Astrology also supplied a number of words founded upon fanciful opinions; but this study having been expelled from the list of sciences, such words now survive, only so far as they have found a place in common language. Thus men were termed _mercurial_, _martial_, _jovial_, or _saturnine_, accordingly as their characters were supposed to be determined by the influence of the planets, Mercury, Mars, Jupiter, or Saturn. Other expressions, such as _disastrous_, _ill-starred_, _exorbitant_, _lord of the ascendant_, and hence _ascendancy_, _influence_, {268} a _sphere of action_, and the like, may serve to show how extensively astrological opinions have affected language, though the doctrine is no longer a recognized science. The preceding examples will make it manifest that opinions, even of a recondite and complex kind, are often implied in the derivation of words; and thus will show how scientific terms, framed by the cultivators of science, may involve received hypotheses and theories. When terms are thus constructed, they serve not only to convey with ease, but to preserve steadily and to diffuse widely, the opinions which they thus assume. Moreover, they enable the speculator to employ these complex conceptions, the creations of science, and the results of much labour and thought, as readily and familiarly as if they were convictions borrowed at once from the senses. They are thus powerful instruments in enabling philosophers to ascend from one step of induction and generalization to another; and hereby contribute powerfully to the advance of knowledge and truth. It should be noticed, before we proceed, that the names of natural objects, when they come to be considered as the objects of a science, are selected according to the processes already enumerated. For the most part, the natural historian adopts the common names of animals, plants, minerals, gems, and the like, and only endeavours to secure their steady and consistent application. But many of these names imply some peculiar, often fanciful, belief respecting the object. Various plants derive their names from their supposed virtues, as _herniaria_, _rupture-wort_; or from legends, as _herba Sancti Johannis_, _St. John's wort_. The same is the case with minerals: thus the _topaz_ was asserted to come from an island so shrouded in mists that navigators could only _conjecture_ (τοπάζειν) where it was. In these latter cases, however, the legend is often not the true origin of the name, but is suggested by it.