PHYSIOLOGY OF LIFE. 87 



demanding his assent as a right, I must solicit a suspen- 

 sion of his judgment as a courtesy ; and, after all, however 

 firmly the hypothesis may support the phenomena piled 

 upon it, we can deduce no more than a practical rule, 

 grounded on a strong presumption. The license of 

 arithmetic, however, furnishes instances that a rule may 

 be usefully applied in practice, and for the particular 

 purpose may be sufficiently authenticated by the result, 

 before it has itself been duly demonstrated. It is enough, 

 if only it hath been rendered fully intelligible. 



In a system where every position proceeds from a 

 scientific preconstruction, a power acting exclusively in 

 length, would be magnetism by virtue of our own defini- 

 tion of the term. In like manner, a surface power would 

 be electricity, as far as that system was concerned, whether 

 it accorded or not with the facts ordinarily so called. But 

 it is incumbent on us, who must treat the subject 

 analytically, to show by experiment that magnetism does 

 in fact act longitudinally, and electricity superficially ; and 

 that, consequently, the former is distinguished from, and 

 yet contained in, the latter, as a straight line is distin- 

 guished from, yet contained in, a superficies. 



First, that magnetism, in its conductors, seeks and 

 follows length only, and by the length is itself conducted, 

 has been proved by Brugmans, in his philosophical Essay 

 on the Matter of Magnetism, where he relates that a 

 magnet capable of supporting a body four times heavier 

 than itself, and which acted as a magnetic needle at the 

 distance of twenty inches, was so weakened by the inter- 

 position of three cast-iron plates of considerable thickness, 

 as scarcely to move the magnetic needle from its place at 



