We shall trace some of the forms in which this Idea has manifested itself in the history of science. In doing so we shall not begin, as in other Books of this work we have done, by speaking of the notion as it is {360} employed in common use: for the relation of Polarity is of so abstract and technical a nature, that it is not employed, at least in any distinct and obvious manner, on any ordinary or practical occasions. The idea belongs peculiarly to the region of speculation: in persons of common habits of thought it is probably almost or quite undeveloped; and even most of those whose minds have been long occupied by science, find a difficulty in apprehending it in its full generality and abstraction, and stript of all irrelevant hypothesis. 2. _Magnetism._--The name and the notion of _Poles_ were first adopted in the case of a magnet. If we have two magnets, their extremities attract and repel each other alternatively. If the first end of the one attract the first end of the other, it repels the second end, and conversely. In order to express this rule conveniently, the two ends of each magnet are called the _north pole_ and the _south pole_ respectively, the denominations being borrowed from the poles of the earth and heavens. 'These poles,' as Gilbert says[1\5], 'regulate the motions of the celestial spheres and of the earth. In like manner the magnet has its poles, a northern and a southern one; certain and determined points constituted by nature in the stone, the primary terms of its motions and effects, the limits and governors of many actions and virtues.' [Note 1\5: _De Magn._ lib. i. c. iii.] The nature of the opposition of properties of which we speak may be stated thus: The North pole of one magnet attracts the South pole of another magnet. The North pole of one magnet _repels_ the North pole of another magnet. The South pole of one magnet repels the South pole of another magnet. The South pole of one magnet _attracts_ the North pole of another magnet. It will be observed that the contrariety of position which is indicated by putting the South pole for the North pole in either magnet, is accompanied by the {361} opposition of mechanical effect which is expressed by changing attraction into repulsion and repulsion into attraction: and thus we have the general feature of Polarity,--A contrast of properties corresponding to a contrast of positions. 3. _Electricity._--When the phenomena of Electricity came to be studied, it appeared that they involved relations in some respects Analogous to those of magnetism.