29 ) 



CHAPTEE II. 



ELECTRIC AND MAGNETIC SCIENCE PRIOR TO THE INTRODUCTION 

 OF THE POTENTIALS. 



THE magnetic discoveries of Peregrinus and Gilbert, and the 

 vortex-hypothesis by which Descartes had attempted to explain 

 them,* had raised magnetism to the rank of a separate science 

 by the middle of the seventeenth century. The kindred science 

 of electricity was at that time in a less developed state ; but it 

 had been considerably advanced by Gilbert, whose researches in 

 this direction will now be noticed. 



For two thousand years the attractive power of amber had 

 been regarded as a virtue peculiar to that substance, or possessed 

 by at most one or two others. Gilbert provedf this view to be 

 mistaken, showing that the same effects are induced by friction 

 in quite a large class of bodies ; among which he mentioned 

 glass, sulphur, sealing-wax, and various precious stones. 



A force which was manifested by so many different kinds of 

 matter seemed to need a name of its own; and accordingly 

 Gilbert gave to it the name electric, which it has ever since 

 retained. 



Between the magnetic and electric forces Gilbert remarked 

 many distinctions. The lodestone requires no stimulus of friction 

 such as is needed to stir glass and sulphur into activity. 

 The lodestone attracts only magnetizable substances, whereas 

 electrified bodies attract everything. The magnetic attraction 

 between two bodies is not affected by interposing a sheet of 

 paper, or a linen cloth, or by immersing the bodies in water j 

 whereas the electric attraction is readily destroyed by screens. 

 Lastly, the magnetic force tends to arrange bodies in definite 



*Cf. pp. 7-9. t De Magnete, lib. ii., cap. 2. 



