8. _General Remarks._--Thus, as we see, the notion of Polarity is applicable to many large classes of phenomena. Yet the Idea in a distinct and general form is only of late growth among philosophers. It has gradually been abstracted and refined from many extraneous hypotheses which were at first supposed to be essential to it. We have noticed some of these hypotheses;--as the poles of a _body_; the poles of the _particles_ of a fluid; _two_ opposite fluids; a single fluid in _excess_ and _defect_; transverse _vibrations_. To these others might be added. Thus Dr. Prout[5\5] assumes that the polarity of molecules results from their _rotation_ on their axes, the opposite motions of contiguous molecules being the cause of opposite (positive and negative) polarities. [Note 5\5: _Bridgewater Treatise_, p. 559.] But none of these hypotheses can be proved by the fact of Polarity alone; and they have been in succession rejected when they had been assumed on that ground. Thus Davy, in 1826, speaking of chemical forces says[6\5], 'In assuming the idea of two ethereal, subtile, elastic {369} fluids, attractive of the particles of each other, and repulsive as to their own particles, capable of combining in different proportions with bodies, and according to their proportions giving them their specific qualities and rendering them equivalent masses, it would be natural to refer the action of the poles to the repulsions of the substances combined with the excess of one fluid, and the attractions of those united to the excess of the other fluid; and a history of the phenomena, not unsatisfactory to the reason, might in this way be made out. But as it is possible likewise to take an entirely different view of the subject, on the idea of the dependence of the results upon the primary attractive powers of the parts of the combination on a single subtile fluid, I shall not enter into any discussion on this obscure part of the theory.' Which of these theories will best represent the case, will depend upon the consideration of other facts, in combination with the polar phenomena, as we see in the history of optical theory. In like manner Mr. Faraday proved by experiment[7\5] the errour of all theories which ascribe electro-chemical decomposition to the attraction of the poles of the voltaic battery. [Note 6\5: _Phil. Tr._ 1826, p. 415.] [Note 7\5: _Researches_, p. 495, &c.]