PROGRESS OF MAGNETIC THEORY. 233 



shows, too, the necessity of bringing facts of all kinds to bear on the 

 hypothesis ; thus, in this case it was requisite to take into account the 

 facts of junction and separation of magnetic bodies, as well as their 

 attractions and repulsions. 



* If we have seen reason to doubt the doctrine of electric fluids as 

 physical realities, we cannot help pronouncing upon the magnetic fluids 

 as having still more insecure claims to a material existence, even on 



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the grounds just stated. But we may add considerations still more 

 decisive ; for at a further stage of discovery, as we shall see, magnetic 

 and electric action were found to be connected in the closest manner, 

 so as to lead to the persuasion of their being different effects of one 

 common cause. After those discoveries, no philosopher would dream 

 of assumino- electric fluids and magnetic fluids as two distinct material 



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agents. Yet even now the nature of the dependence of magnetism 

 upon any other cause is extremely difficult to conceive. But till we 

 have noticed some of the discoveries to which we have alluded, we 

 cannot even speculate about that dependence. We now, therefore, 

 proceed to sketch the history of these discoveries. 



