PROGRESS OF ELECTRICAL THEORY. 213 



electricitv ; to which we may add, the connexion of electricity with 

 magnetism and with chemistry ; a vast field, as yet dimly seen. 

 Now, even with regard to the simplest of these questions, the cause of 

 the retention of electricity at the surface of bodies, it appears to be 

 impossible to maintain Coulomb's opinion, that this is effected by the 

 resistance of air to the passage of electricity. The other questions an. 

 such as Coulomb did not attempt to touch ; they refer, indeed, princi- 

 pally to laws not suspected at his time. How wide and profound a 

 theory must be which deals worthily with these, we shall obtain some 

 indications in the succeeding part of our history. 



But it may be said on the other side, that we have the evidence of 

 our senses for the reality of an electric fluid ; we see it in the spark ; 

 we hear it in the explosion ; we feel it in the shock; and it prodii'-- 

 the effects of mechanical violence, piercing and tearing the bodies 

 through which it passes. And those who arc disposed to assert a 

 real fluid on such grounds, may appear to be justified in doing so, by 

 one of Newton's " Pailes of Philosophizing/' in which he directs the 

 philosopher to assume, in his theories, " causes which are true." The 

 usual interpretation of a " vera causa," has been, that it implies causes 

 which, independently of theoretical calculations, are known to exist by 

 their mechanical effects ; as gravity was familiarly known to exist on 

 the earth, before it was extended to the heavens. The electric fluid 

 might seem to be such a vera causa. 



o 



To this I should venture to reply, that this reasoning shows how 

 delusive the Newtonian rule, so interpreted, may be. For a moment's 

 consideration will satisfy us that none of the circumstances, above ad- 

 duced, can really prove material currents, rather than vibrations, or 

 other modes of agency. The spark and shock are quite insufficient to 

 supply such a proof. Sound is vibrations, light is vibrations ; vibra- 

 tions may affect our nerves, and may rend a body, as when glasses arc 

 broken by sounds. Therefore all these supposed indications of the 

 reality of the electric fluid are utterly fallacious. In truth, this mode 

 of applying Newton's rule consists in elevating our first rude and 

 unscientific impressions into a supremacy over the results of calcula- 

 tion, generalization, and systematic induction. 16 



15 On the subject of this Newtonian Rule of Philosophizing, see further Phil. 

 Ind. Sc. B. xii. c. 13. I have given au account of the history and evidence of 

 the Theory of Electricity in the Reports of the British Association for 1835. I 

 may seem there to have spoken more favorably of the Theory aa a Physical 



