CHAPTER II. 

 PROGRESS OF MAGNETIC THEORY. 



THEORY of Magnetic Action. The assump- 

 tion of a fluid, as a mode of explaining the 

 phenomena, was far less obvious in magnetic than 

 in electric cases, yet it was soon arrived at. After 

 the usual philosophy of the middle ages, the "forms" 

 of Aquinas, the " efflux" of Cusanus, the " vapours" 

 of CostaBus, and the like, which are recorded by 

 Gilbert 1 , we have his own theory, which he also 

 expresses by ascribing the effects to a " formal effi- 

 ciency;" a "form of primary globes; the proper 

 entity and existence of their homogeneous parts, 

 which we may call a primary and radical and astral 

 form" of which forms there is one in the sun, 

 one in the moon, one in the earth, the latter being 

 the magnetic virtue. 



Without attempting to analyze the precise im- 

 port of these expressions, we may proceed to Des- 

 cartes's explanation of magnetic phenomena. The 

 mode in which he presents this subject 2 is, perhaps, 

 the most persuasive of his physical attempts. If a 

 magnet be placed among iron filings, these arrange 

 themselves in curve lines, which proceed from one 

 pole of the magnet to the other. It was not diffi- 

 1 Gilb. lib. ii. c. 3, 4. Prin. Phil, pars c. iv. 146. 



