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266 On the Use of Definitio72s. A most curious assemblage of optical phenomena have attracted attention of late years, which have been grouped under the term "polarisation.'"' The phenomena are some- what complex, and the theory of them was, at least till lately^ unexplained ; so that no very rapid or popular exposition of them was possible. In consequence of this it happened, that when a person to whom the word polarisation was new, enquired the meaning of it, there was generally found some one, who, too well informed to suspect a latent meaning, would answer " It is something of which the philosophers themselves know nothing ; they call it polarisation; they might as well call it iv, an unknown quantity/' Yet those who had attended to the subject a little more patiently, knew that this word, though with something of vagueness, indicated very significantly both the general character of the facts, and the history of the attempts made to explain them. It might be difficult to give a definition of the term ; but it implied a general circumstance belonging to all the experi- ments ; namely " an opposition of properties, associated with an opposition of positions ; a circumstance common to these facts, and to those of magnetic polarity. Now that the undulatory theory of light is conceived to be satisfactorily established, we may, if we please, say that " a ray is po- larised in a certain plane, when it consists of vibrations perpendicular to that plane ; but we may presume that no one will assert, that the indistinctness of ideas which for- merly prevailed upon this subject, existed because it did not occur to any one to propound this definition. The de- finition is a result of the establishment of the theory. As we advance to sciences which are as yet in a more incomplete state, it becomes more and more evident how impossible it is for us to possess exact definitions, except in proportion as our knowledge becomes general and systematic. When did Chemistry acquire that symmetrical nomenclature which has been so much admired? The moment that Lavoisier had established the true theory of the combina- tions of elements with the acidifying principle. His account of the composition of his treatise is remarkable. " While I thought myself employed,*" he says, '^ onlv in forming a nomenclature, and while I purposed to myself nothing