve round in an ellipse, and from west to east. 

 "Place," he said, "a feebly electrified small iron ball, an inch or an 

 inch and a half in diameter, on the centre of a circular plate of resin 

 seven or eight inches in diameter, and then a light body suspended 

 by a very fine thread five or six inches long, held in the hand above 

 the centre of the plate, will begin to move in a circle round the iron 

 ball, and always from west to east. If the globe is placed at a little 

 distance from the centre of the circular plate, the small body will de- 

 scribe an ellipse, the eccentricity of which will be the distance of the 

 ball from the centre of the plate." If the plate of resin is of an ellip- 

 tical shape and the iron ball be placed in its centre, the light body 

 will describe an elliptical orbit of the same form as the plate." These 

 experiments were quite fallacious, although Dr. Mortimer, repeating 

 them after Gray's death, tried and, as he thought, confirmed their ac- 

 curacy. The error of these observers is attributed to the now well- 

 known psychological action by which the movements are determined 

 in the familiar oracle of the " Bible and key," and similar cases. The 

 idea present in the mind of the experimenter, that the suspended body 

 would move from west to east, and that it would describe such and 

 such an orbit, was sufficient to cause an unconscious impulse to be 

 communicated from the finger holding the thread. In fact, it was 

 soon found that when the thread was attached to a support, instead 

 of being held by the finger, these phenomena were no longer observed. 



The foundation of electrical science may be said to have been laid 

 by Gray's experiments; but some important discoveries were soon 

 announced by a French man of science, and he besides supplied the 

 infant science with its first theory. Perhaps this is but another illus- 

 tration of the tendency to generalization which distinguishes the French 

 intellect. In this case the theoretical views have been singularly appro- 

 priate to the facts, for Du Fay's theory has not been superseded even at 

 the present day. Du FAY (1698 1739) was originally a military officer, 

 but after some years' active service he quitted the army, and devoted 

 himself wholly to scientific pursuits. He was an active member of the 

 Academy of Sciences, and was appointed to the superintendence of 

 the Royal Botanical Gardens at Paris, in which capacity he was the 

 predecessor of the celebrated Buffon. Du Fay was first induced to 

 study electricity by the writings of Gray and Hawksbee, and by these 

 he was put in the track of the following discoveries. He found that 

 all bodies (except, he thought, metallic, fluid, or soft ones) could be 

 made electric by friction with a cloth, provided they were previously 



