Our business at present, however, is not with instruments which might be devised for measuring sensible qualities, but with those which have been so used, and have thus been the basis of the sciences in which {356} such qualities are treated of; and this we have now done sufficiently for our present purpose. 24. There is another Idea which, though hitherto very vaguely entertained, has had considerable influence in the formation, both of the sciences spoken of in the present Book, and on others which will hereafter come under our notice: namely, the Idea of Polarity. This Idea will be the subject of the ensuing Book. And although this Idea forms a part of the basis of various other extensive portions of science, as Optics and Chemistry, it occupies so peculiarly conspicuous a place in speculations belonging to what I have termed the Mechanico-Chemical Sciences, (Magnetism and Electricity,) that I shall designate the discussion of the Idea of Polarity as the Philosophy of those Sciences. {{357}} BOOK V. THE PHILOSOPHY OF THE MECHANICO-CHEMICAL SCIENCES. En donnant à ces côtés le nom de _poles_, j'appelerai _polarisation_ la modification qui donne à la lumière des propriétés relatives à ces poles. J'ai tardé jusqu'à présent à admettre ce terme dans la description des phénomènes physiques dont il est question; je n'ai pas osé l'introduire dans les mémoires où j'ai publié mes dernières expériences; mais les variétés qu'offre ce nouveau phénomène, et la difficulté de les décrire, me forcent à admettre cette nouvelle expression, qui signifie simplement la modification que la lumière a subie en acquérant de nouvelles propriétés qui ne sont pas relatives à la direction du rayon, mais seulement à ses côtés considérés à angles droits et dans un plan perpendiculaire à sa direction. Malus (1811), _Mém. de Inst._ tom. xi. p. 106. {{359}} BOOK V. THE PHILOSOPHY OF THE MECHANICO-CHEMICAL SCIENCES. CHAPTER I. ATTEMPTS AT THE SCIENTIFIC APPLICATION OF THE IDEA OF POLARITY. 1. IN some of the mechanical sciences, as Magnetism and Optics, the phenomena are found to depend upon position (the position of the magnet, or of the ray of light,) in a peculiar alternate manner. This dependence, as it was first apprehended, was represented by means of certain conceptions of space and force, as for instance by considering the two _Poles_ of a magnet. But in all such modes of representing these alternations by the conceptions borrowed from other ideas, a closer examination detected something superfluous and something defective; and in proportion as the view which philosophers took of this relation was gradually purified from these incongruous elements, and was rendered more general and abstract by the discovery of analogous properties in new cases, it was perceived that the relation could not be adequately apprehended without considering it as involving a peculiar and independent Idea, which we may designate by the term _Polarity_.