232 HISTORY OF ELECTRICITY. 



The various other circumstances which terrestrial magnetism ex- 

 hibits, the diurnal and annual changes of the position of the compass- 

 needle : the larger secular change which affects it in the course of 



/ o o 



years ; the difference of intensity at different places, and other facts, 

 have naturally occupied philosophers with the attempt to determine, 

 both the laws of the phenomena and their causes. But these attempts 

 necessarily depend, not upon laws of statical magnetism, such as they 

 have been explained above ; but upon the laws by which the produc- 

 tion and intensity of magnetism in different cases are regulated; 

 laws which belong to a different province, and are related to a different 

 set of principles. Thus, for example, we have not attempted to 

 explain the discovery of the laws by which heat influences mag- 

 netism ; and therefore we cannot now give an account of those 

 theories of the facts relating to terrestrial magnetism, which, depend 

 upon the influence of temperature. The conditions of excitation of 

 magnetism are best studied by comparing this force with other cases 

 where the same effects are produced by very different apparent agen- 

 cies ; such as galvanic and thermo-electricity. To the history of these 

 we shall presently proceed. 



Conclusion. The hypothesis of magnetic fluids, as physical reali- 

 ties, was never widely or strongly embraced, as that of electric fluids 

 was. For though the hypothesis accounted, to a remarkable degree 

 of exactness, for large classes of the phenomena, the presence of a 

 material fluid was not indicated by facts of a different kind, such as 

 the spark, the discharge from points, the shock, and its mechanical 

 effects. Thus the belief of a peculiar magnetic fluid or fluids was not 

 forced upon men's minds ; and the doctrine above stated was probably 

 entertained by most of its adherents, chiefly as a means of expressing 

 the laws of phenomena in their elementary form. 



One other observation occurs here. We have seen that the suppo- 

 sition of a fluid moveable from one part of bodies to another, and 

 capable of accumulation in different parts of the surface, appeared at 

 first to be as distinctly authorized by magnetic as by electric phenome- 

 na ; and yet that it afterwards appeared, by calculation, that this must 

 be considered as a derivative result: no real transfer of fluid taking 



* o 



place except within the limits of the insensible particles of the body. 

 Without attempting to found a formula of philosophizing on this cir- 

 cumstance, we may observe, that this occurrence, like the disproof of 

 heat as a material fluid, shows the possibility of an hypothesis which 

 shall very exactly satisfy many phenomena, and yet be incomplete : it 



