52 theories of Magnetifm. [Book I. 



The phenomena of magnetifm (land alone among 

 the wonders of philofophy, unlefs we fuppofe the at- 

 traction of gravitation to be a fpec-ies of genera] mag- 

 netifm, which indifferently affects all the various bo- 

 dies of which this univcrfe is compofed. Certain ana- 

 logies have been traced, or rather imagined, between 

 electricity and magnetifm. Both powers are .excit- 

 ed by friction ; and the magnetic polarity has been 

 compared to the two ftates of pofitive and negative 

 electricity. The analogy is favoured alfo by the pofli- 

 bility of imparting the magnetic virtue to iron by the 

 electric (hock j and the Aurora Borealis, which is ge- 

 nerally accounted an electrical phenomenon, is fuppofed 

 to have fome influence on the variation of the mag- 

 netic needle. Thefe powers, neverthelefs, I mud 

 avow, appear to me effentially different. The phe- 

 nomena of electricity are not at all times exhibited 

 by electrical bodies, but merely when thofe bodies are 

 in a ftate of excitation. When the electrical virtue is 

 imparted from one body to another, the body that 

 imparts it lofes proportionably of its own power, but 

 the magnet rather increafes than diminimes its ftrength 

 by communication. The electric matter is vifible ; 

 whereas the very exiftence of a magnetic fluid is juftly 

 questionable ; befides that electricity, both in its na- 

 ture and effects, bears fo clofe an analogy to another 

 apparently very different power in nature, that there 

 can be no reafon for referring it to one with which 

 it appears to have a very flight, and, moft probably, 

 only a cafual, agreement. 



The polarity of the magnet, as well as the dipping 

 of the needle, are in all probability mere effects of its 

 great property, attraction, fince they appear to be fairly 

 accounted for, frorfr the ftrong and peculiar attraction 

 which the magnet appears to have for the earth, or 



rather 



