The subject of the Tides is, in like manner, destitute of any name which designates the science concerned about it. I have ventured to employ the term _Tidology_, having been much engaged in tidological researches. Many persons possess a peculiarity of vision, which disables them from distinguishing certain colours. On examining many such cases, we find that in all such persons the peculiarities are the same; all of them confounding scarlet with green, and pink with blue. Hence they form a class, which, for the convenience of physiologists and others, ought to have a fixed designation. Instead of calling them, as has usually been done, 'persons having a peculiarity of vision,' we might take a Greek term implying this meaning, and term them _Idiopts_. But my business at present is not to speak of the selection of new terms when they are introduced, but to illustrate the maxim that the necessity for their introduction often arises. The construction of new terms will be treated of subsequently. {285} APHORISM VIII. _Terms must be constructed and appropriated so as to be fitted to enunciate simply and clearly true general propositions._ THIS Aphorism may be considered as the fundamental principle and supreme rule of all scientific terminology. It is asserted by Cuvier, speaking of a particular case. Thus he says[24\4] of Gmelin, that by placing the lamantin in the genus of morses, and the siren in the genus of eels, he had rendered every general proposition respecting the organization of those genera impossible. [Note 24\4: _Règne Animal_, Introd. viii.] The maxim is true of words appropriated as well as invented, and applies equally to the mathematical, chemical, and classificatory sciences. With regard to most of these, and especially the two former classes, it has been abundantly exemplified already, in what has previously been said, and in the _History of the Sciences_. For we have there had to notice many technical terms, with the occasions of their introduction; and all these occasions have involved the intention of expressing in a convenient manner some truth or supposed truth. The terms of Astronomy were adopted for the purpose of stating and reasoning upon the relations of the celestial motions, according to the doctrine of the sphere, and the other laws which were discovered by astronomers. The few technical terms which belong to Mechanics, _force_, _velocity_, _momentum_, _inertia_, &c., were employed from the first with a view to the expression of the laws of motion and of rest; and were, in the end, limited so as truly and simply to express those laws when they were fully ascertained. In Chemistry, the term _phlogiston_ was useful, as has been shown in the _History_, in classing together processes which really are of the same nature; and the nomenclature of the _oxygen_ theory was still preferable, because it enabled the chemist to express a still greater number of general truths. {286}