By the _Form_ of terms, I mean their philological conditions; as, for example, from what languages they may be borrowed, by what modes of inflexion they must be compounded, how their derivatives are to be formed, and the like. In this, as in other parts of the subject, I shall not lay down a system of rules, but shall propose a few maxims. APHORISM XX. _The two main conditions of the Form of technical terms are, that they must be generally intelligible, and susceptible of such grammatical relations as their scientific use requires._ THESE conditions may at first appear somewhat vague, but it will be found that they are as definite as we could make them, without injuriously restricting ourselves. It will appear, moreover, that they have an important bearing upon most of the questions respecting the form of the words which come before us; and that if we can succeed in any case in reconciling the two conditions, we obtain terms which are practically good, whatever objections may be urged against them from other considerations. 1. The former condition, for instance, bears upon the question whether scientific terms are to be taken from the learned languages, Greek and Latin, or from our own. And the latter condition very materially affects the same question, since in English we have scarcely any power of inflecting our words; and therefore must have recourse to Greek or Latin in order to obtain terms which admit of grammatical modification. If we were content with the term _Heat_, to express the _science_ of heat, still it would be a bad technical term, for we cannot derive from it an adjective like {319} _thermotical_. If _bed_ or _layer_ were an equally good term with _stratum_, we must still retain the latter, in order that we may use the derivative _Stratification_, for which the English words cannot produce an equivalent substitute. We may retain the words _lime_ and _flint_, but their adjectives for scientific purposes are not _limy_ and _flinty_, but _calcareous_ and _siliceous_; and hence we are able to form a compound, as _calcareo-siliceous_, which we could not do with indigenous words. We might fix the phrases _bent back_ and _broken_ to mean (of optical rays) that they are reflected and refracted; but then we should have no means of speaking of the angles of _Reflection_ and _Refraction_, of the _Refractive_ Indices, and the like. In like manner, so long as anatomists described certain parts of a vertebra as _vertebral laminæ_, or _vertebral plates_, they had no adjective whereby to signify the properties of these parts; the term _Neurapophysis_, given to them by Mr. Owen, supplies the corresponding expression _neurapophysial_. So again, the term _Basisphenoid_, employed by the same anatomist, is better than _basilar_ or _basial process of the sphenoid_, because it gives us the adjective _basisphenoidal_. And the like remark applies to other changes recently proposed in the names of portions of the skeleton.