IT is not unfrequently stated that the circumstances from which the names employed in science borrow their meaning, ought to be facts and not theories. But such a recommendation implies a belief that facts are rigorously distinguished from theories and directly opposed to them; which belief, we have repeatedly seen, is unfounded. When theories are firmly established, they become facts; and names founded on such theoretical views are unexceptionable. If we speak of the _minor_ {295} _axis_ of Jupiter's _orbit_, or of his _density_, or of _the angle of refraction_, or _the length of an undulation_ of red light, we assume certain theories; but inasmuch as the theories are now the inevitable interpretation of ascertained facts, we can have no better terms to designate the conceptions thus referred to. And hence the rule which we must follow is, not that our terms must involve no theory, but that they imply the theory only in that sense in which it is the interpretation of the facts. For example, the term _polarization_ of light was objected to, as involving a theory. Perhaps the term was at first suggested by conceiving light to consist of particles having poles turned in a particular manner. But among intelligent speculators, the notion of polarization soon reduced itself to the simple conception of opposite properties in opposite positions, which is a bare statement of the fact: and the term being understood to have this meaning, is a perfectly good term, and indeed the best which we can imagine for designating what is intended. I need hardly add the caution, that names involving theoretical views not in accordance with facts are to be rejected. The following instances exemplify both the positive and the negative application of this maxim. The distinction of _primary_ and _secondary_ rocks in geology was founded upon a theory; namely, that those which do not contain any organic remains were first deposited, and afterwards, those which contain plants and animals. But this theory was insecure from the first. The difficulty of making the separation which it implied, led to the introduction of a class of _transition_ rocks. And the recent researches of geologists lead them to the conclusion, that those rocks which are termed _primary_, may be the newest, not the oldest, productions of nature. In order to avoid this incongruity, other terms have been proposed as substitutes for these. Sir C. Lyell remarks[33\4], that granite, gneiss, and the like, form a class {296} which should be designated by a common name; which name should not be of chronological import. He proposes _hypogene_, signifying 'nether-formed;' and thus he adopts the theory that they have not assumed their present form and structure at the surface, but determines nothing of the period when they were produced. [Note 33\4: _Princ. Geol._ iv. 386.]