220 HISTOKY OF MAGNETISM. 



though in a weaker degree than the magnet itself, 8 and lie asserted dis- 

 tinctly that the magnet is merely an ore of iron, (lib. i. c. 16, Quod 

 inagnes et vena ferri idem sunt.) He also noted the increased energy 

 which magnets acquire by being armed ; that is, fitted with a cap of 

 polished iron at each pole.' But we do not find till a later period any 

 notice of the distinction which exists between the niagnetical proper- 

 ties of soft iron and of hard steel ; the latter being susceptible of being 

 formed into artificial magnets, with permanent poles ; while soft iron 

 is only passively magnetic, receiving a temporary polarity from the 

 action of a magnet near it, but losing this property when the magnet 

 is removed. About the middle of the last century, various methods 

 were devised of making artificial magnets, which exceeded in power 

 all magnetic bodies previously known. 



The remaining experimental researches had so close an historical 

 connexion with the theory, that they will be best considered along 

 with it, and to that, therefore, we now proceed. 



CHAPTER II. 

 PROGRESS OF MAGNETIC THEORY. 



OF MAGNETIC ACTION. The assumption of a fluid, as a 

 -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 phi- 

 losophy of the middle ages, the " forms" of Aquinas, the " efflux" of 

 Cusanus, the " vapors" of Costseus, 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 efficiency ;" 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 analyse the precise import of these expres- 

 sions, w r e may proceed tc Descartes's explanation of magnetic pheno- 

 mena. The mode in which he presents this subject 2 is, perhaps, the 



e Lib. i. c. 913. 9 Lib. ii. c. 17. 



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



